ArticlePDF AvailableLiterature Review

Masks and respirators for prevention of respiratory infections: a state of the science review

Authors:
  • University of New South Wales (UNSW) Sydney

Abstract

This narrative review and meta-analysis summarizes a broad evidence base on the benefits—and also the practicalities, disbenefits, harms and personal, sociocultural and environmental impacts—of masks and masking. Our synthesis of evidence from over 100 published reviews and selected primary studies, including re-analyzing contested meta-analyses of key clinical trials, produced seven key findings. First, there is strong and consistent evidence for airborne transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and other respiratory pathogens. Second, masks are, if correctly and consistently worn, effective in reducing transmission of respiratory diseases and show a dose-response effect. Third, respirators are significantly more effective than medical or cloth masks. Fourth, mask mandates are, overall, effective in reducing community transmission of respiratory pathogens. Fifth, masks are important sociocultural symbols; non-adherence to masking is sometimes linked to political and ideological beliefs and to widely circulated mis- or disinformation. Sixth, while there is much evidence that masks are not generally harmful to the general population, masking may be relatively contraindicated in individuals with certain medical conditions, who may require exemption. Furthermore, certain groups (notably D/deaf people) are disadvantaged when others are masked. Finally, there are risks to the environment from single-use masks and respirators. We propose an agenda for future research, including improved characterization of the situations in which masking should be recommended or mandated; attention to comfort and acceptability; generalized and disability-focused communication support in settings where masks are worn; and development and testing of novel materials and designs for improved filtration, breathability, and environmental impact.
Conduct of Scientic Research | Review
Masks and respirators for prevention of respiratory infections: a
state of the science review
Trisha Greenhalgh,1 C. Raina MacIntyre,2 Michael G. Baker,3 Shovon Bhattacharjee,2,4 Abrar A. Chughtai,5 David Fisman,6 Mohana
Kunasekaran,2 Amanda Kvalsvig,3 Deborah Lupton,7 Matt Oliver,8 Essa Tawq,2 Mark Ungrin,9 Joe Vipond10
AUTHOR AFFILIATIONS See aliation list on p. 48.
SUMMARY ...................................................................................................................................................................... 2
INTRODUCTION..............................................................................................................................................................2
Rationale and aim......................................................................................................................................................2
Methodological approach......................................................................................................................................3
THE BASIC SCIENCE OF MASKING.......................................................................................................................... 4
Transmission of respiratory infections...............................................................................................................4
What are masks and how do they work?........................................................................................................10
Standards and certication................................................................................................................................. 13
CLINICAL TRIALS OF MASKS AND RESPIRATORS...........................................................................................15
Methodological challenges in trials and meta-analyses of masks........................................................ 15
Nature of the outcome.....................................................................................................................................16
Seasonal and year-on-year variation.......................................................................................................... 16
Variable primary outcomes............................................................................................................................ 16
Combining dissimilar interventions............................................................................................................. 17
Combining dierent settings..........................................................................................................................17
Combining heterogeneous outcomes..........................................................................................................17
A new meta-analysis: justication of approach........................................................................................ 18
Reanalysis of RCTs of masks in community settings.................................................................................. 18
Reanalysis of RCTs of masks and respirators in health-care settings....................................................27
Comment................................................................................................................................................................... 27
NON-EXPERIMENTAL EVIDENCE ON EFFICACY..............................................................................................27
Observational studies............................................................................................................................................27
Modeling masking..................................................................................................................................................30
ADVERSE EFFECTS AND HARMS OF MASKS....................................................................................................33
Introduction and general adverse eects......................................................................................................33
Discomfort and local irritation...................................................................................................................... 33
Eects during exercise......................................................................................................................................33
Speculated but unconrmed harms in anti-mask discourse..................................................................34
Adverse eects of masks in people in particular risk groups..................................................................34
Children............................................................................................................................................................... 34
Adults with medical conditions..................................................................................................................... 35
Masks and communication................................................................................................................................. 35
SOCIAL AND POLITICAL ASPECTS OF MASKING........................................................................................... 37
Why people mask and why they don’t............................................................................................................37
Communicating information and managing misinformation about masks.....................................38
MASKING AS POLICY.................................................................................................................................................39
Dierent types of mask policies.........................................................................................................................39
Mask policies for targeted personal protection......................................................................................... 39
Mask policies for specic settings..................................................................................................................39
Mask policies for seasonal respiratory infections......................................................................................40
Mask policies for pandemics.......................................................................................................................... 40
Developing and implementing mask policies..............................................................................................40
SINGLE-USE MASKS AND RESPIRATORS: ENVIRONMENTAL IMPACT....................................................41
The scale of environmental harm......................................................................................................................41
Mitigating environmental harm from single-use masks and respirators: what can be done?... 43
Increase public awareness.............................................................................................................................. 43
Improve mask waste management..............................................................................................................43
Recycle mask waste.......................................................................................................................................... 43
Promote reuse and extended use.................................................................................................................. 44
Normalize elastomeric respirators................................................................................................................44
Month XXXX Volume 0 Issue 0 10.1128/cmr.00124-23 1
Editor Ferric C. Fang, University of Washington
School of Medicine, Seattle, Washington, USA
Ad Hoc Peer Reviewer Linsey Marr, Virginia Tech,
Blacksburg, Virginia, USA
Address correspondence to Trisha Greenhalgh,
trish.greenhalgh@phc.ox.ac.uk.
T.G. is a member of Independent SAGE, a UK-
based group of scientists who engage directly
with the public and produce reports and resources
related to COVID-19; this role is unremunerated
(see www.independentsage.org). D.F. received a
grant from the Canadian Institutes for Health
Research (2019 COVID-19 rapid researching funding
OV4-170360); has served on advisory boards related
to inuenza and SARS-CoV-2 vaccines for Seqirus,
Pzer, Astrazeneca, and Sano-Pasteur Vaccines; and
has served as a legal expert on issues related to
COVID-19 epidemiology for the Elementary Teachers
Federation of Ontario and the Registered Nurses
Association of Ontario.
We dedicate this paper to the memories of those who
died in the COVID-19 pandemic. We hope that this
review will prompt a much-needed debate about the
role of masks and respirators in respiratory epidemics
and pandemics, allowing the research community and
policymakers to better address the ongoing challenge of
SARS-CoV-2 and deal with future (unknown) threats.
Published 22 May 2024
Copyright © 2024 American Society for
Microbiology. All Rights Reserved.
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Develop biodegradable and reusable masks.............................................................................................44
Formulate relevant policies and regulations..............................................................................................44
Toward better masks..............................................................................................................................................44
SUMMARY AND CONCLUSION..............................................................................................................................45
ACKNOWLEDGMENTS...............................................................................................................................................48
AUTHOR AFFILIATIONS.............................................................................................................................................48
AUTHOR CONTRIBUTIONS......................................................................................................................................49
Data Availability..........................................................................................................................................................49
Ethics approval............................................................................................................................................................ 49
REFERENCES..................................................................................................................................................................49
AUTHOR BIOS...............................................................................................................................................................61
SUMMARY This narrative review and meta-analysis summarizes a broad evidence base
on the benets—and also the practicalities, disbenets, harms and personal, sociocul
tural and environmental impacts—of masks and masking. Our synthesis of evidence
from over 100 published reviews and selected primary studies, including re-analyzing
contested meta-analyses of key clinical trials, produced seven key ndings. First, there
is strong and consistent evidence for airborne transmission of severe acute respiratory
syndrome coronavirus 2 (SARS-CoV-2) and other respiratory pathogens. Second, masks
are, if correctly and consistently worn, eective in reducing transmission of respiratory
diseases and show a dose-response eect. Third, respirators are signicantly more
eective than medical or cloth masks. Fourth, mask mandates are, overall, eective in
reducing community transmission of respiratory pathogens. Fifth, masks are important
sociocultural symbols; non-adherence to masking is sometimes linked to political and
ideological beliefs and to widely circulated mis- or disinformation. Sixth, while there
is much evidence that masks are not generally harmful to the general population,
masking may be relatively contraindicated in individuals with certain medical conditions,
who may require exemption. Furthermore, certain groups (notably D/deaf people) are
disadvantaged when others are masked. Finally, there are risks to the environment from
single-use masks and respirators. We propose an agenda for future research, including
improved characterization of the situations in which masking should be recommended
or mandated; attention to comfort and acceptability; generalized and disability-focused
communication support in settings where masks are worn; and development and testing
of novel materials and designs for improved ltration, breathability, and environmental
impact.
KEYWORDS masks, respirators, respiratory infections, methodology, meta-analysis,
narrative review, SARS-CoV-2, infection prevention and control, non-pharmaceutical
interventions
INTRODUCTION
Rationale and aim
The ecacy, acceptability, and safety of masks and other face coverings have been
among the most important and contested scientic questions of the coronavirus
disease 2019 (COVID-19) pandemic. Masks have long been used to try to reduce
transmission of respiratory diseases, both in endemic conditions such as tuberculosis
and in epidemics such as European bubonic plague (1619), the Great Manchurian plague
(1910), inuenza (1918–1919), severe acute respiratory syndrome (SARS) (2003), and
Middle East respiratory syndrome (MERS) (2013) (1–8). Current debates relating to masks
have origins which stretch back decades and even centuries (7, 8).
The need for a new review on masks was highlighted by a widely publicized
polarization in scientic opinion. The masks section of a 2023 Cochrane review
of non-pharmaceutical interventions (9) was—controversially—limited to randomized
controlled trials (RCTs). It was interpreted by the press and by some but not all of its
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own authors to mean that “masks don’t work” and “mask mandates did nothing” (10).
Cochrane’s editor-in-chief felt the need to state publicly that in Cochrane’s view, the
review’s ndings did not support such a conclusion (11). Some scholars were quick
to question the review’s methodology, especially key aws in the meta-analysis and
omission of a vast body of non-RCT evidence (12–16).
There are many additional complexities in the mask debate. As we explain in The
Basic Science of Masking, the term “mask” covers a multitude of devices with dierent
material properties; respirators have a more standardized design but are not widely
used even in health-care settings. Some clinical trials of masks and respirators did not
adequately dene or optimize the intervention or maintain its delity, used heteroge
nous interventions and outcomes, or failed to measure whether masks were actually
worn (see Clinical Trials of Masks and Respirators). In non-RCT studies, it has been dicult
to isolate the eect of masking from that of confounding, eect modication, and bias,
such as use of other mitigations, concurrent lockdown, or changes in disease preva
lence (see Non-experimental Evidence on Ecacy). Whatever their protective eects,
masks have some drawbacks, and some people nd it dicult or impossible to wear
them (see Adverse Eects and Harms of Masks). Masks are not just protective devices—
they are cultural and even political symbols about which people may feel strongly.
People’s beliefs about masks may be inuenced by misinformation, which may be widely
circulated (see Social and Political Aspects of Masking). Mask mandates, which require
everyone to wear a mask in certain circumstances, have played out dierently in dierent
jurisdictions and sociocultural settings (see Masking as Policy). Single-use masks and
respirators contribute to non-biodegradable waste and environmental pollution, though
research on recycling, reuse, and novel materials points to some potential solutions (see
Single-Use Masks and Respirators: Environmental Impact).
This review had three main objectives: rst, to summarize the evidence base from
multiple disciplines and study designs for the benets—and also the practicalities,
disbenets, and harms—of masks and masking; second, to examine why the evidence
on these topics is so widely misunderstood, misinterpreted, or dismissed; and third, to
outline an agenda for future research.
Methodological approach
The review was registered on the International Platform of Registered Systematic Review
and Meta-analysis Protocols (number 202410087). An initial scoping search on masks
and masking in respiratory infections identied thousands of studies and more than 100
reviews. In view of this, our chosen review design was an in-depth narrative review in
the hermeneutic tradition, whose primary aim was to make sense of this vast literature
(17). We sought to summarize previous reviews and also, where necessary, to analyze
and critique the key primary studies on which those reviews were based. In a hermeneu
tic review, a thorough literature search is undertaken to identify the most inuential
sources in each tradition. A narrative summary is prepared based on these key sources
and progressively rened by adding further sources as they are identied (17). This
method allowed us to tease out the multiple ways in which masks and masking have
been framed and examined by dierent groups of scientists. A systematic review and
meta-analysis of RCTs on masks in community and health-care settings was conducted
within the larger narrative review. Details of the methods can be found in the section A
New Meta-Analysis: Justication of Approach.
Part of the confusion on this topic can be traced back to philosophical issues such
as ontology (what is the nature of reality?) and epistemology (how can we know that
reality?), on which dierent scholars took widely diering views. The Cochrane review of
non-pharmaceutical interventions, for example, appears to rest on the assumption that
trustworthy evidence on this topic comes primarily or exclusively from RCTs and that if
RCTs have been identied, non-RCT evidence can be ignored (9). An alternative view is
that evidence-based medicine’s “hierarchy of evidence” (with RCTs as the assumed gold
standard) is inappropriate for multifaceted topics such as masking (12, 14, 18, 19). Some
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have argued that the scientic value of the RCT has become inated, particularly among
doctors, leading them to overlook high-quality non-RCT evidence (e.g., mechanistic
evidence about how the virus spreads can inform optimization of intervention design,
and studies of how masking policies played out in real-world settings can provide useful
case studies for policymakers in other settings). Overvaluing the RCT as a design also
allows poor-quality RCTs (e.g., of intervention designs which do not take account of
mechanism and which may therefore mislead rather than inform) to be published in
high-impact journals and gain undue inuence (20, 21). PubMed lists 88 meta-analyses
of trials and other comparative studies of masks undertaken since 2020, with varying
research questions, methods, and interpretations. Importantly, systematic review and
meta-analysis can be subject to signicant biases, aws, and mistakes, just as in any
other research design (22). These synthesis methods are limited by the quality of primary
studies included and should not be considered denitive without critical analysis.
To identify key reviews and primary studies, we recruited authors familiar with
relevant literature in a wide range of disciplines (public health, epidemiology, infectious
diseases, biosecurity, uid dynamics, materials science, modeling, data science, clinical
trials, meta-analysis, sociology, anthropology, psychology, and occupational hygiene).
We began with sources known to these authors and supplemented them by searching
PubMed, Embase, and Social Science Citation Index using key words. We citation-tracked
seminal sources using Google Scholar. We also sent requests to colleagues in relevant
elds and posted on social media (X, Mastodon, and BlueSky).
Table 1 summarizes the study designs which we captured as relevant to a compre
hensive hermeneutic review of masks and masking. Column 2 in Table 1 explains
the potential contribution each design makes; columns 3 and 4, respectively, give the
strengths and limitations of each design. The table is not exhaustive; additional designs
may be relevant.
THE BASIC SCIENCE OF MASKING
Transmission of respiratory infections
To reduce spread of respiratory diseases, we need to understand the mechanisms of
spread. There is strong and consistent evidence that respiratory pathogens including
severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), respiratory syncytial virus
(RSV), inuenza, tuberculosis, and other coronaviruses such as MERS and SARS-1, are
transmitted predominantly via aerosols (32). Infected individuals, whether symptomatic
or not, continuously shed particles containing pathogens, which remain viable for
several hours and can travel long distances (33). SARS-CoV-2 is shed mainly from deep
in the lungs, not the upper respiratory tract, and the viral load is higher in small aerosols
(generated in the lower airways) than in larger droplets (generated in upper airways) (34,
35). Whereas large respiratory droplets emitted when people cough or sneeze fall quickly
by force of gravity without much evaporation, those below 100 µm in diameter become
(bio)aerosols. Even particles tens of microns in diameter at release will shrink almost
immediately by evaporation to the point that under typical conditions they can remain
airborne for many minutes (36). In contrast with droplet transmission, which is generally
assumed to occur via a single ballistic hit, the risk of airborne transmission increases
incrementally with the amount of time the lung lining is exposed to pathogen-laden air,
in other words, with time spent indoors inhaling contaminated air (37).
Respiratory infections may theoretically also be transmitted by droplets, by direct
contact, and possibly by fomites (objects that have been contaminated by droplets) (38),
but the dominant route is via respiratory aerosols. The multiple streams of evidence
to support this claim for SARS-CoV-2 include the patterning of spread (mostly indoors
and especially during mass indoor activities involving singing, shouting, or heavy
breathing), direct isolation of viable virus from the air and in air ducts in ventilation
systems, transmission between cages of animals connected by air ducts, the high
rate of asymptomatic transmission (i.e., passing on the virus when not coughing or
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TABLE 1 Dierent study designs contributing to mask research
Study design Contribution Strengths Limitations
Laboratory studies
Laborator y studies of aerosols and
aerosolized pathogens
Developing and testing hypotheses about
how aerosols (and pathogens in aerosols)
behave
Generates mechanistic evidence about how
transmission occurs and how masks might work
to reduce it; relatively low cost
Requires an understanding of physics,
engineering and uid dynam
ics, plus specialized equipment
(see Transmission of Respiratory
Infections and Table 2 for further
details)
Material and engineering studies Developing and rening physical properties
needed for masks (notably, ltration
standards)
Generates mechanistic evidence on
ltration properties of mask materials;
randomization and controls routinely used as
appropriate, informs engineering standards,
relatively low cost
Requires an understanding
of chemistry, materials and
engineering, plus specialized
equipment (see What Are Masks and
How Do They Work? and Table 2 for
further details)
Randomized controlled trials
RC Ts randomized by individual
participant
Testing hypotheses about the ecacy
of a specic intervention in preventing
wearers from getting infected or testing
the ecacy of source control (preventing
outward transmission from infected to
uninfected people)
High internal validity (can demonstrate ecacy
of a specic
intervention in a particular context and potential
for ecacy in other contexts); a specic
intervention can be developed or selected;
strong design to use when unmeasured
confounding is a concern
Expensive and long duration;
unless informed by mechanistic
understanding, interventions may
be suboptimally designed, leading
to underestimation of ecacy;
cannot measure bidirectional
eects (on wearer and source
control) (23). Some methodologi
cal variations include mismeasure
ment of outcomes (e.g. 24, 25)];
interventions may be heterogene
ously applied across trials (e.g.,
continuous vs intermittent N95
use); variation in disease prevalence
may lead to underpowering; if
outcome is a communicable disease,
outcomes in individuals randomized
to dierent interventions within a
closed setting such as a house
hold or hospital are not independ
ent (cluster randomization must be
used); limited external validity (a
negative result cannot prove lack of
ecacy under other conditions); and
lack of dierence between arms in
the absence of a no-mask control
arm may reect equal inecacy or
equal ecacy; ethical concerns if
researchers are not in equipoise (26)
RC Ts randomized by locality or
organization (cluster RCTs)
Testing hypotheses about the ecacy of a
specic intervention in reducing disease
transmission in a population
High internal validity; a specic intervention
can be developed or selected; bias due
to non-independence of outcomes is less
problematic than in individually randomized
RCTs (27, 28)
May be even more expensive than
RCTs randomized by individual; long
duration. Interventions developed
without mechanistic understanding
or poor choice of intervention
or outcome measures may be
suboptimally designed, resulting in
underestimation of ecacy. May still
be unable to tease apart direct
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TABLE 1 Dierent study designs contributing to mask research (Continued)
Study design Contribution Strengths Limitations
and indirect impacts of masks,
potentially resulting in overestima
tion of ecacy; ethical concerns if
researchers are not in equipoise
Observational designs and modeling
Obser vational studies: cohort and
case-control
Assessing the likelihood of an outcome
based on exposure status
Can be conducted more rapidly, retrospectively,
or prospectively, and at lower cost than RCTs
(14)
Common to all observational studies
is the risk of bias, confounding,
or eect modication (e.g., due to
concurrent policy changes such as
lockdown); known confounders can
be adjusted to some extent using
multivariate regression analysis.
Studies that use surveillance data
may be subject to ascertainment
bias (e.g., if cases are identied only
on the basis of symptomatic illness,
rather than evidence of infection).
There may be confounding by
indication (e.g., masks tend to be
introduced when risk is elevated).
Such biases, if present, would lead to
an underestimate of the protective
eect from masking.
Obser vational studies: ecological
studies and quasi-experiments
Documenting what happens before and
after an intervention is introduced in a
real-world setting (e.g., community mask
mandates).
Can be rapid and low cost; geographical variation
can permit adjustment for unmeasured
confounding (pseudo-randomization) using
instrumental variable analysis
Obser vational studies: “real-world
evidence”
Large observational, epidemiological
studies using data derived from
administrative databases
Can be rapid and low cost
Modeling studies Models are simplied versions of reality,
which can provide insights into complex
phenomena and test hypothetical future
scenarios. There are many dierent
modeling methods (e.g., SEIR, agent
based, and statistical).
Can impute indirect eects of an intervention
(e.g., impact of some people masking on others
who do not mask). Helps optimize interventions
by answering “what-if” questions (e.g., to assess
costs vs benets of modications); useful for
future scenarios (e.g., pandemic planning) or
crisis response. Good models are transparent
and can be reproduced, use evidence-based
parameters, and deal with uncertainty by using
sensitivity analyses.
Depends on the quality of input data,
which be unavailable or suboptimal
(“garbage in, garbage out”), and
on the assumptions and parameters
used. There is always a need
for some simplifying assumptions,
but models that are too simple
may be less accurate. Interpreta
tion and critique of mathemati
cal models of pandemic policies
require advanced interdisciplinary
knowledge, including mathemat
ics, statistics, medicine, behavio
ral science, and economics (29).
Modeling that lacks interdisciplinary
expertise may be less robust.
Social and psychological studies
Sur veys of masking behavior People’s self-reported masking behavior Relatively easy and quick to administer; can gain
large sample sizes (e.g., using online methods)
Subject to recall bias and participant
bias (people who feel strongly
about masks either negatively or
positively may be more likely to
respond). Other predictor variables
are also self-reported, so may
not be reliable. Low-literacy and
marginalized individuals may be
underrepresented.
Obser vational studies of masking
behavior
Direct observation of whether people are
masking (e.g., in a public place)
Relatively easy to undertake as a one-shot
survey; may use video data (e.g., as people pass
by) (30, 31).
Cross sectional studies represent a
snapshot in time, so cannot capture
dynamic changes in behavior.
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TABLE 1 Dierent study designs contributing to mask research (Continued)
Study design Contribution Strengths Limitations
Sur veys of attitudes and intentions People’s stated reasons for masking or not Relatively easy and quick to administer; can gain
large sample sizes (e.g., using online methods)
Responses may not reect attitudes
or reasons (due to, e.g., social
desirability bias). Sampling biases
and low response rates reduce
validity of ndings (non-responders
dier from responders). Low-literacy
and marginalized individuals may be
underrepresented.
Communication studies Identifying how mask-wearing advice can
be communicated and incentivized to
communities
Draws on multiple (qualitative and quantita
tive) methods. Can generate mechanistic
understanding
Communications research on majority
groups may not be directly
transferable to minority or
marginalized groups.
Policy analyses Qualitative analyses of how policy is made
and what good policy is, including what is
morally right and politically feasible
Can help identify the root causes of a problem,
clarify the goals of a proposed policy and assess
its potential eectiveness
Depends on high-quality data on the
options and their outcomes; may
be impossible if such knowledge
is lacking. May produce an overly
sanitized and technical view of
policy.
Sociomaterial analyses Qualitative studies of the symbolic meaning
of masks in dierent cultural and social
settings
Combines multiple methods to examine why
some groups feel strongly (positively or
negatively) about masks; takes into account
social, cultural, historical, and political contexts
of attitudes and practices
Requires advanced training in
sociological theories and methods;
often limited to specic sampled
groups, hence are not usually
generalizable to the broader
population
Social media studies Quantitative and qualitative analyses of
how (mis)information spreads on social
media
Combines multiple methods to track the spread
of (mis)information on particular platforms and
the sentiments demonstrated in these media
May require advanced technical
skill, such as machine learning,
and computing power; qualitative
studies require advanced training in
interpretation and analysis
Health economic studies Cost-benet and cost-eectiveness of
interventions
Estimates the direct (and some indirect) nancial
costs and benets of a particular approach to
masking; cost-eectiveness analysis requires a
common unit of outcome measurement such
as quality-adjusted life years gained or lost; can
use a government payer or societal perspective
May have limited external validity
since costs are locality or country
specic, and change over time;
subject to the same limitations of
modeling studies, such as quality
of data and model assumptions.
Studies that consider only the
government payer perspective or
only the acute infectious episode
underestimate true societal costs.
Metrics such as quality-adjusted life
years are dicult to operationalize in
children.
Evidence syntheses
Systematic review Review undertaken according to explicit
and reproducible criteria
If undertaken well, tends to identify all or
most relevant studies and produces accurate
assessment of key biases and contribu
tions of included primary studies; prospec
tive registration (e.g., on the international
prospective register of systematic reviews
[PROSPERO]) increases transparency
Data extraction and evaluation
of primary evidence depend on
reviewers’ judgments, especially if
multiple reviewers share awed
assumptions and biases, can lead
to spurious ndings that are given
undue credibility because of the
“systematic” kitemark. Inappropriate
application of rigid evidence
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sneezing), and transmission in quarantine hotels when individuals in dierent rooms
shared corridor air but did not meet or touch any common surface (32, 39, 40).
Historically, droplet modes of transmission were erroneously categorized as a discrete
mode and considered dominant for many respiratory diseases, including inuenza,
measles, smallpox, SARS-1, and tuberculosis. This was partly because inuential
infectious disease physicians in the early 20th century (notably Charles Chapin) initiated
a number of awed assumptions and logical fallacies in their pronouncements and
publications (Box 1) (41–43). We summarize these errors here because they underpin a
linked set of awed arguments relevant to the masking debate which persist, particularly
within the infection prevention and control (IPC) community, to this day (44, 45).
Chapin’s statements in a 1914 article (without empirical evidence) that respiratory
diseases are transmitted primarily by “spray” and that airborne transmission is “negligi
ble” (page 430) (46) became a bibliographic virus, reproduced in numerous subsequent
books and articles. Then as now, the idea that airborne transmission had to be “demon
strated” but droplet transmission could be “assumed” was rarely challenged. Airborne
TABLE 1 Dierent study designs contributing to mask research (Continued)
Study design Contribution Strengths Limitations
hierarchies may lead to systematic
biases (e.g., omission of all non-RCT
evidence) (22).
Meta-analyses (some limited to
RCTs, some focusing on or including
observational evidence)
Testing various hypotheses, depending on
primary studies
Combining ndings from primary studies may
increase power of estimates; relatively low cost;
prospective registration increases transparency.
A meta-analysis is only as robust
as the primary studies on which
it is based and the methodology
used. If primary studies are few and
poor quality, or if dierent methods
and outcomes are combined
inappropriately, conclusions will be
unreliable. When a specic bias
mechanism is common in primary
studies, pooled estimates of eect
will reproduce and magnify this
bias. If interventions or outcomes
that are dissimilar are combined
for meta-analysis, results may not
be informative or valid. Reviewers
lacking key content expertise may
miss, misunderstand, or misinterpret
key data or concepts.
Narrative reviews Broad-ranging syntheses drawing on
multiple sources of evidence
Goal is typically to draw together and make
sense of a complex literature; can combine
mechanistic and probabilistic evidence
Traditionally criticized for being
non-systematic, though high-quality
narrative reviews have a systematic
and rigorous methodology, with a
focus on interpretation (17).
Environmental impact reviews Narrative reviews which summarize
epidemiological, chemical, engineering,
and toxicological evidence
Combines multiple methods to produce depth
and detail on the specic issue of environmen
tal impact
Requires multiple sources of
high-quality evidence; such data
may not be available or may be
suboptimal
Historical studies Narrative reviews drawing on data from
past outbreaks
Provides rich case studies of historical
events and how they unfolded over time
Can be hard to conrm whether
interventions were informed by
eld-specic expertise; data sources
may be incomplete; transferability
to present-day outbreaks may be
questioned
aRCT, randomized controlled trial; SEIR, susceptible, exposed, infected, recovered.
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transmission is uniquely dicult to demonstrate empirically. To do so requires sophisti
cated equipment (e.g., air needs to be concentrated or passed through an air sampler
prior to analysis, and this is hard to do without damaging the virus). IPC physicians
unfamiliar with these techniques attempted and failed to isolate viable virus from air
samples obtained in simple “cough bags” and interpreted their ndings (erroneously)
as evidence that there was no viable virus in the air (47). Yet numerous studies, many
done back in 2020, identied SARS-CoV-2 RNA or viable virus in the air in hospital rooms,
and on ceiling air vents. Lednicky et al. made the point that the method of air sampling
determined the likelihood of detecting viable virus, as most air samplers use high shear
force, which destroys viable virus (48).
Large droplet transmission is generally assumed to occur only at close quarters
(within 1–2 m), but it does not follow that any transmission within this distance must
be via large droplets. Similarly, just because aerosols travel beyond 2 m, it does not
follow that airborne transmission occurs only at these longer distances. While some IPC
clinicians (among others) conate close-contact transmission with droplet transmission,
basic physics tells us that the closer one is to a source of airborne pathogens, the more
likely one is to inhale those pathogens (just as the closer one sits to a smoker, the more
smoke one inhales), and that close contact involves exposure to aerosols of all sizes.
Because some droplets above 100 µm in diameter are smaller than the lumen of
the smallest bronchioles (approximately 1 mm) and follow ballistic trajectories (e.g., in
sneezes), it is sometimes assumed that they can be carried deep into the lungs on these
trajectories, allowing SARS-CoV-2 to infect its target cell (type II alveolar pneumocytes)
directly via inhalation of large droplets. In fact, access to the alveoli via the air requires
transport as aerosol particles below 5 µm (33, 49, 50). For the same reason a motorcy
cle can negotiate a tighter corner than a laden truck moving at similar speed, larger
particles collide with the walls of the upper respiratory tract and are deposited there and
prevented from penetrating further by mucociliary clearance mechanisms (49).
Droplet transmission was given undue credence by an incorrect cut-o for the size of
a droplet (51). A 1934 article by engineer William Wells (whose wife and co-investigator
was a physician) correctly hypothesized that tuberculosis was airborne and correctly
proposed a cut-o of 100 µm to distinguish between droplets and aerosols (52). Wells’
work was ignored in the medical literature until 1962, when he demonstrated the
airborne nature of tuberculosis in a series of elegant experiments involving guinea
pigs (53). In another experiment, Wells exposed rabbits to ne aerosols (below 5 µm)
and coarse aerosols (above 5 µm) containing tuberculosis bacteria; only those exposed
to the ne aerosols became sick (54). This was because, like SARS-CoV-2, tuberculosis
infects the alveolar pneumocytes, and in Wells’ experiment, the larger particles, even
though airborne, did not reach the target cells. Flawed conation of the maximum size
at which a particle is infectious (below 5 µm) with the maximum size at which it can
be airborne (100 µm) led to many in the IPC community (including the World Health
Organization [WHO] and the U.S. Centers for Disease Control and Prevention [CDC])
coming to view 5 µm as the maximum size of an airborne particle (42). This reinforced
the false conclusion that many of the viral-laden particles entering the lungs were
droplets, not aerosols.
Another assumption among infectious disease physicians in early 2020 was that
because the most well-established airborne viruses such as measles and chickenpox
had very high reproduction number, R0 (estimates of up to 18), the novel coronavirus
SARS-CoV-2 (with an estimated R0 of 2–5) was therefore not signicantly airborne. The
R0 is not, however, a scientic indicator of mode of transmission (55, 56). As the example
of tuberculosis illustrates, a pathogen may be airborne but have low transmissibility,
as indicated by the average R0 value (57), especially when R0 is overdispersed (i.e.,
skewed) (58). Furthermore, as the SARS-CoV-2 virus has evolved, there is evidence that
the transmissibility of this virus is now substantially higher than the original estimate
(59).
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The so-called “aerosol-generating medical procedure” (AGMP) is based on the
assumption that a patient with a respiratory infection will generate bioaerosols only
during certain medical procedures such as bronchoscopy or physical therapy (60). The
patient is assumed to be non-infectious unless actively provoked. In fact, breathing,
speaking, shouting, and singing all generate similar or higher levels of aerosols to
many so-called AGMPs (35, 61). Further, breathing is continuous and speaking occurs
repeatedly over long periods, so the cumulative aerosol exposure is much higher from
routine human emissions than from a single medical procedure.
An assumed droplet and contact mode of transmission leads to prevention policies
that center on handwashing and surface cleansing, maintaining 2-m physical distancing,
wearing medical masks (whose waterproof backing is designed to stop droplets) within
that 2-m distance (especially when attending an infected patient), using physical barriers
(e.g., plastic screens) and providing health-care workers with higher-grade respiratory
protection only when undertaking AGMPs. However, if the virus is transmitted signif
icantly by the airborne route, dierent prevention policies are needed, oriented to
controlling air quality in indoor spaces (e.g., ventilation and ltration), reducing indoor
crowding and time spent indoors, wearing masks whenever indoors, careful attention to
mask quality (to maximize ltration) and t (to avoid air passing through gaps), taking
particular care during indoor activities that generate aerosols (e.g., speaking, singing,
coughing, and exercising), and providing respirator-grade facial protection to all sta
who work directly with patients (not just those doing AGMPs) (62, 63).
The misconceptions listed in Box 1 have been complicated by dierences in
terminology among aerosol scientists and IPC clinicians. IPC clinicians make a distinction
between “airborne” and “aerosol” and have tended to use the term “droplet nuclei” to
refer to small airborne particles, terms that are not used by aerosol scientists. The detail
of this linguistic impasse is beyond the scope of the article, but it has contributed to
persistent misunderstandings and a dismissal of much evidence by the WHO during
a key period of the COVID-19 pandemic (62). A recent attempt to “standardize” the
language of airborne transmission (64) has been criticized as naïve and unworkable (65).
In sum, despite claims to the contrary, the evidence on airborne transmission of
SARS-CoV-2 is clear, consistent, and denitive. It has been built from many dierent kinds
of evidence including a variety of laboratory-based designs (Table 2).
What are masks and how do they work?
Imposing any ltering material in the respiratory path will remove particulates or
aerosols (83). Face coverings for respiratory protection occupy a broad continuum from
simple one-layer improvised cloth coverings (84) to medical (or surgical) masks and
BOX 1: FLAWED ASSUMPTIONS AND LOGICAL FALLACIES ABOUT AIRBORNE
TRANSMISSION
The following incorrect assumptions have led to awed conceptual models and
ineective policies (see text for details and references):
1. Absence of direct evidence in favor of airborne transmission can be taken as
evidence refuting airborne transmission.
2. Because contact and droplet transmission can occur only during close contact, all
close-contact transmission must be contact and droplet.
3. Because large droplets are smaller than the lumen of the smallest bronchioles,
they can reach the key target cell for SARS-CoV-2 in the alveoli.
4. Particles above 5 µm in diameter are droplets and not aerosols.
5. Aerosols are produced in signicant numbers from infectious patients only when
aerosol-generating medical procedures (AGMPs) are done.
6. Only respiratory diseases with a high R0 (such as measles) are airborne.
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respirators, which are certied to a national or international standard such as NIOSH-42
CFR Part 84, EN149:2001 + A1:2009, or CSA/CAN Z94.4–18 (85). Respirators have been
developed and used mainly in the occupational health context (i.e., to protect workers
from hazards in their jobs).
All masks and respirators need to be assessed for four key properties. First, ltration
ecacy. This is inuenced by the lter material, ltration methods, air velocity, and
pore size (86, 87). The most challenging particle size range for masks and respirators
to lter is 0.05–0.5 µm, since such particles are most able to evade both mechani
cal and electrostatic ltration methods, posing an increased infection risk (87). This
ineciency is particularly critical for virus-laden aerosols and particles, underscoring
the need to incorporate reliable ltration capabilities in mask and respirator design.
Accumulated dust and microbial growth on the mask can reduce ltration eciency
(88). Respirators are regulated on ltration eciency but also on their ability to achieve
a specied minimum protection factor or t factor when worn, but surgical masks are
not. Surgical masks for medical use are typically certied based on synthetic blood
penetration resistance (ASTM F1862), bacterial ltration eciency (ASTM F2101), and
submicron particulate ltration eciency (F2299), but these tests are performed on a
sample coupon in a test xture with no t factor or protection factor requirements when
worn. This reects a key dierence between surgical masks and certied respirators: the
respirator must meet standards when worn in an occupational setting, integrated into a
complex system which includes the wearer, the work tasks, and all respiratory hazards.
Second, t and seal. Poorly tting masks and respirators allow air and micro-organ
isms to bypass the lter, leading to inhalation of unltered air and signicantly reduc
ing their eectiveness. Improper t can also compromise the mask’s ability to contain
exhaled droplets, undermining source control and leading to issues like fogging of
TABLE 2 Summary of how laboratory studies have contributed to mask researcha
Type of study Goal Examples of key ndings
Laboratory studies of aerosol
dynamics
Developing and testing hypotheses about the
physical behavior of aerosols
Data on, for example, rate of evaporation in particles of
dierent size (66)
Laboratory studies of aerosol
measurement systems
Developing and testing hypotheses about how
aerosols may be captured and quantied
Specic techniques for generating and measuring
aerosols in the laboratory (67–69)
Laboratory studies of pathogen
behavior within aerosols
Developing and testing hypotheses about how the
evolving aerosol environment aects pathogens
Insights into how an airborne pathogens’ viability
changes over time and in dierent ambient conditions
(70)
Laboratory and animal studies of
aerosol-generating processes
Developing and testing hypotheses about generation
of aerosols by host organisms
Demonstration that many regular activities such as
breathing, speaking, and singing generate similar levels
of aerosol to conventional AGMPs (71, 72)
Laboratory studies of uid dynamics
and airow
Developing and testing hypotheses about the airow
that transports infectious aerosols
Ecacy of dierent approaches to ventilation, comparing
turbulent vs laminar ow, toilet plumes, cough plumes
(73, 74)
Laboratory studies of how materials
interact with suspended particles
Developing and testing hypotheses about how
particles are captured on surfaces
Insights into processes of, for example, ltration,
deposition, adsorption, absorption, and electrostatics
(75–77)
Laboratory studies of how pathogens
interact with materials
Developing and testing hypotheses about how
materials aect pathogens
Insights into, for example, killing, persistence, permanent
capture, and retention vs later release of pathogens (78)
Laboratory studies of respiratory
dynamics
Developing and testing hypotheses about how air
ows within the respiratory tract
Insights into how alveoli behave in ination vs deation
(79)
Laboratory studies of respiratory
aerosol deposition
Developing and testing hypotheses about how
aerosols move within the respiratory tract
Demonstration that particles above 5–20 µm do not reach
the alveoli (33, 80)
Laboratory contribution to studies of
PPE eectiveness
Testing how PPE protection performs under
“real-world” conditions and how well PPE blocks
aerosols
Data on comfort and compliance; durability quantitative
performance assessment via, for example, t factor,
workplace protection factor, and assigned protection
factor (81); aerosol-blocking ability of dierent PPEs (82)
aAGMP, aerosol-generating medical procedure; PPE, personal protective equipment.
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goggles and glasses, which can aect visibility and safety (89, 90). Respirators are
designed with t and seal in mind; masks, generally, are not. A systematic review of
t testing studies of masks and respirators identied (from 137 studies) numerous factors
inuencing t: brand/model, style, gender, ethnicity, facial dimensions, facial hair, age,
reuse, extensive movement, seal check, comfort and usability assessment, and training
(91). Facial hair was a particular key factor contributing to poor t.
Third, breathing resistance. This is a key factor inuencing wearer comfort and safety
(92). Lower resistance ensures greater comfort but must be balanced against ltration
eciency.
Fourth, potential for contamination. Filter materials can, at least hypothetically,
accumulate viruses and bacteria over time, particularly if they lack anti-microbial
properties. While viable SARS-CoV-2 can be detected on plastic (including the outer
layer of a surgical mask) for several days (93–95), to our knowledge, no study has
ever demonstrated direct contagion with SARS-CoV-2 from a contaminated mask or
respirator. A simulation study using articial skin showed that viable virus was not
transferred from mask to skin (though viral RNA was), suggesting that the risk of
contagion may be less than previously assumed (96). Nevertheless, many masks and
respirators are designed with anti-microbial properties (see Toward Better Masks). The
sequence of donning and dong of personal protective equipment (PPE) is important in
reducing risk of self-contamination (97).
These properties are inter-related. Materials not designed for ltration typically
provide poorer ltration and may have higher breathing resistance (though broadly
speaking, resistance tends to correspond with ltration). A layered cotton bandana, for
example, provides some protection against large particulates but may resist airow and
lose ltration eectiveness as respiratory moisture saturates the material (98, 99). Hao
et al. found 7.1% ltration eciency for 0.3-µm particles through four layers of bandana
(100). While improvised or uncertied masks provide some reduction in dose exposure
against aerosol pathogens (101), they may not do so reliably or repeatably (102). The low
water and uid resistance of cloth masks reduces the ltration eectiveness compared to
surgical masks or respirators, highlighting the need for more research (86).
A broad category of ear-loop style or tied rectangular pleated masks is referred to
variously as surgical, medical, or procedure masks. Surgical masks are primarily intended
for interception of liquids and ballistic droplets as either source or exposure control.
Certied surgical or medical masks require objective testing, including against uid
penetration and skin irritation, and for bacterial ltering eciency under standards
such as ASTM F2100-21. Standards for such masks do not require achieving a leak-free
seal on the wearer’s face. Many consumer masks appear similar and may be labeled
“surgical” masks, but if not formally certied, their performance cannot be predicted.
While surgical masks reduce respiratory aerosols (103), culture-positive SARS-CoV-2 has
been detected in exhaled air from loose-tting surgical masks (104). Unsurprisingly,
masks and respirators with integrated exhaust valves allow respiratory aerosols from the
wearer to escape into the air even when t is good (105–108).
A respirator, or more correctly, a “ltering facepiece respirator” (FFP) or “air-purifying
respirator”, is designed to seal to the face and ensure that all inhaled and exhaled air
passes through the material rather than short-circuiting it. Respirators may be half or full
face and “disposable” (single-use) or elastomeric (reusable with a replaceable lter). To
achieve certication, respirators must be independently tested to conrm they meet the
standard, typically including a manufacturing quality management system and ability to
achieve a low-leak seal (109). Respiratory protection standards generally prescribe not
only the minimum level of protection required under dierent conditions but also an
eective workplace respiratory protection program (110), an important consideration
that was not incorporated into the design of most RCTs (see Clinical Trials of Masks and
Respirators).
The pore size in typical non-woven materials used in surgical masks and respirators
is larger than many viruses like SARS-CoV-2 (65–125 nm in diameter) (111). However,
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viruses travel through the air as passengers in or on respiratory aerosols, rather than
alone. In addition, masks and respirators are not simple sieves. They use a variety of lter
media which work in multiple ways, inuenced by the size of the particle (85):
Sedimentation of larger particles (1–10µm) under the inuence of gravity.
Inertial impaction, in which larger (above 1 µm) particles are blocked by bers in
the lter matrix.
Interception, in which medium-sized particles (smaller than 0.6 µm) following the
airow collide with bers.
Diusion, in which smaller lighter particles (0.02–0.40 µm) impact bers with
Brownian motion.
Electrostatic eects, in which particles passing near electrostatically charged bers
are attracted and captured; eective for all particle sizes, but particularly for
smaller particles (0.1–0.5 µm).
Electret-type lter material (possessing a non-dispersing electrostatic charge) is in
widespread use because of high performance derived by integrating these mechanisms.
Standards and certication
The International Standards Organization (ISO) denes standards as providing for
“achievement of the optimum degree of order in a given context” based on the
“consolidated results of science, technology and experience, and aimed at the promotion
of optimum community benets” (112).
Respiratory protection standards for respirators are designed to provide adequate
protection in realistic use settings; they include metrics like bench-test ltration and
sizing to t a broad range of face types. They typically require a worker to achieve a
minimum t factor through objective testing (e.g., conducting a set of standard exercises
while facing a challenge contaminant) before using a respirator (113). When a quantita
tive t factor of over 100 is achieved for a half-face respirator (a 100-fold reduction of
the challenge contaminant penetrating the respirator), the respirator is considered to
meet legal requirements under occupational health and safety legislation (114). While t
testing and training are important and legally mandated for occupational health, some
studies have shown similar levels of performance in non-t-tested respirators used by
untrained individuals, and this arrangement still outperforms other, uncertied types of
face coverings (115, 116). Standards for protection against bioaerosols vary, depending
on whether the hazard is fully known and quantied, compared with situations when the
risk level cannot be fully quantied (e.g., novel diseases), when a precautionary approach
should be taken (117).
Schmitt and Wang summarized measured t factors for a variety of types of face
coverings. Their ndings are shown diagrammatically in Fig. 1 (118).
Certied respirators are rated by the degree of ltration eciency provided, for
example, “N95” reects a minimum 95% reduction in 0.3-µm particles in a non-oily
atmosphere; “P100” reects a 99.97% reduction to the same exposure including oily
contaminants. While the standard species 0.3 µm as the minimum particle size for
testing, these respirators lter smaller and larger particles more eciently (0.3 µm is used
as the “most penetrating particle size” for ltration) (129).
Certied respirators are also distinguished from uncertied face coverings by the
assigned protection factor and achievable t factor.
The certication of surgical masks for particle/bacterial ltering eciency (P/BFE)
does not reect equivalence to respirators as the ltration is typically compromised by
poor face seal (77, 103). The ASTM F2100-21 P/BFE certication, for example, requires
at least 95% ltration against 0.1-µm particles and at least 98% against aerosolized
Staphylococcus aureus, but this is on a sample of the mask clamped in a xture, not
on a representative face. In terms of ltering aerosols, N95 respirators outperform
surgical masks between 8- and 12-fold (77). The eectiveness of certied surgical mask
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material against transmission when used as a lter was demonstrated in a hamster
SARS-CoV-2 model (130). Infected hamsters were separated from non-infected ones by a
partition made of surgical mask material; when the partition was in place, transmission of
SARS-CoV-2 was reduced by 75%.
In addition to protecting the wearer, respirators provide very eective source control
by dramatically limiting the amount of respiratory aerosols emitted by infectious
individuals (131). In one study, risk of infection was reduced approximately 74-fold when
infected, and susceptible individuals both wore well-tting FFP respirators compared to
when both wore surgical masks (132). Figure 2 reproduces Bagheri et al.s demonstration
of the dramatic decrease in total inward leakage for dierent types of respirators and
surgical masks.
Eective selection of respiratory protection requires knowledge of the hazard and
what level of exposure is acceptable. Cheng et al. modeled the critical concept of dose-
exposure, with selection of respiratory protection based on the pathogen load of a space
(133). In pathogen-rich spaces such as medical centers, respirators and enhanced
ventilation are particularly crucial to reduce the high-transmission risk. Cheng et al.
acknowledged that control of viral load using ventilation reduces the challenge load
faced by any type of face covering. This use of engineering controls to reduce exposure is
well established in industrial hygiene.
The design of masks and respirators continues to evolve; it is discussed further in
section 8.3.
FIG 1 Synthesis of the measured t factors and protection factors for various types of face coverings, based on the 12 primary studies shown in the key.
Reproduced under Creative Commons license from Schmitt and Wang (118). “Protection factor” and “t factor” are similar constructs. Both provide quantitative
estimates of protection based on objective testing (t factor takes more account of real conditions of use). Studies shown used one or the other. References:
O’Kelly et al. (a)(119), Coey et al. (120), Lee et al. (121), Oberg and Brosseau (77), Duncan et al. (122), Pauli et al. (123), Lindsley et al. (124), De-Yñigo-Mojado et al.
(114), O’Kelly et al. (b)(125), Fakherpour et al. (126), Lawrence et al. (127), and van der Sande (128).
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CLINICAL TRIALS OF MASKS AND RESPIRATORS
Methodological challenges in trials and meta-analyses of masks
There are numerous methodological challenges when designing high-quality RCTs of
masks vs a control intervention and in combining such studies in meta-analyses. General
quality criteria for RCTs of complex interventions relate to issues such as sampling
strategy and size, setting, optimization and piloting of the intervention, concealment
of allocation, compliance (and intention-to-treat vs per-protocol analyses), primary and
secondary outcome measures, follow-up, and assessing adverse eects and harms (134–
FIG 2 Median of the total inward leakage over all participants for dierent cases. Adapted under Creative Commons license from Bagheri et al. (132). Further
details are given in the original source. FFP2 w/o adj., participant wears FFP2 respirator without adjustment; FFP2 with adj., FFP2 with adjustment to t; FFP2 +
surgical, FFP2 with additional surgical mask over it; FFP2 adh. tape, adhesive tape applied to increase t of FFP2; surg., surgical mask.
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136). These criteria are extremely dicult to meet in trials of masking. Some scholars
have argued that RCTs have major limitations in answering questions about the ecacy
of masking (137). For example, real-world eects (such as poor compliance) in an
intention-to-treat analysis of a trial conducted in a non-pandemic context may not
reect eectiveness in a pandemic, where it is more likely that masks would be worn as
intended, such as during an emerging pandemic where risk perception and compliance
are high and no other mitigation is available.
Below, we list key challenges in interpreting the results of RCTs and meta-analyses.
Nature of the outcome
The outcome in mask trials is an infectious disease which can transmit from person
to person; risk of transmission is higher in hospitals and other closed settings. In RCTs
of interventions against non-communicable diseases (e.g., hypertension), the interven
tion aects only the person receiving it. In trials involving a contagious infection, the
intervention may aect people other than those receiving it by preventing onward
transmission. This “herd protection eect” of N95 respirators has been described in
health-care settings (138). It means prevention of infection in one individual can prevent
a chain of subsequent transmission. To account for this, any clinical trial of respiratory
protection in health-care and other closed settings should ideally be cluster randomized
without appreciable contact between clusters. We acknowledge that this ideal design
may not always be feasible, but ndings from less optimal designs should be interpreted
accordingly.
Seasonal and year-on-year variation
Respiratory virus activity (especially inuenza) is highly variable by season and from year
to year (139). In contrast, SARS-COV-2 is not clearly seasonal. A trial of masking that is
conducted when the prevalence of the disease under investigation is extremely low (see,
e.g., reference 140) may falsely conclude that masking is ineective because few people
in either arm develop the disease (hence, the study is likely to be underpowered). Ideally,
any trial of prevention of inuenza should be run over more than 1 year to overcome the
risk of low seasonal inuenza activity in a single year.
Variable primary outcomes
Mask trials may use clinical, laboratory-conrmed, or a combination of outcomes. Using
“inuenza-like illness” (ILI) (141) as a clinical outcome is problematic, particularly in
studies that rely on self-reporting (with or without subsequent laboratory conrmation)
because this relatively insensitive denition requires a fever of at least 38oC in addition
to one respiratory symptom such as cough or sore throat, and most adults do not have
documented fever in the presence of a conrmed respiratory viral infection (142). Using
ILI as the primary outcome measure may reduce the statistical power of a study. Some
RCTs have used a denition of “clinical respiratory illness” (CRI), generally dened as
two or more respiratory symptoms (cough, nasal congestion, runny nose, sore throat or
sneezes) or one respiratory symptom and a systemic symptom (chill, lethargy, loss of
appetite, abdominal pain, muscle or joint aches), which appears more sensitive (115).
Another limitation of these RCTs is including only inuenza and/or COVID-19 as the
primary outcome. Trial power may be reduced if tests are not conducted at the peak
of viral shedding or are done incorrectly. Not accounting for inuenza vaccination may
also result in misclassication of people testing positive on serology as infected. In most
mask trials, laboratory conrmation was through PCR, but some studies used serology
as well. Inuenza and SARS-CoV-2 have a higher risk of morbidity and mortality than
other seasonal respiratory viruses measured in some RCTs. However, in reality, a wide
range of pathogens cause respiratory illnesses and are transmitted in a similar way
regardless of how severe the illness is. Although several RCTs report “other respiratory
viruses” as an outcome, separate analysis is typically provided for inuenza and, more
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recently, for COVID-19. A pooled analysis of two large RCTs reported signicantly lower
rates of laboratory-conrmed viral respiratory infections, even those assumed to be
spread predominantly by large droplets, among participants who used a continuous N95
respirator (143).
Combining dissimilar interventions
Meta-analyses commonly and erroneously combine dissimilar interventions, such as a
trial of advice to wear a mask when going out of the home combined with a trial of
advice to wear a mask inside the home in the presence of a sick family member (144).
Some community trials examined masks only in combination with another intervention
such as hand hygiene. Such trials do not allow estimation of the ecacy of masks alone,
though the rationale for such designs is pragmatic (hand hygiene should accompany
mask use to prevent self-contamination) (145). Almost all meta-analyses of N95 use in
health-care combine trials on intermittent and continuous use of the device. However,
crucially, intermittent use of an N95 respirator by health-care sta when caring for
infected or potentially infected patients or conducting an AGMP is a dierent interven
tion to wearing a respirator continuously during an entire work shift. In some RCTs,
participants used masks and respirators only when they worked within 1-2m of patients
with suspected or conrmed respiratory illness or when doing an AGMP, but not at other
times (25, 146). This assumes health workers can accurately self-identify situations of
exposure risk and fails to consider wider airborne exposure (including to asymptomatic
or presymptomatically infected sta or patients) in health-care settings (48). The only
trial to compare continuous and intermittent N95 use showed that continuous wearing
of an N95 is protective, but intermittent use is not, and that intermittent N95 use and
surgical masks were equally inecient (147). This is consistent with trials that compared
intermittent N95 use with surgical masks and found no dierence (25, 146). Other
examples of combining dissimilar interventions include combining masking to protect
the uninfected wearer with masking of sick people (“source control”) to protect their
contacts and combining studies of masking with studies of masking plus handwashing
(9).
Combining dierent settings
Meta-analysis should combine only studies that were done in similar settings, but widely
cited meta-analyses of mask trials combined dissimilar settings such as health-care
and community settings (9). Even among community settings, there is heterogeneity.
Households with an infected member are the highest-risk community setting, but a wide
range of non-household community settings are commonly combined with household
studies in meta-analyses (9). Moreover, in health-care settings, masks are used for
occupational protection, and it is a responsibility of the employer to protect the health
of employees (148). Masks may also provide source control to protect patients from
asymptomatically infected health workers, but to our knowledge no study has investiga
ted this. A meta-analysis of aged care outbreaks did show that universal masking of sta
was protective and resulted in smaller outbreak attack rates (149).
Combining heterogeneous outcomes
When combining studies in a meta-analysis, the outcome being combined should be
homogeneous. Mask trials may use clinical, laboratory-conrmed, or a combination
of outcomes. Widely cited meta-analyses of mask trials combined dissimilar outcomes
(9). The use of a denition of ILI (141) as a clinical outcome is problematic because
most adults do not have fever (a prerequisite for the ILI denition) in the presence
of a conrmed respiratory viral infection (142). Some RCTs have used a denition of
CRI, which does not mandate fever and is more sensitive (115). A common mistake in
meta-analyses is combining studies with dissimilar laboratory outcomes, such as RCTs
whose outcomes were inuenza conrmed by PCR plus serology with other RCTs that
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used only PCR (9). For inuenza, serology may be positive following either infection or
vaccination, and seropositivity is far more common than PCR positivity in such trials (25,
146). This aw in the review process gives disproportionate weight to trials that used
a less specic outcome (serology) than those that use PCR only. This can substantially
distort ndings because positive serology is numerically far greater than positive PCR
results (9, 146). Furthermore, the interpretation of a single high titer compared to rising
titers with paired sera may lead to misclassication. If participants are being vaccinated
against the pathogen being tested, analysis should account for this (including the timing
of vaccination relative to the positive test) to avoid misclassication bias.
A new meta-analysis: justication of approach
To address our methodological concerns about previous studies (notably, the mask
section of the 2023 Cochrane review [9]), we separated dissimilar settings, interventions,
and outcome measures for a new meta-analysis of published RCTs. Preferred Reporting
Items for Systematic Reviews and Meta-analyses (PRISMA) guidelines were followed
(PRISMA owchart is available on request from CRM). Meta-analysis of single proportions
was performed using R 4.1.3, using the ‘metafor’ and ‘meta’ packages. A random eects
model was used throughout. Heterogeneity was assessed using I2 and Q statistics. The
restricted maximum-likelihood estimator method was used to estimate between-study
variance (τ2). Statistical signicance was dened as a two-sided P-value of <0.05. To
examine the ecacy of masks and respirators in community and health-care settings, we
searched PubMed and Embase for RCTs in these settings. We grouped community RCTs
into “primary prevention” (masking to protect the wearer) (27, 144, 150–156) and source
control (masking to protect others in the community) (157–159). Because health-care
settings present dierent risks and issues, we examined the eectiveness of masks and
respirators in community and health-care settings separately. For the meta-analyses
reported here, we excluded RCTs of source control and analyzed only primary prevention
RCTs. In community RCTs, we separately analyzed RCTs of mask and RCTs of masks
plus hand hygiene. We analyzed outcomes of CRI, ILI, and laboratory (PCR)-conrmed
inuenza and other respiratory viruses (including inuenza and COVID-19). Serological
data were excluded to remove heterogeneity of outcome. We found six RCTs in health-
care settings comparing various types of masks and respirators (24, 25, 115, 146, 147,
160). We analyzed data on targeted (intermittent) and continuous use of respirators
separately.
For all trials, we extracted the following data: study design, year, type of interven
tion, primary and secondary outcomes, total number of participants, participants with
each outcome, main results, and limitations as reported by authors. We tabulated study
characteristics and performed meta-analyses using the subgroup analysis approach by
outcome measures (161). We used the random eects model to estimate the respective
pooled risk ratios (RRs) and 95% condence intervals (CIs). We prepared forest plots
to show pooled estimates and corresponding 95% CIs. Reanalysis of RCTs of Masks in
Community Settings reports the ndings of this reanalysis for community settings, and
Reanalysis of RCTs of Masks and Respirators in Health-care Settings reports ndings for
health-care settings.
Reanalysis of RCTs of masks in community settings
Table 3 summarizes published RCTs conducted in community settings, noting that in
some, the intervention being tested was “policy or guidance” to wear a mask.
Figure 3 shows the forest plot comparing medical masks with controls in community
settings. There was a signicant dierence between arms for ILI (inuenza-like illness
or COVID-like illness), which was signicantly lower in the mask arm compared to the
control arm (RR 0.89, 95% CI 0.87–0.91). There was no signicant dierence in any other
outcome between these two arms. Figure 4 shows the forest plot comparing medical
masks plus hand hygiene with no-mask control. Incidence of PCR-conrmed inuenza
was signicantly lower in the masks plus hand hygiene arm (RR 0.63, 95%CI 0.42–0.92).
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TABLE 3 RCTs of masks and respirators in community settingsb
Author, year Design, methods Population, intervention, comparison Outcomes Results Comments, limitations
Cowling et al.,
2008 (153)
Cluster RCT (by household)
in Hong Kong
Households where one member had ILI were
randomized to three arms: medical masks,
hand hygiene, and control (n = 350); masking
by index case and household contacts
Self-reported inuenza symptoms,
laboratory-conrmed inuenza (by
culture or RT-PCR) in household;
nose and throat swabs taken from
each household contact, except for
asymptomatic children under the age
of 2, at baseline and days 3 and 6
Rates of laboratory-conrmed
inuenza (OR 1.16, 95%CI 0.31–
4.34) and ILI (OR 0.88, 95%CI
0.34–2.27) were not signicantly
dierent in masks vs control arm.
Pilot study which informed Cowling et al.
(2009), underpowered, compliance 45%
in index cases and 21% in household
contacts; compliance data showed that
some index cases in control and hand
hygiene arms used medical masks
Cowling et al.,
2009 (145)
Cluster RCT (by household)
in Hong Kong
Households where one member had ILI were
randomized to three arms: medical masks
plus hand hygiene, hand hygiene, and control
(education).
Both index cases and household contacts used
masks (n = 794); masking by index case and
household contacts.
Self-reported inuenza symptoms,
laboratory-conrmed inuenza (by
RT-PCR) in household; all household
members tested with throat swab
regardless of symptoms at baseline,
days 3 and 6
Rates of laboratory-conrmed
inuenza not signicantly
dierent in three arms. Signicant
dierence if masks + hand
hygiene within 36 hours of illness
onset (OR 0.33 and 95% CI 0.13–
0.87). Hand hygiene alone was
not signicant.
No separate medical mask arm, making
it dicult to estimate ecacy of masks
alone; pragmatic trial design, as hand
hygiene should accompany mask use;
compliance 49% in index cases and 26%
in household contacts. Compliance data
showed that some index cases in the
control and hand hygiene arms used
medical masks
MacIntyre et al.,
2009 (144)
Cluster RCT (by household)
from 2006 to 2007 in
Sydney, Australia
Households where a child had ILI were
randomized to three arms: medical masks, P2
respirators (equivalent to N95), and control
(n = 286); masking (medical masks and P2
respirators) by household contacts.
Self-reported ILI, laboratory-conrmed
respiratory infection by multiplex
respiratory PCR; index case and
contacts tested at baseline. Household
contacts tested if symptoms developed
over 2 weeks of daily follow up.
In the intention-to-treat analysis,
no signicant dierence in
any outcome; adherent use of
respirators or medical masks
signicantly reduced risk of ILI
(HR 0.26, 95%CI 0.09–0.77).
Low compliance: 21% of household
contacts wore masks often/always.
Adherence with mask wearing was low
and unsustained (25%–30% by day 5).
Aiello et al., 2010
(150)
Cluster RCT of college
students (by university
residence house) in Ann
Arbor, USA, during 2006–
2007 inuenza season
Three arms: medical masks, medical masks +
hand hygiene, control (n = 1297). Intervention
arms started after lab conrmation of inuenza
infection on the university campus. On the
basis of the size and demographic similarity of
residential halls, seven halls were included as
intervention or control units. No index cases or
contacts were specied for mask intervention.
Self-reported ILI, laboratory-conrmed
inuenza (by culture or RT-PCR). Study
nurses collected throat specimens
from students with ILI for laboratory
examinations.
Intention-to-treat analysis found
no signicant dierence in ILI
in the three arms. Signicant
reduction in ILI in the medical
masks plus hand hygiene arm in
weeks 4–6 (P < 0.05)
Not all ILI cases (n = 368) were laboratory
tested (n = 94); no data on compliance.
Week 4–6 data reects a period of
higher inuenza circulation.
aLarson et al
(154)
Block-randomized
household RCT in
Manhattan, USA
Three arms: HE, HE + hand sanitizer, HE + hand
sanitizer+ medical masks (n = 2,788)
Households where an ILI occurred in any
household members, inclusive of ill person and
caretaker (if the index case was a child younger
than 18 years of age). Masking after the
household contact or caretaker was within 3
ft of a person with an ILI for 7 days or until
Self-reported ILI, self-reported upper
respiratory infection (URI), laboratory-
conrmed inuenza (by culture or
PCR). Project sta made a home visit
within 24–48 hours to the household
of a reported ILI case to collect
a sample for laboratory testing for
inuenza.
No signicant dierence in rates
of URI, ILI, or laboratory-con
rmed inuenza between the
three arms; signicantly lower
secondary attack rates of URI/ILI/
inuenza in HE + hand sanitizer+
medical mask arm (OR 0.82,
95%CI 0.70–0.97).
No separate medical mask arm, making
it dicult to evaluate ecacy of masks
alone; pragmatic trial design, as hand
hygiene should accompany mask use.
Low compliance and around half of
households in the masks arm used
masks within 48 hours. Unlike other
trials, there was no index case at home,
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TABLE 3 RCTs of masks and respirators in community settingsb (Continued)
Author, year Design, methods Population, intervention, comparison Outcomes Results Comments, limitations
symptoms disappeared. The ill person was also
advised to wear a mask within 3 ft of other
household members, if possible.
so exposure was less likely, resulting in
lower statistical power.
Simmerman et
al., 2011 (155)a
Cluster RCT (by household)
in Bangkok, Thailand, from
2008 to 2009. Index case
was a child with inuenza,
and subjects were their
household members.
Households where a child had ILI were
randomized to three arms: hand hygiene, hand
hygiene + medical masks, control (n = 885);
masking by index case and household contacts
Self-reported ILI, laboratory conrmed
inuenza (by PCR or serology) in family
members. Index cases in the outpatient
department were identied by a nasal
swab rapid test. If the test was positive,
one additional nasal swab and one
throat swab were collected from the
index case.
During home visits, one nasal swab and
one throat swab were collected from
the index case and from all household
contacts. For serology, blood samples
were collected on days 1 and 21.
No signicant dierence in
secondary inuenza infection
rates in hand hygiene arm (OR
1.20, 95%CI 0.76–1.88) and hand
hygiene plus medial masks arm
(OR1.16, 95%CI 0.74–1.82).
No separate medical mask arm, making
it dicult to evaluate ecacy of masks
alone; pragmatic trial design, as hand
hygiene should accompany mask use.
The inuenza A (H1N1) pandemic
occurred during the study, resulting
in mask use substantially increasing
among the index cases (4%–52%) and
families (17.6%–67.7%) in control arm.
Aiello et al., 2012
(151)
Cluster RCT of college
students (by university
residence house) in
Michigan, USA, during
the 2007–2008 inuenza
season
Three arms: medical masks, medical masks +
hand hygiene, control (n = 1,111). Masking
in the intervention arms started, follow
ing laboratory conrmation of inuenza on
campus. All residents used the intervention.
Clinically diagnosed and laboratory
conrmed inuenza (by RT-PCR)
College students responded to
questions on ILI symptoms via weekly
surveys.
Throat swabs were collected from
students who reported ILI, and the
swabs were tested for inuenza.
No overall dierence in ILI and
laboratory-conrmed inuenza
in three arms. A signicant
reduction was observed in ILL in
medical masks + hand hygiene
arm in weeks 3–6 (P < 0.05).
Masks alone were not protective.
Compliance was good. Masks were worn
for approximately 5 hours/day. ILI was
self-reported. Authors concluded that
eect in masks + hand hygiene arm
may have been due to hand hygiene,
as medical masks alone were not
signicant.
Suess et al., 2012
(156)
Cluster RCT (by house
hold) in Berlin, Ger
many, during 2009/2010
pandemic inuenza season
and 2010/2011 inuenza
season
Household members of inuenza-positive cases
were randomized to
three arms: medical masks, medical masks +
hand hygiene, control (n = 218).
Participants in both mask arms were asked to
wear masks at all times when the index patient
was in the room. Masking by index case and
household contacts
Laboratory-conrmed inuenza
infection and ILI. Nasal wash, using
isotonic saline into one nostril of
the participants, was conducted. Nasal
swabs were collected by using virus
transport swabs. Specimens were
analyzed by RT-PCR.
Intention-to-treat analysis showed
no signicant dierence in rates
of laboratory-conrmed inuenza
and ILI in all arms. Risk of
inuenza was signicantly lower
if data from two intervention
arms (masks and masks plus
hand hygiene) were pooled and
intervention was applied within
36 hours of onset of symptoms
(OR 0.16 and 95% CI 0.03–0.92).
Compliance with masks was low
(approximately 50% of the participants
wore masks “mostly” or “always”).
Monetary benets provided. Only
household contacts used a mask
(index case did not mask). The short
incubation period of inuenza (2–3
days) means applying an intervention
after a household member has inuenza
may not have ecacy.
Alfelali et al.,
2020 (152)
Cluster RCT (by pilgrim tent)
over three consecutive Hajj
seasons (2013, 2014, and
Two arms: masks, no masks (n = 6,388); masking
randomized by tents that accommodated up to
150 pilgrims for up to 5 days during Hajj in
Laboratory-conrmed viral respiratory
infections and clinical respiratory
infections. Study tents were visited
In both intention-to-treat and
per-protocol analyses, masks
were not eective
Compliance was low in both arms -
overall (24.7% participants used masks
daily, while 47.7% used masks
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TABLE 3 RCTs of masks and respirators in community settingsb (Continued)
Author, year Design, methods Population, intervention, comparison Outcomes Results Comments, limitations
2015) in Makkah, Saudi
Arabia
2013, 2014, and 2015. Pilgrims were supplied
with masks (per protocol: to be worn for 24
hours daily for 4 days during Hajj, if possible).
twice daily. Nasal swabs were obtained
from those who had u-like symptoms.
Respiratory viruses were detected using
a real-time PCR test. For the analysis,
pilgrims who used at least one
facemask daily during Hajj were
considered to have masked that day.
against laboratory-conrmed viral
respiratory infections or clinical
respiratory infection
intermittently). Moreover, in control arms
a few participants also used masks daily
(14.3%) or intermittently (34.9%).
Bundgaard et al.,
2021 (140)
RCT in Denmark, from April
to May 2020
Two arms: recommendation to wear masks
when outside the home, no recommenda
tion (n = 4,862) Community-based study,
no prerequisite for exposure to infection
Conducted in a low-incidence period of
COVID-19 (3 April–2 June 2020) when Danish
authorities did not recommend mask use in the
community
SARS-CoV-2 infection by antibody
testing, PCR, or hospital diagnosis;
PCR positivity for other respiratory
viruses. Participants received materials
and instructions for antibody testing
and for collecting an oropharyng
eal/nasal swab sample for PCR testing
at 1 month and whenever they had
COVID-19 like symptoms. Participants
registered symptoms and results of
tests in the online REDCap system.
No signicant dierence in any
outcome
Primary outcome includes both
PCR-positive SARS-CoV-2 infection and
antibody-positive SARS-CoV-2 (IgM or
IgG). Sample size powered to detect
a 50% reduction of infection, so
was underpowered to detect smaller
dierences. Fewer than 50% used
masks as recommended. Study funded
by Salling Foundation, with Salling
being the largest retailer in Den
mark. Antibody testing was done
by participants themselves. No ethics
approval was sought.
Abaluck et al.,
2022 (27)
Cluster RCT (by village),
in rural Bangladesh from
November 2020 to April
2021
Three arms: surgical masks, cloth masks, and
no masks (n = 336,010). Household members
in the intervention villages were asked to use
masks (surgical masks and cloth masks) when
they are outside and around other people,
during the study period, which was a COVID-19
pandemic period. Community-based study, no
prerequisite for exposure to infection.
Symptomatic SARS-CoV-2 seropreva
lence and symptoms consistent
with COVID-19 illness. Participants
reported COVID-19 like symptoms that
were experienced by any house
hold member in the previous week
and previous month. Blood samples
from participants with COVID-19-like
symptoms were collected and tested
for IgG antibodies against SARS-CoV-2.
Mask use was ecacious in
reducing COVID symptoms
and symptomatic seropreva
lence of SARS-CoV-2; benets
were greater in older people
(35% reduction in symptomatic
seroprevalence). Eect size 30%–
80% larger for surgical masks
compared to cloth masks
Large, community-wide study;
geographically contiguous villages used
as intervention vs control to reduce bias
caused by dierent rates of transmis
sion by location. Only serologically
positive cases of SARS-CoV-2 were
included. Surgical masks were reused,
and cloth masks could be washed and
reused. Mask wearing increased from
13% to 43% in intervention villages;
results reect protective eects despite
low compliance. Social distancing was
unchanged by interventions. Only
symptomatic people tested, so infection
rate underestimated and may have
biased results toward the null.
aLarson et al., 2010, was not included in the forest plot as it did not have a separate mask arm. Simmerman 2011 was not included in the forest plot as serology results could not be separated from PCR in the laboratory-conrmed
results.
bHE, health education; OR, odds ratio; RT-PCR, reverse transcription PCR.
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These results suggest that masks plus hand hygiene is probably a more eective
infection prevention intervention than masks alone in community settings, but masks do
appear to give some protection against ILI. Hand hygiene may protect against direct
FIG 3 Forest plot of community trials: medical masks vs control (no masks). For references, see Table 3. There is some heterogeneity between studies. In some
trials, infection or ILI symptoms in an index case was a pre-requisite for recruitment of family members (e.g., Cowling 2008, Suess 2012 and MacIntyre 2009)
while in other trials, the intervention was triggered when a rst case was conrmed in a non-household setting (Aiello 1 2010, Aiello 2 2012). In others (Abaluck
2022, Bundgaard 2021) there was no pre-requisite for exposure and the intervention was applied to general community (Abaluck 2022). The rate of infection is
expected to be higher in settings such as households with an index case. Two primary prevention trials (Cowling 2008 and Suess 2012) also included "source
control" – i.e., the index case wearing a mask in addition to their contacts). This was a mixed intervention, but we included them in the analysis because
they also included primary prevention in contacts. a. Cowling 2008, PCR positive case numbers are calculated from rates provided in the paper in Table 2 and
approximated to nearest whole number (e.g., Medical/surgical mask arm: 0.07*61 = 4 cases, Control arm: 0.06*205 = 12 cases). b. Cowling 2008, Clinical denition
1 in the paper included fever 38°C, hence placed under Inuenza-like illness; case numbers are calculated from rates provided in the paper in Table 2 and
approximated to nearest whole number (e.g., Medical/surgical mask arm: 0.18*61 = 11 cases; Control arm: 0.18*205 = 37 cases).
FIG 4 Forest plot of community trials: hand hygiene + medical masks vs control. For references, see Table 3. Among the community trials there is some
heterogeneity between studies. In some trials, infection or ILI symptoms in an index case was a pre-requisite for recruitment of family members (e.g., Cowling
2009, Suess 2012), while in other trials, the intervention was triggered when a rst case was conrmed in a non-household setting (Aiello 1 2010, Aiello 2 2012). a.
Cowling 2009, Clinical denition 1 in the paper included fever ≥ 38°C, hence placed under Inuenza-like illness.
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TABLE 4 RCTs of masks and respirators in health-care settings
Author, year Design and methods Population, intervention, and
comparison
Outcomes Results Comments and limitations
Loeb et al., 2009
(25)
Non-inferiority RCT
conducted in eight
tertiary care hospitals in
Ontario, Canada, from
2008 to 2009
Nurses: targeted use (selected
circumstances such as conduct
ing an aerosol-generating medical
procedure) of t-tested N95
respirators compared to medical
masks (n = 422)
Laboratory-conrmed inuenza infection
by PCR or seroconversion (fourfold rise
in hemagglutinin titers) Participants were
assessed for u-like symptoms twice
weekly. If a symptom was reported, the
study nurse was notied, and a nasal
specimen was collected.
No signicant dierence in outcomes.
Inuenza cases in medical masks arm
23.6% vs 22.9% in respirator arm
(absolute risk dierence −0.73%; 95%CI
−8.8%-7.3%).
No control arm, hence lack of dier-
ence between arms could indicate equal
ecacy or equal inecacy. Nurses who
could not pass a t test were excluded. No
data on compliance. Trial was terminated
early due to inuenza A(H1N1)pdm09, as
respirator use became mandatory. Trial
was “non-inferiority”, but there was no
gold standard of ecacy prior to this for
comparison, because the superiority of
any tested intervention was not previously
demonstrated in any RCT (and could not
be demonstrated in this RCT without a
control arm).
MacIntyre et al.,
2011 (115)
Cluster RCT in 15 hospitals
in Beijing, China, from
2008 to 2009.
Three arms: medical mask, t-tested
N95 respirator, and non-t tested
N95 respirator. All interventions
were used continuously. A
convenience control group was also
included (n = 1,441)
Self-reported CRI, self-reported ILI,
laboratory-conrmed respiratory viral
infection including inuenza, by multiplex
respiratory PCR Trained nurses and
doctors collected two pharyngeal swabs
from participants with ILI or CRI.
Pharyngeal swabs tested by PCR for
respiratory viruses
In intention-to-treat analysis, rate of CRI
was signicantly lower in non-t-tested
N95 respirators compared to medical
masks (OR 0.48 (0.24–0.98). The medical
mask and t-tested N95 arms were
non-signicant. Both N95 arms combined
were signicantly protective.
Self-reported compliance 68-86%. Lack of
power for PCR-conrmed inuenza. The
convenience control group was selected
from hospitals which do not routinely use
masks, as the ethics committee deemed it
unethical to allocate subjects to no mask.
The study often cited on the need for t
testing, but the low t test failure rate in
this study is specic to the N95 used in the
study, and cannot be generalized to other
N95s.
MacIntyre et al.,
2013 (147)
Cluster RCT in 68 wards
(19 hospitals) in Beijing,
China, from 2010 to 2011
Three arms: continuous use of N95
respirators, targeted use of N95
respirators for high-risk situations,
continuous use of medical masks (n
= 1,669)
Self-reported CRI, self-reported ILI,
laboratory-conrmed viral infection,
including inuenza by multiplex
respiratory PCR Swabs of tonsils and
posterior pharyngeal wall collected from
participant(s) who had ILI or CRI
symptoms. Swabs tested by PCR for
respiratory viruses
Rates of CRI (HRa 0.39, 95%CI 0.21–
0.71) and bacterial colonization (HR 0.40,
95% CI 0.21–0.73) were signicantly lower
in continuous use of N95 respirator arm.
Self-reported compliance 57%–82%. Lack of
power for PCR-conrmed inuenza. Results
consistent with references (25) and (146),
showing equal inecacy of masks and
targeted (i.e., non-continuous) N95, but
adds to the evidence base because the
latter two trials did not have a control or
other arm for comparison.
MacIntyre et al.,
2015 (160)
Cluster RCT in
14 secondary- and
tertiary-level hospitals in
Hanoi, Vietnam, in 2011
Three arms: medical masks, cloth
masks, and no-mask control (n =
1,607)
CRI, ILI, and laboratory-conrmed viral
respiratory infection, including inuenza,
by multiplex respiratory PCR. Swabs from
tonsils and posterior pharyngeal wall
In Intention-to-treat analysis, ILI rate
signicantly higher in cloth mask arm (RR
13.00, 95% CI 1.69–100.07) vs medical
mask arm. Post-hoc analysis by actual
Mask use was high in the control group, so
a post hoc analysis was done comparing all
participants who used only a medical mask
(from the control arm and the
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TABLE 4 RCTs of masks and respirators in health-care settings (Continued)
Author, year Design and methods Population, intervention, and
comparison
Outcomes Results Comments and limitations
taken from participants who had CRI or ILI
symptoms. Swabs tested by RT-PCR for
respiratory viruses.
mask use showed signicantly higher ILI
rates (RR = 6.64, 95%CI 1.45–28.65) and
laboratory-conrmed virus (RR = 1.72,
95%CI 1.01–2.94) in those using cloth
masks vs medical masks.
medical masks arm) with all participants
who used only a cloth mask (from the
control arm and the cloth masks arm).
Self-reported compliance with mask use
and hand hygiene was reported. There
was a lack of inuenza circulation during
the study period. A subsequent study
using data collected from this trial showed
that poor washing of the cloth masks
contributed to poor outcomes and that, if
machine washed, they performed equally
to a medical mask (164).
Radonovich et
al., 2019 (146)
Cluster RCT at 137
outpatient sites at 7
US medical centers from
2011 to 2015
Two arms: targeted use of medical
masks and targeted use of N95s (n
= 5,180)
Laboratory-conrmed inuenza (PCR or
serology), acute respiratory illness,
laboratory-detected respiratory infection,
ILI Swabs of the anterior nares
and oropharynx were obtained from
participants. who reported respiratory
symptoms.
During each 12-week intervention period,
two random swabs were obtained from all
participants, typically while asympto
matic.
Swabs were tested for inuenza A or B
using PCR.
Serum samples obtained each year from
all participants for inuenza hemagglu
tinin levels before and after peak viral
respiratory season
No signicant dierence in any outcome
between medical masks and targeted N95
Outpatient study with no control arm.
Intervention comprised wearing the mask
or respirator when within 6 ft (1.83m)
of patients with suspected or conrmed
respiratory infection. Approximately 65%
of participants in respirator and mask arms
reported wearing their device “always.” A
post hoc analysis found that the presence
of preschool-aged children in the home
was associated with a higher risk of
respiratory infections among participating
health-care workers (165).
Loeb et al., 2022
(24)
Non-inferiority RCT in 29
health-care facilities in
Canada, Israel, Pakistan,
and Egypt (n = 1,004)
Two arms: medical masks, t-tested
N95 respirators
SARS-CoV-2 tested by reverse transcrip
tase PCR. Nasopharyngeal swabs were
obtained from symptomatic participants.
Blood tests at baseline and end of
follow-up for IgG antibodies.
Other outcomes: acute respiratory illness,
lower respiratory infection or pneumonia,
and work-related absence.
In the intention-to-treat analysis, there
was no dierence in RT-PCR–conrmed
SARS-COV-2 in the medical mask arm
compared to the N95 respirator arm (HR
1.14 ,95% CI 0.77 to 1.69). Other outcomes
were also non-signicant.
Combined data from Canada and Israel
reported lower rates of COVID-19 in
Prespecied analyses (which do not support
the published conclusion of non-inferior
ity) were omitted (166). Non-inferiority was
redened during the Omicron wave, after
95% of the study period was complete,
to accept a hazard ratio of up to 2
(approximately doubling the prespecied
margin) as constituting clinically important
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TABLE 4 RCTs of masks and respirators in health-care settings (Continued)
Author, year Design and methods Population, intervention, and
comparison
Outcomes Results Comments and limitations
Participants were assessed for COVID-19-
like symptoms twice weekly.
N95 arm, compared to medical mask
arm (4.6% vs 9.5%), but dierence was
not statistically signicant. Correspond
ing rates of COVID-19 in combined data
of Pakistan and Egypt were 11.3% and
10.9%.
inferiority in the medical mask arm. This
means anything up to a 99% increase
in relative risk associated with a medical
mask was considered unimportant. The
study’s sample size (1,004) was too low
to identify clinically important dierences
in risk and likely to generate a null result.
Over 4,200 participants would have been
necessary to identify a hazard ratio of 1.5
with 90% power and a one-sided alpha
of 0.025 (164). Rolling recruitment with
addition of Pakistan and Egypt almost
a year later (not included in initial trial
registration) and during the Omicron
period, compared to Canada and Israel
(pre-Omicron). Most trial outcomes were
from the Egypt site. Trial registration
species N95 was intermittent (targeted)
use, but authors later stated it was
continuous. Therefore, intervention delity
and consistency is unclear. Substantial
changes to protocol were made on
multiple occasions as the study unfolded.
Other criticisms have been summarized in
a preprint (166).
aHR, hazard ratio.
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contact transmission and transmission through contaminated masks. Pathogens may be
present on the outer surface of masks, resulting in self-contamination (162). Guidance for
the public recommends hand hygiene before and after mask use (163).
FIG 5 Forest plot of trials in health-care workers: any use of N95 vs medical masks. For references, see Table 4. a. MacIntyre 2011 combined values for t-tested
and not-t tested arms = All N95 arm. b. MacIntyre 2013 (targeted N95 arm) vs control arm was continuous use of medical masks.
FIG 6 Forest plot of trials in health-care workers: continuous use of N95 vs medical masks. For references, see Table 4. a. MacIntyre 2011 combined values for
t-tested and not-t tested arms = All N95 arm.
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Reanalysis of RCTs of masks and respirators in health-care settings
Table 4 lists the included studies in our reanalysis of RCTs of masks and respirators
in health-care settings. Figure 5 and 6 show the results of our meta-analysis on these
studies.
The forest plot in Fig. 5 includes all RCTs which compared N95s (regardless of
whether they were used intermittently or continuously) with medical masks in health-
care settings. Incidence of ILI was signicantly lower in the N95 arm (RR 0.80, 95% CI
0.65–0.99).
Figure 6 shows the same primary studies as Fig. 5, dierently analyzed to sepa
rate continuous from intermittent use of N95s. This important reanalysis shows that
continuous use of N95 respirators compared to medical masks in health-care settings
was signicantly protective against CRI (RR 0.48, 95%CI 0.35–0.65). Rates of ILI, laboratory
(PCR)-conrmed respiratory viruses, and laboratory (PCR)-conrmed inuenza were also
lower in the continuous N95 arm, but dierences were not statistically signicant.
Comment
The ndings presented in Reanalysis of RCTs of Masks in Community Settings and
Reanalysis of RCTs of Masks and Respirators in Health-care Settings dier from those
of some previous systematic reviews and meta-analyses, which did not acknowledge or
fully take account of the heterogeneities listed in Methodological Challenges in Trials
and Meta-Analyses of Masks and gave greater weight to trials with a large number of
seropositive subjects (9, 167–170); those analyses generally concluded that the evidence
for masking was weak. However, our ndings broadly align with other reviews which
did attempt to address the methodological issues listed in Methodological Challenges in
Trials and Meta-Analyses of Masks. Kim et al., for example, found that in studies where
adherence to masking was measured, high adherence conferred signicantly greater
protection against respiratory viruses [odds ratio (OR) 0.43, 95% CI 0.23–0.82] (171).
Kollepara et al. noted that almost all published RCTs of mask ecacy were underpow
ered; they too found a dose-response eect with adherence and concluded that “The
studies [of mask ecacy] that did not nd statistically signicant eects prove only that
masks cannot oer protection if they are not worn” (172).
We did not perform a meta-analysis of RCTs of source control, some of which looked
at ecacy endpoints and one of which looked at the amount of virus in exhaled breath
with and without masks, due to the small number of trials and our focus in this review on
primary prevention (173, 174). Leung et al. found that surgical masks had a limited eect
on rhinovirus compared to other viruses, highlighting specic dierences in respiratory
viruses (174). The policy implications of masks are most relevant for potential pandemic
pathogens such as inuenza or novel coronaviruses, but data on other respiratory viruses
transmitted through respiratory aerosols are informative, regardless of the severity of the
infection and varying degrees of airborne transmission.
NON-EXPERIMENTAL EVIDENCE ON EFFICACY
Observational studies
Early in the COVID-19 pandemic, when there were no RCTs of masks for SARS-COV-2,
Chu et al. conducted a systematic review and meta-analysis of 44 observational studies
involving SARS-1, MERS, and SARS-CoV-2. (175) They found that masks and respirators
reduced the risk of infection by 85% (adjusted odds ratio [aOR] 0.15, 95% CI 0.07–0.34),
more in health-care settings (RR 0.30, 95% CI 0.22–0.41) but also in the community
(RR 0.56, 95% CI 0.40–0.79; pinteraction = 0.049). They attributed this greater eect in
health-care settings to the predominant use of N95 respirators in those settings. In a
subanalysis, they showed that respirators were, overall, 96% eective (aOR 0.04, 95% CI
0.004–0.30) compared with masks, which were 67% eective (aOR 0.33, 95%CI 0.17–0.61;
pinteraction = 0.090) (175). Chu et al. concluded in 2020 that use of face masks “could
result in a large reduction in risk of infection … with stronger associations with N95
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or similar respirators compared with single-use surgical masks or similar,” though they
described the evidence base at the time as “low certainty” (175). Li et al., who included a
sensitivity analysis, concluded that masking was likely to be eective in the community
and highly eective in the protection of health-care workers during outbreaks; they
called for further RCTs to conrm these ndings (176).
To update and extend this early review of observational studies, we supplemented
key studies of which we were aware through a Medline search using terms (mask OR
respirator) AND (COVID-19 OR SARS-CoV-2 OR pandemic) AND epidemiology AND year
of publication >2019, a strategy which identied 199 unique publications. Abstracts
were reviewed to identify relevant studies for inclusion, which were supplemented with
studies of which we were previously aware, including three reviews (14, 175, 177).
Masking was often one component of bundled prevention strategies (178), so we sought
to focus on studies in which mask and respirator eects could be isolated from other
contemporaneous interventions.
Evidence which consistently demonstrated the ecacy of cloth masks, medical
masks, and respirators against infection with SARS-CoV-2 emerged early in the pan
demic, from classical epidemiological (cohort and case-control) studies (179–186),
database-derived real-world evidence (187, 188), and ecological studies and quasi-
experiments related to policy change (189–198). A community-based case-control study
performed in California found a dose-response relationship between both mask or
respirator quality and frequency of use and reduction in SARS-CoV-2 risk: the aOR for
SARS-CoV-2 infection associated with mask use was 0.44 (95% CI 0.24–0.82); surgical
mask aOR was 0.34 (95% CI 0.13–0.90), and respirator use aOR was 0.17 (95% CI 0.05–
0.64) (179).
In cohort studies in schools, COVID-19 risk in students’ family members and
conrmed SARS-CoV-2 risk among elementary school attendees were reduced by 30%–
40% when teachers wore masks (180, 181), supporting the hypothesis that masks
have some ecacy as source control. Mask eects were multiplicative with the 30%–
50% reduction in risk that was achieved through ventilation improvement in schools
(180). A university-based cohort study that evaluated transmission probability between
identied SARS-CoV-2 cases and their contacts found that the risk of transmission to
contacts when neither individual used a mask was vefold higher (aOR 4.9, 95% CI
1.4–31.1) than when both were masked, implying an eectiveness of around 80% for
infection prevention (95% CI 29%–97%) (182).
Cohort and case-control studies in health-care workers provided early evidence of a
substantial reduction in SARS-CoV-2 risk associated with consistent and continuous use
of respirators for prevention. Wang et al. noted early in 2020 that the adjusted odds
of occupational acquisition of SARS-CoV-2 was over 400 times lower (lower-bound CI
98) among hospital sta who wore N95 respirators while on duty in respiratory, ICU
and infectious diseases departments than among those working in other departments
(who did not wear any kind of face covering continuously), notwithstanding the higher
risk of SARS-CoV-2 exposure in the former (183). In a case-control study from Thailand
completed and published in 2020, Duong-Ngern and colleagues found that consistent
use of even cloth or surgical masks by health-care workers reduced SARS-CoV-2 infection
risk by 77% (95% CI 40%–91%) after adjusting for other risk factors (184). In a longitudi
nal cohort study, Dörr et al. found that risk of infection with SARS-CoV-2 was reduced by
use of a respirator compared to a mask when in contact with COVID-19 patients, after
adjustment for both frequency of exposure and vaccination status (adjusted eective-
ness 44%, 95% CI 26%–57%) (185). Hutchinson and colleagues evaluated clusters of
hospital-acquired SARS-CoV-2 infections during the emergence of the Delta variant
(June–October 2021) in Sydney, Australia, and found that all four documented health-
care worker clusters and all workplace-acquired SARS-CoV-2 infections during this period
occurred on general wards where surgical masks were used as PPE. By contrast, no
clusters and no workplace-acquired infections occurred in critical care areas where
respirators were used as PPE (186).
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As with the above (and other) classical epidemiological studies, database-driven
real-world evidence strongly supports ecacy of masks at the population level,
especially but not exclusively when mandated (199). Rader et al., using online survey
data from approximately 380,000 individuals in the United States in 2020, found that
a 10% increase in likelihood of self-reported mask use was associated with a 3.5-fold
increase (95% CI 2.0–6.4) in the probability of the local reproduction number being
reduced below 1 (187). Leech and colleagues noted that in European jurisdictions during
the rst year of the COVID-19 pandemic, spontaneous mask wearing was common
and was only moderately increased by government mandates (188). They performed a
Bayesian analysis to estimate actual mask (as opposed to mandate) eect, independent
of other public health measures such as restrictions on mobility and public gatherings,
using multi-jurisdictional survey data. They found that universal masking appeared to
independently reduce transmission by 25% (95% CI 6%–43%), with this estimate proving
robust in numerous sensitivity analyses.
Ecological studies and quasi-experiments have demonstrated the impact of changes
in mask policy at the level of health-care institutions, schools, and jurisdictions. The
rst such analysis evaluated the impact of jurisdictional mask mandates in the United
States using a dierence-in-dierences approach and found a progressive decline in
epidemic growth rates (to 2% per day 21 days after mandate introduction) when
compared to a referent period of 1–5 days prior to mandate introduction (189). Bollyky
et al. found that mask use in US states was a predictor of lower infection incidence
during the COVID-19 pandemic but was not independently associated with decreased
mortality (196). In this study, vaccine mandates were associated with decreased risk of
death, but mask mandates were not (196). A study by Krishnamachari and colleagues
found that earlier implementation of mask mandates in US states was associated with
lower COVID-19 incidence than later implementation (197). Using a structural equation
modeling approach, Chernozhukov et al. (198) estimated the impact of workplace mask
mandates in the United States and also simulated counterfactuals in which national
mask mandates were promptly instituted early in the pandemic (March 2020). They
estimated that cumulative cases and deaths during the rst pandemic wave (to June
2020) could have been reduced by 21% (9%–32%) and 34% (19%–47%), respectively,
which would have been equivalent to 34,000 deaths prevented during that time period
(198). A dierence-in-dierences analysis performed in Boston following the February
2022 lifting of school mask mandates found that removal of mandates was associated
with a surge in SARS-CoV-2 incidence of 45 cases per 1,000 students and sta. The study
was likely subject to negative confounding, as higher-risk school districts maintained
mask mandates longer, suggesting that true mask eects may be even greater (190).
Ferris and colleagues used a quasi-experiment resulting from a change in mask and
respirator policy at a teaching hospital in the United Kingdom to demonstrate the
eectiveness of respirator use for prevention of health-care associated COVID-19 (191).
The hospital in question was divided into “red” wards, which accepted infected patients,
and “green” wards, which did not. At baseline, both red and green wards used surgical
masks for worker protection. COVID-19 incidence on green wards correlated well with
local community incidence, implying that infections were less likely to be occupationally
acquired. By contrast, incidence on red wards was initially 31-fold higher (95% CI 5.9 to
innity), implying an occupationally acquired attributable fraction of 97% (83%–100%).
With a switch to FFP3 respirators on red wards, incidence fell to below that seen on
green wards; model tting via maximum likelihood estimation provided estimates of
FFP3 respirator eectiveness of 52%–100% against occupationally acquired COVID-19
infection.
In Ontario, Canada, a quasi-experiment on mask mandates was created when each
of 34 health regions introduced mask mandates in a staggered fashion in the sum
mer of 2020. Karavainov and colleagues used statistical modeling and found that
mask mandates were likely to have reduced transmission by approximately 24% when
adjusted for other public health control measures as well as population mobility (192).
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The publicly reported case counts used by Karavainov et al. are likely to be strongly
inuenced by high rates of testing in older adults (those aged over 70) and by out
breaks in long-term care institutions (193). We developed an approach to adjusting case
counts for undertesting in younger individuals (194) and found that when we repeated
Karavainov et al.s analysis using test-adjusted case counts, mask mandate eects were
far stronger (eectiveness, depending on the modeling approach used, ranged from 44%
to 86%) (195).
The protective eects of masks and respirators consistently demonstrated against
SARS-CoV-2 transmission across observational studies were summarized in a 2021
systematic review by Talic et al. (177). This evidence is all the more remarkable as there
are several reasons to expect that the direction of biases in observational epidemiologi
cal studies of masks and respirators would be toward the null (i.e., published estimates
are likely to underestimate actual eectiveness). Social desirability bias may lead to
overreporting of mask use, resulting in non-dierential misclassication of exposures
(200). Mask mandates may occur in conjunction with enhanced surveillance activities
and case nding (195). The indirect eects of masks as source control, which dimin
ishes risk for the population as a whole, would also result in diminished eectiveness
estimates due to protection of unmasked individuals. Kollepara and colleagues have
also demonstrated that variability in mask adherence is an important determinant of
mask eectiveness but is seldom considered when calculating sample size. The result
is underpowered observational studies, which are unlikely to demonstrate statistically
signicant protection by masks even if a strong protective eect exists (172).
Modeling masking
Mathematical models of communicable diseases are used to describe and simulate
epidemic processes, either by describing observed disease patterns in populations
as non-specic logistic growth processes (201, 202) or by representing underlying
mechanisms aecting these patterns such as transmission probability, duration of
infectivity, eectiveness of immunity, and eectiveness of interventions designed to
prevent transmission (203). They should be distinguished from statistical models (such
as regression models), which make simplifying assumptions about the statistical form
and relationships between data elements to draw probabilistic conclusions about gaps
between observed and expected ndings.
In the context of masks for communicable disease control, mathematical models can
be applied in several dierent ways, including (i) “what if” scenarios, which explore the
expected impact of masks and mask directives in specic populations; (ii) model-tting
studies, in which mask eects are inferred at a population level by tting mechanis
tic models to empirical data; (iii) exploring multiple inuences, for example, masks’
bidirectional impact on transmission (i.e., disrupting both transmission and acquisition of
infection), in a context that also considers other inuences such as population behavior
and mixing; these studies typically include sensitivity analyses, which vary the input
parameters to produce best-case and worst-case estimates, thereby taking account of
uncertainty; and (iv) integrative models, that is, mathematical platforms designed to
integrate data on communicable disease dynamics with data from other disciplines such
as the physics of aerosol behavior. We consider examples of each category in turn below.
We identied recent mathematical models of SARS-CoV-2 or inuenza and com
munity-level mask eect using a targeted PubMed search with terms “mathematical
model,” (“mask” or “respirator”), (“SARS-CoV-2” or “novel coronavirus” or “inuenza”), and
(“pandemic” or “epidemic” or “outbreak”). This search identied 30 citations that were
reviewed for relevance. We supplemented these with other relevant papers known to the
authors, producing a nal sample of 28 studies (23, 39, 45, 204–228).
The largest category in our dataset was “what if” analyses—studies which used
mathematical models to project the impact of masking on the contours of pandemics,
epidemics, and outbreaks (204–212, 222, 223, 225–228). Such studies depend heavily on
input parameters derived from mechanistic, observational, and experimental research.
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Results are somewhat predictable, since when masks and respirators are parameterized
as being suciently eective to bring the reproduction number below 1 (either by
themselves (204, 212) or in combination with vaccination (207)), it is a mathematical
tautology that they will be eective. While “what if” models can serve as a tool for
management and communication of uncertainty, they are not crystal balls that can
predict outcomes or determine policy.
“What if” models can also be used as platforms to estimate the expected economic
attractiveness of mask or respirator use under uncertainty. Mukerji and colleagues (226)
reviewed the available literature on model-based health economic analyses of masks
and respirators for prevention of infection. However, of the seven studies they identi
ed, four used either simple back-of-the-envelope calculations or xed-risk models to
estimate economic attractiveness. Such approaches are inappropriate when evaluating
the economic attractiveness of communicable disease control interventions (229), as the
knock-on eects of reduced transmission cannot be estimated. Interestingly, modeling
by Tracht et al. that did incorporate dynamic transmission projected mask use during a
pandemic to be broadly cost-saving across a range of mask eectiveness estimates (225).
Two additional studies that used transmission models presented results in a manner that
prevented estimation of mask cost-eectiveness (227, 228).
Few studies identied for this review used model tting to estimate mask eective-
ness. Mohammadi and colleagues used modeling to estimate the reduction in SARS-
CoV-2 transmission associated with masking to be between 39% and 100% (213). Other
work, such as that by Yang and colleagues, did not attempt to disentangle mask eects
from the impact of other contemporaneous public health interventions (214), highlight
ing a limitation of mathematical model-based parameter estimation, which mathemati
cal models may share with statistical models of observational data.
We identied three modeling studies looking at the eect of multiple inuences
under conditions of uncertainty using sensitivity analysis (23, 215, 216). Iboi et al.
evaluated the interplay between mask ecacy, mask compliance, and control of a
SARS-CoV-2 outbreak in Nigeria, and identied parameter combinations for which
masking would drive the reproduction number below 1 (215).
Fisman et al. explored the relationship between bidirectional mask eects and the
tendency of populations to self-assort based on behaviors (such as masking), such
that masked and unmasked individuals are more likely to interact with one another
than with individuals from the other group (23). These authors found that for realistic
values of mask ecacy, masks could drive epidemic reproduction numbers below one in
combination with other disease control measures, but the tendency of unmasked people
to assort with one another made disease control more challenging (Fig. 7) (23).
Watanabe and Hasegawa, using network-based modeling (a mathematical model
that explicitly incorporates the structure of contact networks, with infection moving
along “edges” or connections between individuals), came to similar conclusions (216).
We identied a small empirical literature which combined communicable disease
modeling (including eects of masks and other interventions aimed at disrupting
transmission of airborne respiratory pathogens) with data from aerosol science, which
has a long tradition of using mathematical models to predict the eects of an infective
individual on risk among others sharing air in an indoor space (45, 217–221). Aerosol
scientists conventionally use the Wells-Riley equation (or variants of it) to simulate the
infective “quanta” produced by an infective individual (217–219), such that infectivity is
proportional to Q/V, where Q is quantum production and V is the ventilation rate in a
given indoor space; Q can be further decomposed into component parameters based on
the known natural history of a given infectious disease (217). By contrast, infectious
disease dynamic models typically represent airborne infectious diseases by modeling
“mass action,” where the rate of infection among susceptible individuals is a function of β
× I, where I is the number of infectives in the population and β is an infectivity constant
(220). The Wells-Riley expression for infectious quanta can be used as a stand-in for β in
infectious disease models, in a manner that allows infectious aerosols to be represented
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mechanistically in models and which would permit incorporation of available data from
aerosol science into models of infectious disease dynamics at the population level.
Noakes and her colleagues have pioneered integration of these model types (218, 219,
221), with application to relatively small indoor spaces such as hospital wards. Expanding
such models to incorporate heterogeneity, population behavior, and larger population
sizes represents an exciting frontier for infectious disease modeling.
FIG 7 Model of mask eectiveness in dierent levels of population assortativity. Reproduced under Creative Commons license from Fisman et al. (23).
Assortativity is the tendency of individuals to interact preferentially with those who are most like themselves. The basic reproduction number (number of
secondary cases produced by a primary case in the absence of immunity or control interventions) is on the left-side vertical axis; mask uptake in the population
is on the right-side vertical axis with lower uptake at the top of the gure, high uptake at the bottom of the gure, and intermediate uptake in the middle
of the gure. Mask eectiveness in reducing transmission from masked, infectious individuals is on the bottom horizontal axis with the highest eectiveness
to the right and lower eectiveness to the left. Assortativity (the tendency of like to mix with like) is on the top horizontal axis and ranges from random
(non-assortative) mixing on the left to highly assortative mixing (with individuals strongly preferring to interact with people like them) at the right. Pink-shaded
areas indicate expected eective reproduction numbers above 1, where epidemic growth will continue. Blue-shaded areas represent combinations of parameter
values where the reproduction number is reduced below 1 (the threshold where an epidemic will continue to grow). This is easier to achieve with higher mask
eectiveness, a lower baseline reproduction number, higher mask uptake, and lower assortativity. The more unmasked people preferentially associate with other
unmasked people, the less likely epidemic control is to occur.
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ADVERSE EFFECTS AND HARMS OF MASKS
Introduction and general adverse eects
Adverse eects of infection control measures must always be compared with the
counterfactual scenario of potential increased infections. The existence of adverse eects
is not an absolute contraindication to mask wearing, just as adverse eects are not
necessarily a contraindication to taking a medication for a health condition, but these
eects need to be understood, weighed up and addressed. They can be divided into
generic adverse eects of masks on all wearers, which need mitigation at a universal
level such as better mask design (this section); eects on wearers in particular risk
groups, which need targeted exemptions and mitigations (see Adverse Eects of Masks
in People in Particular Risk Groups); eects experienced by some people when those
around them are masked, which may need individualized solutions (see Masks and
Communication); and harms to the environment (see Single-Use Masks and Respirators:
Environmental Impact). It is important to distinguish speculative harms (which, if not
refuted by evidence, should be taken into account by a precautionary approach to
policy) and empirically demonstrated harms, which should generally carry more weight
in policy.
Systematic and comprehensive narrative reviews did not identify serious adverse
eects from mask wearing at a whole-of-population level (230–234). These reviews
found minor side eects (discussed below) and other more speculative harms including
documented viral contamination of masks (but no studies demonstrating transmission of
infection from such masks). We consider the documented adverse eects below.
Discomfort and local irritation
Mask wearers commonly experience local irritation of the skin and eyes, pressure eects,
and a mechanical acne (“maskne”) resulting from contact dermatitis and disruption of
skin microbiota (235). Risk of all these increases with duration of use (235) and appears to
be high in health-care workers (230, 235). Headache is also common, particularly among
those with a history of headache, and increases with duration of mask use (236). Thermal
discomfort may occur, particularly in health-care workers wearing full PPE (237–239).
Potential mitigations include local measures (topical treatment, cushioning tapes, and
moving mask straps away from the ears) and workplace measures such as regular and
timely “air breaks” (235, 240, 241). In the longer term, there is a need for improved
designs that meet safety standards while ensuring that masks are comfortable to wear
for extended periods (241), a topic that is discussed further in Toward Better Masks.
Eects during exercise
The eect of mask wearing on respiratory function has been extensively researched.
Statistically signicant but transient and clinically insignicant changes in gas exchange
and pulmonary function have been detected in healthy individuals wearing masks or
respirators during strenuous exercise (234, 242, 243) and during manual work (244).
Many though not all primary studies found modest reductions in exercise perform
ance, but participants often reported discomfort and subjective increase in breathing
resistance. Accurate measurement of the physiological eects of mask wearing is
challenging in several respects (245). Wearing spirometry apparatus over a face mask
(242) can generate artifactual errors in estimates of breathing pressure and perceived
respiratory eort that may exaggerate the impacts of masking on pulmonary function
(246, 247). Overall, these ndings suggest that individuals may choose to avoid high-
intensity exercise while wearing a tightly tting respirator. A concern raised early in
the pandemic about the risk of cardiac dysrhythmias during exercise (248) was not
conrmed in subsequent systematic reviews (240).
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Speculated but unconrmed harms in anti-mask discourse
Although speculative claims have been made for widespread serious harms (especially
to cardiorespiratory and metabolic health from masking), consistent ndings across what
is now a large body of research have shown a reassuring absence of clinically meaning
ful serious harms (230, 232, 240). Furthermore, no serious safety incidents have been
documented despite periods of high community uptake of masking during pandemic
waves (31, 249). Risk compensation behaviors (paying less attention to other protective
measures when masked) have been proposed as an adverse eect of masking, but
studies which looked for such behaviors did not nd them, and indeed masking may
lead to wearers adopting more protective behaviors (250). An exception is the work of
Wadud et al., who found that community mobility increased after implementation of
mask mandates (251) and characterized this change as risk compensation. However, as
noted by Cooper et al. (252), trade-os between (eective) mask mandates, on the one
hand, and mobility and in-person work hours, on the other hand, mean that rather than
being regarded as a negative consequence of population masking, mask eects could
be framed as the potentiation of normal activities, be they economic (e.g., work), social
(e.g., family visits), or educational (e.g., school attendance) without the worsening in
population health status that would occur in their absence.
It is worth noting that the scientic literature contains peer-reviewed articles which
selectively cite awed empirical studies to support the argument that masks are
universally harmful. In 2021, for example, a research letter in JAMA Pediatrics claimed
that masks increased the carbon dioxide content of inhaled air in children aged 6–17
years (253). Shortly after publication, this letter was retracted by the journal following
complaints by scientists, citing “fundamental concerns” with the study methodology
and uncertainty regarding the validity of the ndings and conclusions (254), but it
has attracted over a million views. That evidently awed study was later republished
in a dierent journal (255). Similarly, a recent systematic review of harmful eects of
masking on breathing (256) was quickly retracted by the journal because of concerns
with scientic validity (257), but it continues to be circulated on social media.
Adverse eects of masks in people in particular risk groups
Masking can be particularly challenging and even contraindicated in certain groups. We
consider some of these groups here and acknowledge that there may be others.
Children
“Children” range in age from 0 to 17 years. Infants are a special case: they have narrower
airways, which may exacerbate the increased work of breathing during mask wearing,
and they are less able to remove a mask if experiencing discomfort or obstruction. Masks
are therefore not recommended for children under 2 years (258). Empirical studies are
occasionally cited selectively in narrative reviews to support a strongly held view that
masks are dangerous in older children. In some cases, the primary studies cited did not
show the adverse eects claimed or did not include children. A 2021 narrative review
examining the impacts of masking on respiratory infections in children aged 7–14 years
did not identify any adverse eects, but only two randomized trials on children (124
participants) were included (259). Additional trials on school-aged children published
since that review have shown no adverse impact of masking on cognitive performance (n
= 133) (260).
While the absence of serious adverse eects in RCTs of masks in children is reassuring
up to a point, the studies were small and had methodological limitations. However,
despite widespread uptake of mask wearing in school-aged children around the world
(e.g., masking of children was widespread in Asia in some settings and situations before
the COVID-19 pandemic), there have, to our knowledge, been no formal reports of
incidents of harm. An observational study of cloth masks in US primary schools (261) (n =
1,000 students in pre-K, kindergarten, rst, or second grades) reported appropriate mask
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usage for an average of 77% of the school day over a 4-week period. No serious harms
were reported, and there were fewer than 15 reports each of stress, ears hurting/head
aches, communication diculties, feeling hot, and diculty breathing (the last resolved
after a short break from the mask).
Several commentators have speculated that wearing a mask would increase hand-to-
face contact among children. Empirical evidence has shown the opposite: either no
dierence or a decrease in hand-to-face contact among children wearing masks (262,
263).
Children’s own opinions on masks are highly relevant and rarely elicited. When
they are, the responses tend to be positive. In a survey of 42,767 Canadian adoles
cents during the 2020/2021 school year, 81.9% supported wearing masks in indoor
public spaces; 67.8% supported wearing them in school (23.1% were neutral and 9.1%
opposed) (264). Coelho et al. conducted a survey and held focus groups to understand
children’s experience of masking (265); acceptability appeared high, but some children
reported barriers to communication, learning, and socialization. The authors recommen
ded strategies to address these barriers, including the use of clear face masks. A
recent systematic review of children’s views on masks covered 30 primary studies and
concluded that acceptability appeared high, but comfort, t, style (including age-appro
priate designs), communication, and environmental issues were of high priority for
children; the review also found that parents were sometimes more opposed to masks
for their children than the children themselves were (266). These ndings challenge
statements by people speaking on behalf of children and claiming that masks are
unacceptable to them (267, 268).
Adults with medical conditions
Table 5 summarizes empirical evidence on particular medical conditions, based mainly
on previous reviews, along with suggested mitigations (230, 232, 233, 240). While
primary studies were small, few in number and of variable scientic quality, common
sense suggests a need for exibility and precaution. Note that some of the conditions
listed in Table 5 substantially increase vulnerability to respiratory infections, underscor
ing the point that the risks of masking should be balanced against the benets.
During mask mandates, people who cannot wear a mask may opt to wear a
distinctive lanyard in public or use a ashcard to communicate their exemption status.
We found no research studies on the use or acceptability of such interventions.
Masks and communication
Masks reduce the intelligibility of verbal communication because they attenuate speech
sounds, particularly in the high-frequency range (283, 284), and present a barrier to
lipreading. They also aect non-verbal communication by reducing the visibility of
social cues and emotions (285), which can increase the stress of social interactions.
Masks are challenging for sign language users because they impede access to essential
components of grammar and meaning that are conveyed by facial expressions and lip
movements. These disruptive impacts on social connection and information sharing
have been most frequently investigated in health-care settings (286), but they are
relevant to many other situations including in-person education (287, 288). While most
adverse eects of masks aect the wearer, communication diculties mainly aect other
people.
Communication diculties arising from masking can be a cause of frustration in
everyone, but some groups may experience signicant disadvantage and distress when
others are masked. They include
D/deaf people and others who need to see full faces to access communication; this
group includes but is not limited to people who identify as disabled (286, 289).
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People who are neurodiverse, have cognitive diculties, or have experience of
trauma, and others who experience distress from seeing or interacting with
masked people (290).
Infants and young children in childcare and early education settings who may
experience harms to speech and socialization from interacting with masked adults.
However, parents and primary caregivers, the most important source of these
developmental skills (291), would not normally be masked in the home. Harms to
language outcomes have not been empirically demonstrated, but well-designed
trials and longitudinal studies are needed to identify best practice for communica
tion and language learning during periods of high respiratory infection risk (292).
The above impacts are best considered on a case-by-case basis and must be balanced
against infection risk and vulnerability of wearers. Even if communication is more
dicult, it may not be in a person’s best interests for people interacting with them to be
TABLE 5 Medical conditions requiring caution with masking
Condition Possible adverse impact of masking and relevant studies Suggested mitigation
Allergic rhinitis Mask-induced worsening of rhinorrhea (269), though masks
may also reduce exposure to environmental allergens (e.g.,
pollen) (270)
Experiment with dierent designs and situations. Exemption
may be needed when mandates are in place.
Alzheimer’s disease Impossible to achieve consistent t and adherence [people
with severe Alzheimer’s did not wear masks properly or at all
during pandemic peaks (271, 272)]
Focus on other preventive measures, including indoor air
quality, reduced mixing, and masking of sta and visitors
Chronic lung disease Subjective diculty in breathing because of increased
breathing resistance, especially during exercise (234);
in severe lung disease, theoretical (but not empirically
demonstrated) risk of compromised gas exchange.
People with mild and well-controlled asthma can usually
mask normally; those with more severe respiratory
conditions should be assessed individually. If necessary,
avoid indoor crowded situations. Low-breathing resistance
respirators may be better tolerated. Exemption may be
needed.
End-stage kidney disease Decrease in oxygenation and increased respiratory eort, of
uncertain clinical signicance (based on a single small study
conducted during the 2003 SARS-1 outbreak) (273)
Assess individually, taking account that such people may be
vulnerable to severe complications if infected. Avoid indoor
crowded situations.
Epilepsy Risk of hyperventilation, which could theoretically trigger a
seizure (based mainly on expert opinion) (274, 275).
Avoid indoor crowded situations. Mask should be removed
from anyone having a seizure. Exemption may be needed.
Facial conditions Facial trauma or surgery and painful conditions of the face
(e.g., trigeminal neuralgia) may make masking dicult
or painful [no empirical studies but often mentioned in
guidance (233)].
Assess individually; exemption may be needed.
Heart failure Possible deterioration of cardiopulmonary function during
exercise (276)
Test to see if mask is tolerated during indoor exercise. If
symptomatic in such situations, exercise outdoors.
Laryngeal or tracheal
surgery
People with laryngectomy or tracheotomy are at greatly
increased risk of respiratory infections, and some are
immunocompromised (e.g., during cancer treatment) (277).
Mask should be worn over the tracheotomy.
Mental health condi
tions (e.g., anxiety,
autism, depression, and
claustrophobia)
Worsening of anxiety, panic, and sense of suocation (278–
280). People who have experienced trauma may feel
profound distress while masking (281).
Experiment with dierent designs (an elastomeric respirator
with high breathability may be less symptom inducing).
Take frequent breaks. Grounding techniques can be helpful
for trauma-related anxiety. Exemption may be needed.
Pregnancy-related
conditions
Pregnancy is a high-risk state for complications of COVID-19
(including miscarriage); empirical evidence on masking in
pregnancy is limited (240). A single-challenge study in 20
pregnant health-care workers showed changes in some
physiological variables (e.g., tidal volume) with respirator
materials (282). That study had major design aws (e.g.,
breathing was not through an actual respirator but through
a tiny segment of N95 lter material cut from a respirator).
While denitive evidence is lacking, masking during
strenuous exercise or demanding physical work when
pregnant is not advised. In other situations, advantages
of masking while pregnant appear to outweigh disadvan
tages.
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unmasked. Instead, better communication strategies and other approaches to support
ongoing masking may be more appropriate (285); see Marler and Ditton (293) and Grote
and Izgaren (286) for solutions informed by speech and language therapy expertise
and lived experience respectively. Amplication by lapel microphones remains eective
in the presence of masking (283) and is a readily available technological solution for
classrooms and other settings, reducing the need for increased vocal eort and the risk
of vocal fatigue (284). Written communication, ashcards, and speech-to-text apps can
be helpful, but video relay may be a better option for rst-language sign users (294).
Some masks have a clear panel which allows the lower half of the face to be seen (295),
although this solution may be limited by lack of availability (286) and fogging (296).
When the above strategies are not adequate or appropriate, a decision may be made
to suspend a mask requirement (see Masking as Policy).
SOCIAL AND POLITICAL ASPECTS OF MASKING
Why people mask and why they don’t
The reasons why people mask (or not) can be broadly divided into psychological,
sociocultural, sociomaterial, and socioeconomic.
Psychological research, summarized in a number of reviews (232, 297–299), considers
the inuence of personality traits, emotions, mental health and well-being, attitudes, and
cognition on acceptance and uptake of mask wearing. Uncertainty and other stressors
such as grief, anger, social isolation, and exposure to trauma have a signicant impact
on mental health during epidemics (299). Public health measures such as recommenda
tions or mandates to mask can help alleviate stress and feelings of loss of control by
oering positive steps for action, but they can also be experienced as restrictions on
autonomy and freedom, leading to psychological reactance and non-adherence (297,
299). The psychological need for autonomy (the ability to have free will and choice
over one’s actions), relatedness (feeling socially connected to others), and competence
(the feeling that we are eective and capable and have mastery over our circumstan
ces) helps explain people’s reluctance to mask (232). Psychological reactions may help
explain non-compliance and anger toward those who do comply (232), which sometimes
manifests as public protests and the formation of groups to push against universal
masking policies (300, 301). For those wearing masks, being subjected to racism or
other acts of discrimination, social exclusion, and aggression can cause a variety of
psychological reactions including fear, rejection, loneliness, and anxiety (302–305).
The psychological phenomenon of pandemic fatigue—feelings of burnout or
emotional exhaustion in response to continuing pandemic-related stressors and
uncertainties—has been suggested as an explanation for the decreasing willingness of
people to engage in COVID-19 protective behaviors, including masking over time (306).
Pandemic fatigue is associated with traits such as narcissism, entitlement, perception of
greater auence, pessimism, and apathy, as well as having previously been infected with
and successfully recovered from SARS-CoV-2, and being vaccinated (307). These factors
are not necessarily xed, however. Both compliance with and rejection of public health
recommendations can change, responding to aspects in pandemic conditions such as
the stringency of restrictions, case numbers, and personal experience of infection (308),
as well as changes in perception of risk (309). The concept of pandemic fatigue has
been invoked by governments and health authorities as a political strategy to delay the
introduction of or loosen COVID-19 protections and frame COVID-19 safety as a matter
of personal responsibility and choice, though a review of empirical studies suggests that
such fatigue is less of an issue than is often claimed (298).
Sociocultural research on masks, summarized in a book (301), has shown how
preexisting cultural norms powerfully inuence uptake of masking during epidemics
and outbreaks. Countries such as Japan, Korea, and China accept universal masking as a
community and personal preventive health practice and used this approach long before
the COVID-19 pandemic. These countries also accept masking in health-care settings. In
contrast, most countries in the Global North and some in Africa depict mask wearing as
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strange and perhaps suspicious. For social groups and cultures in which mask wearing
as a public health practice is unfamiliar or stigmatized, developing social norms to
encourage use is particularly important.
Throughout the COVID-19 pandemic, the meanings and practices related to masks
have shifted substantially. The sociocultural, economic, geographical, and political
contexts in which masks have been produced, promoted, and worn (or not) are crucial
to understand these dynamics. The vast consumer culture that sprang up to cater to a
universal masking market, involving hand-crafted and mass-produced decorative masks
for special occasions as well as increased production and promotion of medical-style
masks, is evidence of how quickly the symbolic meanings and practices related to a
simple object can change and diversify well beyond its original role in a specic cultural
context (301, 310).
As COVID-19 has shifted from a headline-grabbing global crisis to one of many
ongoing challenges to human health, masking and support for it have declined, raising
societal and ethical questions about whether and for how long the healthy majority
should compromise their “freedom” to help protect the clinically vulnerable (for whom
a COVID-19 infection could be life-threatening) and prevent the long-term sequelae of
the condition (311). Given that health-care facilities remain high-risk settings for both
sta and patients (312), masking of health-care workers has become a management-vs-
unions issue, especially in relation to whether and when more costly respirator-grade
respiratory protection is needed (148, 313).
Support for and adherence to mask-wearing recommendations or mandates are
inuenced by sociodemographic attributes such as age group, gender, ethnicity/race,
and spiritual beliefs and by the stringency of public health policy (mandates achieve
higher levels of adherence than recommendations) (31, 314). Masking attitudes and
behavior are also strongly inuenced by social group membership and identity. In
particular, right-wing political views and libertarian identity in the United States are
strong predicators of unwillingness to mask (315), and people who place high value on
altruism and social solidarity are more supportive of universal masking (316).
Sociomaterial research has explored the role of cultural meaning in supporting
people’s eorts to use masks (301, 310, 317). The symbolic meanings of face masks in
popular culture, as evidenced in such phenomena as street art portrayals, public health
signage, and online shops selling bespoke masks, are major contributors to the public
dialogue. For those unaccustomed to mask wearing, habits of use—such as purchasing
masks, having them ready to place on the face, learning how to don and do, what to do
with the mask when it is removed from the face—must be learned and incorporated into
everyday routines. Some people nd the physical sensation of having a mask on the face
uncomfortable or conning, as they adjust to becoming more aware of their own breath
and learn new ways to respire and to relate to and communicate with others with their
noses and mouths obscured; for others, masking and seeing others mask makes them
feel safe and protected, part of a community supporting each other.
Research from a socioeconomic and political economy perspective has demonstrated
how susceptibility to and outcomes from COVID-19 are strongly patterned by social
determinants such as family income, housing, workplace conditions and precarity, all
of which inuence people’s ability to obtain and consistently use masks (318). When
resources are available, and people feel part of communities of practice in which the
majority of their fellow citizens are following public health advice, adherence to rules and
recommendations for mask wearing are high, but when this is not the case, unwilling
ness to mask is socially supported and reinforced (298, 303, 305).
Communicating information and managing misinformation about masks
The COVID-19 pandemic generally has been characterized by misinformation and
disinformation (inadvertent and deliberate dissemination of false and misleading
information respectively) (319). How public forums are used to convey messages and
information about public health preventive practices such as mask wearing are vital to
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public acceptance or rejection. When government and public health leaders nationally or
globally regularly advocate for and model mask wearing in public, this can contribute to
developing and promoting strong public support. Conversely, when leaders fail to wear
masks or make negative comments about them, these actions also signicantly detract
from the public health messaging (301).
Since the early months of the COVID-19 pandemic, there have been multiple
examples of major health agencies and government leaders (up to and including the
World Health Organization) promoting incorrect or misleading narratives about how
SARS-CoV-2 is transmitted and the best modes of prevention. These include downplay
ing the value of universal masking, or even taking a specic position against masks,
overemphasizing droplet-oriented measures such as hand hygiene, and failing to convey
the superior benets of respirators over cloth or medical masks, leading to public
confusion and overreliance on handwashing and hand sanitizing in the community (62).
In recent years, the spreading of anti-science misinformation and disinformation via
social media has fomented challenges to public health measures, including conspiracy
and apocalyptic theories that actively challenge mask-wearing recommendations or
mandates (320–324). The circulation of conspiracy and anti-mask theories damages
public health messaging and undermines the trust the public holds in authorities,
eroding the social license for regulations and mandates. Exponents of such viewpoints
attract high levels of media attention, which amplies their controversial stance,
generating concerns about civil unrest and fostering dissent. In such circumstances,
adherence to masking recommendations is increased if public health authorities and
other trusted sources provide clear and frequent public communication about the
importance of masking in high-risk settings such as crowded indoor spaces (325).
MASKING AS POLICY
Dierent types of mask policies
Mask policies are stated government and organizational positions on how masks should
be used for prevention and control of respiratory infections (and also other hazards such
as air pollution (326)). There are four overlapping kinds.
Mask policies for targeted personal protection
These are recommendations for mask use by individuals for whom the consequences
of infection are likely to be signicantly worse than for others (including people who
are elderly, pregnant, immunocompromised, or living with severe long-term conditions
(327)) and those for whom avoiding infection is particularly important (e.g., those
about to have surgery, travel, or participate in critical activities such as high-stakes
sporting events). The scope of mask use would depend on individual risk assessment and
management, but would typically be indoors and either intermittently or continuously,
depending on the risk assessment. Requirements are likely to be increased in specic
settings and during times of elevated infection risk; those at highest risk may be advised
to wear respirator-grade protection. These policies may be developed or adapted by
various interest groups and could be specied as requirements for some highly regulated
activities such as elite competitive sports.
Mask policies for specic settings
These are recommendations and requirements for mask use in workplaces (148) or
public places such as health-care settings (328, 329) and long-term care facilities (149,
302) where risk of transmission is elevated or there are vulnerable people. Such policies
should be developed and interpreted in the context of existing legal standards (e.g.,
those that cover protection in workplaces) and respiratory protection policies including
formal t-testing of respirators (110). Mask policies in specic settings are more likely
to be eective and sustainable if all interested parties work collaboratively to develop
them. In health care, for example, interested parties include IPC clinical teams, employers
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(responsible for occupational health and safety and maintaining workforce capacity,
particularly during peaks of infection (148)), independent worker safety experts, sta
unions (who advocate for workers’ conditions and safety), and patient advocacy groups
(e.g., for the clinically vulnerable). The question of what kind of mask or respirator to
provide in health-care settings and whether use should be continuous or intermittent is
covered in Reanalysis of RCTs of Masks in Community Settings. Mask policies may also
be needed in a range of non-health settings, particularly where sta and visitors are at
higher risk of infection, for example, indoor, crowded environments with poor ventilation
such as hospitality venues, and public transport (330).
Mask policies for seasonal respiratory infections
An increase in seasonal respiratory infections such as inuenza and RSV above a
preset threshold locally or nationally could warrant setting-specic or more wide
spread mask use, along with other measures aimed at reducing infections and disease
burden. Masking and physical distancing introduced to control COVID-19 infection were
associated with a reduction in seasonal inuenza (331), lending support for the benets
of such policies for preventing transmission of respiratory infections more broadly.
Mask policies for pandemics
These policies follow the same principles as for seasonal respiratory infections but may
be stricter, introduced earlier, and continued for longer, depending on the risk assess
ment (see next section). If there is early evidence that a pandemic will be particularly
severe (e.g., fatality risk above a dened threshold), the pandemic response should
attempt to eliminate, rather than merely reduce, the infectious hazard (332). The use of
masks and respirators is particularly important in the early stages of a pandemic, when
drugs and vaccines are unavailable and to protect health workers. A proactive elimina
tion response, along with border controls, could exclude the emerging disease from a
country for a prolonged period, as was achieved by a few countries in the rst 1–2 years
of the COVID-19 pandemic (333, 334). In such circumstances, mask requirements could
be relaxed within a country while elimination is sustained but would still be required
for border, health, and quarantine facility workers having contact with infected people
entering the country and for local outbreaks. An eective pandemic mask policy can
build on individual, setting-specic, and seasonal policies, all of which help to establish
mask use as a familiar form of infection control. However, pandemic management needs
additional preparedness, planning, and development during interpandemic periods
(335). As this paper went to press, the U.S. Centers for Disease Control and Prevention
issued interim guidance recommending respirators for worker protection against the
emerging threat of Novel Inuenza A Virus (“bird u”), which is spreading among cattle
and has infected humans (336).
Developing and implementing mask policies
Mask policies may be voluntary, based on a recommendation, or mandated, based on an
occupational or legal requirement. They must balance the risk of potential harms from
the policy, ranging from unpopularity and inconvenience through to possible adverse
impacts of masking on individuals or groups. The main rationale for mandating mask
use for control of respiratory infections, especially major epidemics and pandemics,
is that the masking behavior of an infected person has serious consequences for the
health of other individuals (132) and will aect the pace of exponential growth in
cases, possibly leading to health services becoming overwhelmed. Furthermore, a legal
mandate generally increases adherence over and above a mere recommendation (337).
Substantial asymptomatic and presymptomatic transmissions of a pathogen such
as SARS-CoV-2 are also a strong rationale for universal masking (40, 338). Pandemic
control measures that keep people apart are eective but can be highly disruptive.
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Masking enables the continuation of normal activities while reducing the probability of
transmission.
Such measures should be based on a thorough risk assessment, be proportionate to
the hazard, and be maintained for the minimum period needed to control the outbreak.
Key steps include a risk assessment to estimate the level of threat; risk management
decisions about who (if anyone) should mask under what circumstances, taking account
of contextual factors; and an implementation plan for supporting and enforcing mask
use, which may include mandates (Table 6).
New pandemic threats pose the largest challenge for risk assessment as key
parameters such as infection fatality risk, reproduction number, and extent of trans
mission from people who have no symptoms (or mild symptoms which they may
not recognize as important) are unknown. Decisions must be made with limited and
uncertain information. In such situations, there are strong arguments for applying the
precautionary principle (341), which has four elements: taking preventive action in the
face of uncertainty (i.e., before denitive evidence is available); shifting the burden of
proof to the proponents of a potentially hazardous activity; exploring a wide range of
alternatives to possibly harmful actions; and increasing public participation in decision
making (355). On the basis of evidence presented in The Basic Science of Masking,
Clinical Trials of Masks and Respirators, and Non-Experimental Evidence on Ecacy,
universal masking has a particular role in preventing transmission and containing
outbreaks caused by predominantly airborne pathogens which may cause asymptomatic
infection, have a presymptomatic phase, or cause mild symptoms in some people,
especially where people gather in shared indoor environments. It is also a simple and
safe intervention that may reduce the need for more invasive interventions such as
lockdowns.
SINGLE-USE MASKS AND RESPIRATORS: ENVIRONMENTAL IMPACT
The scale of environmental harm
In previous pandemics, reusable cloth masks (and to a lesser extent, single-use paper
masks) predominated (356); cloth masks performed similarly to surgical masks in ecacy
tests of the day (357). From the 1970s, these products were steadily supplanted
(especially in health-care settings) by single-use synthetic masks, which were seen as
more convenient and comfortable and reected a wider trend to a “disposable” culture
in health-care settings (356). The COVID-19 pandemic saw a substantial rise in the
manufacture, distribution, use, and improper disposal of environmentally unfriendly
surgical masks and single-use respirators (358, 359). A number of reviews published since
2022 have documented the environmental impact of these products (360–370). One
estimated that 15 trillion face masks are used globally every year, resulting in 2 megatons
of waste (360), though the contribution of masks to plastic waste is small compared to
other sources such as food packaging or beverage bottles.
Single-use masks and respirators are typically made from synthetic polymers
including polypropylene, polyester, polyurethane, polyacrylonitrile, polycarbonate,
polyethylene, polystyrene, and polymeric nanobers and microbers, which are not
biodegradable. Rather, they break down over a period of up to 20–30 years through
photodegradation and thermo-oxidative degradation, producing microplastics (<5 mm),
which, under various environmental conditions, accumulate and enter ecosystems
(361, 362, 366, 367). This is a particular problem in the marine environment, where
microplastic waste ingested by marine animals leads to physical harm, toxic eects,
and potential entry into the human food chain through seafood consumption (371,
372). In soil, discarded single-use masks and respirators can generate microbers and
nanobers, negatively impacting soil biological systems, plant growth and invertebrates
(373, 374). They also leach harmful chemicals, including metals (cadmium, antimony,
copper), heavy metals (lead), antioxidants, dyes, plasticizers and ame retardants into
the environment, harming wildlife and disrupting the balance of ecosystems (375, 376).
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Some have predicted that biomedical waste (of which PPE is a major component) may
soon overwhelm existing waste management systems and produce backlogs (377).
TABLE 6 Developing and implementing mask policies for respiratory pathogensa
Stage Details
Stage 1: assess the risk posed by the
pathogen
Consider:
Epidemiological pattern: notably endemic, seasonal, or pandemic.
Transmission dynamics: including the proportion of transmission that occurs by the airborne route and the
extent of asymptomatic and presymptomatic transmission (40).
Probability of exposure: both generally and in specic settings (330).
Consequences of exposure: which depends on infectivity and pathogenicity of the agent, fatality risk, morbidity
risk including long-term health eects and impact of repeated exposure, and wider eects on health-care system
and societal functioning (339).
Unequal distribution of risk: with heightened vulnerability for particular demographic or clinical risk groups (340).
The precautionary principle (i.e., the need to take account of risks that are not fully known) (see text) (341).
Stage 2: develop the risk management
policy
Consider:
Goal: usually disease control (mitigation or suppression), but may be elimination in specic situations (see text).
Proportionality: mask policies are justied if the risk assessment (see stage 1) shows that the infection is
likely to have signicant negative impact in terms of mortality, morbidity, hospitalization, long-term illness and
disability, and health systems (e.g., overwhelming services), social (e.g., loss of schooling), and economic (e.g., lost
productivity) consequences (342, 343), and there is evidence that masking is likely to be eective in this outbreak.
Scope of policy: identify what kind of mask policy (see Dierent Types of Mask Policies) is justied in the
circumstances. Combining protection for the wearer with source control is likely to provide the best levels of
protection (see What Are Masks and How Do They Work?). Policy design should include consideration of both
individual-level ecacy (high-ltration masks, correctly worn) and population coverage (how widely masks are
being worn and by whom).
Costs and cost-eectiveness: Ideally, mask policies should include economic analysis to compare the costs of
mask use (including supply, communication, support for use, and monitoring and enforcement) and consequen
ces compared to other alternatives that could achieve similar levels of disease control (though this consideration
is less relevant for elimination strategies) (344, 345).
Potential adverse eects and unintended consequences: adverse eects include harm to specic groups (e.g.,
D/deaf people unable to communicate; see Social and Political Aspects of Masking). Some individuals may be
unable to tolerate masks; hence, exemption policies are needed. Unintended consequences include resistance
and even civil disobedience (300), which may make the policy dicult or impossible to enforce except in highly
controlled environments such as health-care settings or airports.
Stage 3: implement and monitor the
policy
Consider:
Likelihood of voluntary adherence: if the society has a history of high voluntary adherence to mask use (346, 347)
or higher levels of collectivism (348, 349), policy may be eectively enforced through recommendation. If not, a
mandate may be necessary.
Mandates, potentially with legal support: requiring individuals to wear a mask over their mouth and nose may be
ethically and legally justied to protect the wearer and those around them in high-risk occupational settings such
as hospitals and when exposed to harmful substances. For epidemic and pandemic situations, mask policies need
to be based in law, for a formally declared public health emergency, with the aim of limiting spread (343, 350).
Mechanisms to encourage adherence and act on non-compliance: Provide clear and proportionate sanctions for
non-compliance (e.g., workplace sanctions for employees, registration requirements for health-care providers,
exclusion of un-masked visitors from specied settings, and legal enforcement for mask wearing in dened public
places) (343, 350). Criminalization and other punitive measures are a last resort because of potential harms such
as undermining trust and disadvantaging marginalized populations (351);
Measures to support mask use: provide clear and consistent information and education about when, where,
and how to use masks (352), and emphasize benet to others as well as self. Role-modeling from political and
public health agency leaders is a crucial contributor to encourage mask wearing, as are government social
marketing campaigns. These measures not only promote mask wearing but also maintain the social license for
masking recommendations and mandates (301). Combine top-down and bottom-up measures (353). Ensure a
range of eective masks and respirators are available in dierent sizes, shapes, and designs. Making these highly
accessible and minimizing cost by direct provision or subsidies are likely to support their uptake, particularly for
disproportionately aected communities (354).
Measures to minimize inequities: ensuring equity across demographic, socioeconomic and ethnic groups will
require developing active partnerships with diverse populations and communities, particularly those who are
underserved and disproportionally aected by respiratory infections, and providing resources and support as
needed (354);
Resources and mechanisms to monitor the policy: including sustaining the response, adjusting it as knowledge
and circumstances change (including building in regular policy review mechanisms which incorporate ongoing
evidence updates from research, surveillance, and evaluation), and deciding when to discontinue it.
aA mask policy which will usually be part of an overall infection prevention and control policy and strategy, along with other public health and social measures.
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Disposal of these potentially toxic products via incineration (one approach to
large amounts of waste in health-care facilities) releases particulate matter, heavy
metals, carbon monoxide, carbon dioxide, and other greenhouse gases (378). However,
carcinogenic dioxins, released with burning of plastics, are not released in the burning of
polypropylene and other non-halogen containing polymers.
With increasing concerns about the worsening climate crisis, estimates have been
made of how much single-use masks contribute to greenhouse gas emissions. One study
gave the global warming potential of the mask at 21.5 g/CO2eq, comprising 40.5% for
raw materials, 30% for packaging, 15.5% for production, and 7.4% for transportation
(379); other studies produced lower estimates (379–381). However, these studies did not
factor in the consequences of mask use (e.g., protection against infection) nor compar
isons with more common consumer items. To determine the true climate impact of
masks, one would have to compare the estimated increase in greenhouse gas emissions
and other harms from mask use with the estimated decrease in emissions from, for
example, averted hospitalizations and medication prescriptions.
Mitigating environmental harm from single-use masks and respirators: what
can be done?
While environmental concerns were, perhaps understandably, put aside in the early
months of the COVID-19 pandemic, action to address the environmental impact of the
ongoing response to this and other respiratory diseases is now urgent (358). Several
reviews have proposed measures for addressing the growing threat of mask pollution
(360, 363, 364, 366, 369, 370). The recommendations below are based on those reviews.
Increase public awareness
The environmental hazards posed by discarded masks need to be made clear, and
information and resources need to be provided for more environmentally friendly
disposal. Without public understanding and support, little change is likely to occur.
Improve mask waste management
Many environmental reviews reproduce a widespread but probably awed assumption
that because masks may be contaminated with infectious organisms, all mask waste
should be treated as a signicant biohazard with substantial waste worker protection
and special waste incineration (360, 368, 371, 382). While it is hypothetically possible
that discarded single-use masks and respirators may act as fomites and contribute
to virus spread (see What Are Masks and How Do They Work?), we believe that the
biohazard of masks has been substantially overestimated in the past because of an
assumed droplet mode of transmission. Scaling back the special biohazard measures
in mask waste disposal would reduce their environmental impact. In addition, as Wang
et al. recommend, dedicated mask disposal bins and systematic decontamination will
maximize opportunities for recycling (360).
Recycle mask waste
A reliable and scalable approach to recycling (or “upcycling”) such products has proved
challenging for scientists. Diculties include risk of contamination (e.g., with blood
or non-mask waste), the multiple materials they contain (polymeric lters, aluminium
nosepiece, and elastic loops), and adverse cost-benet balance. However, studies have
begun to demonstrate that shredded mask waste can be incorporated into roads
and pavements, building materials (e.g., concrete), membranes and ltration materi
als, various kinds of fuel (through a high-temperature thermochemical process called
pyrolysis), battery electrodes, adsorbents, and various specialist chemical products, in
each case improving performance of the end material (360, 363, 364, 366, 369, 370).
While few of these solutions are ready to be implemented at scale, they oer some hope
for the future.
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Promote reuse and extended use
Reuse and extended use are partial solutions that could potentially be implemented
promptly and in a highly cost-eective way. Surgical masks are not designed for reuse
or even prolonged use, with a recommended maximum duration of 6 hours (383), but
respirators are designed for prolonged use; a single respirator can be worn repeatedly
and for long periods and still be eective (384, 385). A UK analysis showed that reusing
respirators once reduced waste by 65%–80% when compared to single-use respirators
but still generated more waste than single-use surgical masks, though this narrowed and
then reversed with the number of reuses (386). Another study found similar results (387).
Further research is needed, however, on when and how to decontaminate respirators
prior to reuse. Although several decontamination methods like ultraviolet irradiation
and hydrogen peroxide vapor have been explored (383), these can compromise t and
ltration performance, leading some manufacturers to advise against reuse (87, 388).
Normalize elastomeric respirators
Elastomeric respirators provide a high level of protection as well as comfort and t, are
reusable, and have a lightweight solid frame and replaceable lters. They are a promising
solution for both community and health-care facility use (389, 390). While their initial
cost ($30–$150) is high compared to a single-use device, their long-term nancial and
environmental costs are lower. Depending on the model, they can be reused for 5 years
or more, resulting in up to 96% less waste over single-use or decontaminated and reused
options (386, 387).
Develop biodegradable and reusable masks
Biodegradable polymers range from natural types like cellulose, chitin, and silk broin to
semi-synthetic and synthetic varieties such as polylactic acid (PLA) and polyvinyl alcohol
(PVA) (391). PLA and PVA, in particular, have shown signicant potential as lter materials,
with studies demonstrating their eectiveness in creating high-performance, environ
mentally friendly masks (391, 392). Wang et al. summarize additional studies on these
and a range of additional materials including gluten, banana stems, and biodegradable
nanobers (see Toward Better Masks) (360). While cloth masks provide a lower level of
protection than surgical masks (179), they can be reused. There is ongoing research into
developing novel anti-microbial fabrics impregnated with graphene, copper, silver, or
zinc nanoparticles, as well as design of fabric masks with improved t that can perform as
well as a surgical mask (92, 393, 394).
Formulate relevant policies and regulations
While 175 countries around the world have committed to ending plastic pollution, there
are currently no international laws, regulations, or restrictions regarding mask disposal
(360). Measures by national governments are needed, along with incentives (e.g., tax
breaks) for those developing more environmentally friendly alternatives.
The above approaches should be pursued alongside other protections against
airborne pathogens, notably attention to indoor air quality. If all indoor spaces were
optimally ventilated, for example, protection would be less reliant on individuals’ ability
and willingness to mask (395, 396).
Toward better masks
The design of masks and respirators continues to evolve, especially in relation to
novel materials designed to address both the limitations (ltration, breathability, and
susceptibility to contamination) of traditional materials (see What Are Masks and How
Do They Work?) and their environmental risks (see Mitigating Environmental Harm from
Single-Use Masks and Respirators: What Can Be Done?). Nanobers, produced using
electrospinning techniques from various polymers, are an important advance. They oer
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superior performance due to their light weight, adjustable surface chemistry, small
pore sizes, and large surface areas. In particular, nanobers permit creation of a highly
ecient and breathable lter that is thinner and lighter than traditional polypropylene
lters, with ltering performance comparable to highly eective electret type lters
(397–400). These properties mean that masks and respirators made of nanobers can
reduce moisture, heat, and pressure build-up, enhancing their t and comfort, while
also minimizing communication issues and possessing the potential to physically block
viruses (401, 402). They also allow the use of polar liquids such as ethyl alcohol to
decontaminate the device (401, 402).
In response to the COVID-19 pandemic, there has been a signicant push to develop
masks and respirators which are capable of not just trapping but destroying pathogens,
though the contribution of such technologies to reducing transmission relative to more
traditional designs is debated. As a reviewer of an earlier draft of this article pointed out,
“Pathogen-trapping eciency is orthogonal to pathogen-killing ability. A mask could be
made of highly eective antimicrobial materials, but if the material is only marginally
eective at physically capturing pathogens in the rst place, then the antimicrobial
capability is moot.
Traditional non-woven fabrics with electrostatic ltration tend to lose their pathogen-
trapping eciency as the electrostatic charge diminishes over time (403). To address
this, a novel stream of research focuses on embedded anti-microbial materials, including
carbon-based (e.g., activated carbon, carbon nanotube, graphene, and carbon aerogel)
and biopolymers, which can kill or inactivate pathogens upon contact (86, 249, 404–406).
Non-reusable versions of these nanocomposite materials incorporate elements like metal
(such as copper, silver, and zinc) nanoparticles and quaternary ammonium compounds,
which have shown high ecacy in inactivating viruses and bacteria (249, 404, 405). More
environmentally friendly anti-microbial materials involve coatings or treatments that can
be reactivated or remain eective after multiple uses, such as salt-based functionaliza
tion, photoactive materials, metal ions and metal-organic frameworks embedded within
the lter matrix (86, 404).
While these developments have theoretical potential, research continues on the
safety, costs, and environmental impact (especially with metal derivatives) of these novel
products.
Other developments which may improve the performance and acceptability of
masks in the future include masks with advanced detection and decision-making
capacities (407); 3D printing technology to allow customizing of masks for individual
facial structures, ensuring better seals and reduced leakage (408); and hydrogel patches
applied to the edges to ensuring a snug t without compromising comfort (409).
Lastly, and somewhat speculatively at the present time, the integration of smart
technologies into masks creates the potential for enhanced detection and monitoring
(e.g., by identifying pathogens, monitoring air quality, and even tracking the wearer’s
physiological biomarkers) (86, 410, 411). More prosaically, existing designs need to
be produced in a broader range of sizes and shapes to accommodate dierent facial
dimensions, anthropomorphic types, and presence of facial hair (91), and greater
attention should be paid to color and style to improve acceptability, especially to
younger generations (266).
SUMMARY AND CONCLUSION
This review was commissioned partly because of controversy around a Cochrane review
which was interpreted by some people as providing denitive evidence that masks don’t
work (9). Our extensive review of multiple streams of evidence from dierent disciplines
and study designs builds on previous cross-disciplinary narrative reviews (233, 412) and
aligns with the recent call from philosophers of science to shift from a “measurement
framework” (which draws solely or mainly on RCTs) to an “argument framework” (which
systematically synthesizes evidence from multiple designs including mechanistic and
real-world evidence) (19). Using this approach, we have demonstrated a more nuanced
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set of conclusions, summarized below, and have revealed why certain inaccurate
assumptions and defective reasoning about the science of masks and masking seem
to have become widely accepted among certain groups.
We began by reviewing basic science evidence on the transmission of SARS-CoV-2
and other respiratory pathogens and showed that there is strong and consistent
evidence that they spread predominantly by the airborne route. We also showed
that masks are eective, and well-tting respirators are highly eective, in reducing
transmission of respiratory pathogens, and that these devices demonstrate a dose-
response eect (the level of protection increases as adherence to masking increases).
We then provided a methodological critique of clinical trials of masks in the control of
respiratory disease epidemics and outbreaks, including listing common design aws. We
summarized evidence from RCTs, including repeating methodologically awed meta-
analyses, and showed that respirators are signicantly more eective than medical or
cloth masks, especially (and to the extent that) they are actually worn in all potentially
hazardous circumstances.
We also reviewed an extensive body of observational and modeling evidence which
showed that, overall, masking and mask mandates are eective in reducing community
transmission of respiratory diseases during periods of high community transmission. The
observational ndings are particularly striking since various inherent limitations of such
designs are likely to bias ndings toward the null.
Our review of adverse eects and harms of masks found strong evidence to refute
claims by anti-mask groups that masks are dangerous to the general population. We also
found that masking may be relatively contraindicated in individuals with certain medical
conditions and that certain groups (notably D/deaf people) are disadvantaged when
others are masked.
We summarized evidence from multiple countries and cultures which shows that
masks are important sociocultural symbols about which people care deeply (positively
or negatively). We also showed that adherence (and non-adherence) to masking is
sometimes linked to political and ideological beliefs and to widely circulated mis- or
disinformation, and hence hard to change.
In a section on mask policy, we described how governments and organizations need
explicit policies on using masks for prevention and control of respiratory infections,
covering personal protection of at-risk groups; protection in specic settings, including
workplaces and healthcare facilities; seasonal respiratory infections; and pandemics.
These policies need to be based on sound risk assessment, risk management, and
implementation principles.
Finally, we reviewed environmental impacts from single-use masks and respirators
and highlighted novel materials and designs with improved performance and less
environmental risk.
We believe this evidence supports several important conclusions and implications for
further research.
First, the claim that masks don’t work is demonstrably incorrect, and appears to be
based on a combination of awed assumptions, awed meta-analysis methods, errors
of reasoning, failure to understand (or refusal to acknowledge) mechanistic evidence,
and limitations in critical appraisal and evidence synthesis. Masks and respirators work
if and to the extent that they are well-designed (e.g., made of high-ltration materials),
well-tting and actually worn. The heterogeneity of available mask RCTs does not appear
to have been fully understood by some researchers who have conducted high-prole
meta-analyses of the same. It is time for the research community to move on from
addressing the binary question “do masks work?” through unidisciplinary and episte
mologically exclusionary study designs and pursue more nuanced and multi-faceted
questions via interdisciplinary designs.
A fruitful avenue for future research, for example, would be the combination of
experimental, observational and modeling data to rene our understanding of when
universal masking should be introduced during respiratory epidemics and how best to
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promote and support masking policies in dierent situations and settings, and especially
for groups at increased risk, during such outbreaks. Research on ventilation, ltration and
other measures to improve indoor air quality was beyond the scope of this review [it has
been covered elsewhere (330, 395)], but there is scope for cross-disciplinary modeling to
bring the science of indoor air into more direct dialogue with that of infectious disease
transmission and masking to address the question of when and in what circumstances
indoor masking can be deemed unnecessary (or, alternatively, advised or mandated)
based on air quality. As noted in Modeling Masking, some research groups have begun
to contribute to this interdisciplinary knowledge base.
Second, given that masking is an eective (though not perfect) intervention for
controlling the spread of respiratory infections, and that it may be particularly important
in the early stages of pandemics (when the pathogen may be unknown and drugs and
vaccines are not yet available), improving understanding among scientists, clinicians,
policymakers and the public about the eectiveness of masks and respirators is an
urgent priority. The continuing recalcitrance of many (though not all) in the infection
prevention and control community on this issue could prove a major threat to public
health in future pandemics, particularly since such individuals often hold inuential
positions on global and national public health decision-making bodies.
Third, mask policies should better reect the actual risks and harms of masks rather
than being overly inuenced by speculative risks (such as retention of carbon dioxide)
that have no empirical foundation, or by adverse eects aecting certain dened groups
(e.g., some people with autism) which could be covered by exemptions. Rather, the focus
should be on supporting eective mask use by addressing well-described and widely
experienced adverse eects of masking such as communication diculties, physical
discomfort, and skin reactions. Communication is a vital human need, so communication
resources and best practice guidelines should be integral to mask policy and operational
in every setting where masking is required or advised. Physical adverse eects of masks
should be addressed by better mask design, which should be a priority for research.
Fourth, there is scope for research centered on helping people nd masks that they
nd comfortable, aesthetically appealing, and which t them well. If masking is to be
normalized in certain risk situations, there needs to be a range of masks and respirators
available in dierent sizes, shapes, colors, and designs to take account of the many
physical and sociocultural factors aecting uptake, t, and use. This stream of research is
especially important for people who are clinically vulnerable (e.g., immunosuppressed),
who may need to mask much or all of the time and, in some cases, lifelong.
Fifth, research should continue into novel materials which could lead to masks with
improved comfort, lower breathing resistance, and good quality reusable products which
will greatly reduce waste and environmental pollution. Plastic-backed medical masks
that are ill-tting, uncomfortable, and made of non-biodegradable materials should be
phased out and replaced with masks and respirators that meet a higher standard for
ltration ecacy, breathability, t, and environmental sustainability. Research should
also be directed at maximizing options for recycling mask waste.
Finally, as the COVID-19 pandemic continues into a fth (and, quite possibly,
subsequent) year, the grave danger posed by ideologically driven anti-mask narratives
to public and global health should be acknowledged and systematically addressed.
Anti-mask sentiment is increasing, along with anti-vaccine sentiment (413), and this
bodes ill for both the current and any future pandemics. While there are no simple
solutions to the problem of widespread disinformation, clear and consistent messag
ing from public health bodies on masks and other mission-critical topics would help
considerably.
These suggestions for further research are summarized in Box 2.
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BOX 2: SOME SUGGESTIONS FOR A NEW GENERATION OF RESEARCH ON MASKS
AND RESPIRATORS
1. Interdisciplinary and multi-method designs which go beyond “do masks work?”
and ask nuanced, multi-faceted questions such as “what kind of masks should
be introduced in respiratory epidemics and pandemics, at what stage, for whom,
how and with what support?”
2. Studies of how to address the mismatch between the strong and consistent
evidence base on the eectiveness of masks and respirators and the lack of
acceptance of this evidence by inuential scientists, clinicians and policymakers.
3. Studies to improve the quality of communication when [some people are]
wearing face coverings.
4. Studies to optimize acceptability, t and comfort of masks and respirators and
minimize side eects such as skin reactions and headache. We recommend a
wider range of mask materials, designs and styles, including consideration of
specic need groups.
5. Studies of new materials and combinations of materials for masks and respirators,
with a view to optimizing ltration ecacy, breathability, t and environmental
sustainability.
6. Studies of how to address the widespread, sinister and growing phenomenon of
anti-mask misinformation and disinformation on social and mainstream media.
The time is well overdue for international policy bodies to acknowledge the totality of
evidence on the science of masks and masking and to show leadership in providing such
messaging to policymakers, clinicians, and the public.
ACKNOWLEDGMENTS
This multi-author, interdisciplinary review has been shaped by our formal and informal
interactions with numerous colleagues and critics, too numerous to mention individually.
We thank them all for contributions which have informed, inspired and challenged us
over the years. We also thank the original authors of Figures 1, 2, and 7 for publishing
their work under a Creative Commons license and allowing us to reproduce these visuals.
Two reviewers and the editors provided excellent and detailed feedback which allowed
us to improve the manuscript.
AUTHOR AFFILIATIONS
1Nueld Department of Primary Care Health Sciences, University of Oxford, Oxford,
United Kingdom
2Biosecurity Program, The Kirby Institute, University of New South Wales, Sydney,
Australia
3Department of Public Health, University of Otago, Wellington, New Zealand
4School of Mechanical and Manufacturing Engineering, University of New South Wales,
Sydney, Australia
5School of Population Health, University of New South Wales, Sydney, Australia
6Dalla Lana School of Public Health, University of Toronto, Toronto, Ontario, Canada
7Centre for Social Research in Health and Social Policy Research Centre, Faculty of Arts,
Design and Architecture, University of New South Wales, Sydney, Australia
8Professional Standards Advocate, Edmonton, Canada
9Faculty of Veterinary Medicine; Department of Biomedical Engineering, Schulich School
of Engineering; Alberta Children's Hospital Research Institute, University of Calgary,
Calgary, Alberta, Canada
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Month XXXX Volume 0 Issue 0 10.1128/cmr.00124-2348
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10Department of Emergency Medicine, Cumming School of Medicine, University of
Calgary, Calgary, Alberta, Canada
AUTHOR ORCIDs
Trisha Greenhalgh http://orcid.org/0000-0003-2369-8088
C. Raina MacIntyre http://orcid.org/0000-0002-3060-0555
Michael G. Baker http://orcid.org/0000-0002-1865-1536
Shovon Bhattacharjee http://orcid.org/0000-0003-1241-641X
Abrar A. Chughtai http://orcid.org/0000-0003-4203-7891
David Fisman http://orcid.org/0000-0001-5009-6926
Mohana Kunasekaran http://orcid.org/0000-0001-5779-7979
Amanda Kvalsvig http://orcid.org/0000-0002-4184-001X
Deborah Lupton http://orcid.org/0000-0003-2658-4430
Matt Oliver http://orcid.org/0000-0002-7197-3032
Essa Tawq http://orcid.org/0000-0001-8147-079X
Mark Ungrin http://orcid.org/0000-0002-7410-1491
Joe Vipond http://orcid.org/0000-0002-1475-9814
AUTHOR CONTRIBUTIONS
Trisha Greenhalgh, Conceptualization, Data curation, Formal analysis, Investigation,
Methodology, Project administration, Writing – original draft, Writing – review and
editing | C. Raina MacIntyre, Conceptualization, Data curation, Formal analysis, Investi
gation, Methodology, Supervision, Validation, Writing – original draft, Writing – review
and editing | Michael G. Baker, Data curation, Writing – original draft, Writing – review
and editing | Shovon Bhattacharjee, Data curation, Writing – original draft, Writing –
review and editing | Abrar A. Chughtai, Data curation, Formal analysis, Investigation,
Methodology, Writing – original draft, Writing – review and editing | David Fisman,
Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Writing
– original draft, Writing – review and editing | Mohana Kunasekaran, Data curation,
Formal analysis, Writing – review and editing | Amanda Kvalsvig, Conceptualization, Data
curation, Formal analysis, Investigation, Writing – original draft, Writing – review and
editing | Deborah Lupton, Data curation, Formal analysis, Investigation, Writing – original
draft, Writing – review and editing | Matt Oliver, Data curation, Formal analysis, Investiga
tion, Writing – original draft, Writing – review and editing | Essa Tawq, Data curation,
Formal analysis, Writing – review and editing | Mark Ungrin, Conceptualization, Data
curation, Formal analysis, Investigation, Methodology, Writing – original draft, Writing –
review and editing | Joe Vipond, Data curation, Formal analysis, Writing – original draft,
Writing – review and editing
DATA AVAILABILITY
The meta-analysis datasets will be made available to bona de researchers from
academic institutions on reasonable request.
ETHICS APPROVAL
The review was based on publicly available published sources, so no specic ethical
approval was needed.
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AUTHOR BIOS
Professor Trisha Greenhalgh is a
researcher at the University of Oxford,
Oxford, UK. A medical doctor and
social scientist, she leads a programme
of interdisciplinary research at the
interface between medicine and the
social sciences. She has brought this
interdisciplinary perspective to bear on
the research response to the COVID-19
pandemic, looking at diverse themes including remote clinical
assessment of the patient with acute COVID-19 in the commun
ity, the science and social science of face coverings, and policy
decision-making in conditions of uncertainty. She is a mem
ber of Independent SAGE, an interdisciplinary academic team
established to provide independent advice on the pandemic
direct to the lay public. Her many awards include Order of
the British Empire from Her Majesty the Queen for Services to
Evidence-Based Medicine.
Professor C. Raina MacIntyre is Head
of the Biosecurity Program at the
Kirby Institute, UNSW Sydney, Australia.
She leads research on prevention and
detection of epidemic infections, with a
focus on vaccines and facemasks. She
has conducted several RCTs of masks
and N95 respirators. She has received many awards including
the Sir Henry Wellcome Medal and Prize from the Association
of Military Surgeons of the US. She is on the editorial boards
of Vaccine, BMJ Open and Epidemiology & Infection. She has
served on committees for the US National Academies of Science,
Engineering and Medicine and WHO.
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Professor Michael G. Baker is a public
health physician, epidemiologist, and
Professor in the Department of Public
Health, University of Otago, Wellington.
He leads a group focussed on infec
tious disease and environmental health
research. Michael took a leading role
in shaping New Zealand’s COVID-19
pandemic response, particularly the
elimination strategy. He has a strong
interest in science communication and evidence translation and
is the inaugural director of the Public Health Communications
Centre Aotearoa. He has been a member of the New Zealand
Government’s COVID-19 Technical Advisory Group throughout
the pandemic and serves on World Health Organisation expert
groups.
Dr. Shovon Bhattacharjee is a Research
Fellow at UNSW Sydney, Australia,
and is aliated with both the School
of Mechanical and Manufacturing
Engineering and the Kirby Institute. His
research operates at the intersection of
materials science and healthcare, aiming
to develop and leverage functional materials for environmen
tal health safety and personalized healthcare applications. This
interdisciplinary pursuit is particularly focused on the discovery
and integration of potential materials, cutting-edge technolo
gies, and innovative strategies. These eorts are dedicated
to advancing the design and functionality of next-generation
Personal Protective Equipment (PPE) and medical devices,
targeting improvements in environmental and occupational
health and safety.
Dr. Abrar Chughtai is a medical
epidemiologist and is working as
a Senior Lecturer in the School
of Population Health, UNSW Sydney,
Australia. He is also Director of the
Master of Infectious Diseases Intelli
gence (MIDI) Program at School. His
research interests include epidemiology
and control of infectious diseases, focusing on emerging and
re-emerging infections. He has published more than 160 journal
papers and one book chapter during the last 10 years. Abrar
provided leadership during COVID-19, through publications,
media interactions and a committee membership. During 2021,
he was on secondment to NSW Health COVID-19 Emergency
Operation Center.
Professor David Fisman is a physician-
epidemiologist with research interests
that fall at the intersection of applied
epidemiology, mathematical modeling,
and applied health economics. He
completed a residency in Internal
Medicine at McGill and Brown Univer
sities, before completing a fellowship
at the Beth Israel Deaconess Medical
Centre in Boston, and an MPH at Harvard School of Public Health.
He was also an AHRQ fellow in Health Policy (1998-2001) at
Harvard Centre for Risk Analysis, and has held faculty appoint
ments at McMaster, Princeton and Drexel Universities. He is
an author of over 260 scientic articles, and a Fellow of the
Canadian Academy of Health Sciences and the College of
Physicians of Philadelphia. He co-leads the Pandemic Readiness
Stream at the University of Toronto's new Institute for Pandemics.
Dr. Mohana Kunasekaran is a research
associate in the Biosecurity Program
at the Kirby Institute, UNSW Syd
ney, Australia. Her research inter
ests include multivariable methods,
systematic review and meta-analysis and
the epidemiology of infectious diseases,
particularly in aged care settings. Her
work is focused on enhancing the aged care sector’s capacity
for outbreak control by utilising data on facility design and past
outbreaks to guide decision-making.
Dr. Amanda Kvalsvig is an epidemiolo
gist and Research Associate Professor
at the University of Otago Wellington,
Aotearoa New Zealand. She has a clinical
paediatrics background. Since early 2020
her work has been strongly focused
on generating evidence to inform NZ’s
pandemic response, including border
measures, the Alert Level system, children and schools, equitable
policy, pandemic strategy and preparedness, and face masks.
Other research areas include meningococcal disease, respiratory
infection surveillance and control, and the syndemic relation
ships between infectious diseases, chronic conditions, and
structural inequities. She is deaf with cochlear implants and
disability advocacy is an important aspect of her work.
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Professor Deborah Lupton is SHARP
Professor in the Centre for Social
Research in Health and the Social
Policy Research Centre at UNSW
Sydney, Australia. Her background is in
sociology, communication and cultural
studies, with a particular focus on the
sociocultural aspects of health, risk and
medicine. She is one of the world's
leading sociologists of the COVID-19
pandemic. She edited a special section on the pandemic for
Health Sociology Review (2020), co-edited a book collection, The
COVID-19 Crisis: Social Perspectives (2021), and authored/co-auth
ored The Face Mask in COVID Times: A Sociomaterial Analysis
(2022) and COVID Societies: Theorising the Coronavirus Crisis
monographs. She has also published numerous articles and book
chapters on the social impacts and everyday experiences of the
pandemic.
Matt Oliver is a commissioner involved
in the regulation of utilities, an
aerospace/electrical engineer with long
history in regulation, legal analysis,
forensic engineering, theology and
restorative justice. Experience assess
ing legal and technical causation and
investigating failure modes in complex
underdetermined systems led to pandemic eorts involving
ventilation, ltration, respiratory protection, risk assessment,
professional & research ethics and public policy. A graduate
of the Royal Military College of Canada, he retired from the
Canadian Armed Forces as a senior ocer. Matt is one of
Canada’s three Indigenous groups – Red River Métis – and
teaches about the collision between Western and Indigenous
cosmologies.
Dr. Essa Tawq is a medical doctor by
background. He has an MPH degree
from Tulane University, USA and a
PhD degree from Victoria University of
Wellington, New Zealand. Essa worked
for more than 20 years in clinical and
public health settings in Afghanistan
and New Zealand. In April 2023, Essa
joined the Kirby Institute, UNSW Sydney, Australia, and has
been involved in research on COVID-19, personal protective
equipment (PPE), health impacts of bushres, and eects of u
vaccination on cardiovascular disease.
Dr. Mark Ungrin is an Associate
Professor in the Faculty of Veterinary
Medicine, Department of Biomedical
Engineering, and Alberta Children's
Hospital Research Institute at the
University of Calgary. An interdiscipli
nary biomedical researcher, his interests
centre on the practical, safe and
eective translation of biomedical
research and associated supporting
technologies into benets to health,
with emphasis on ensuring rigour, reproducibility and e-
ciency. His work impacts global practice in multiple areas with
contributions to international standards, community outreach,
and technologies in use around the world and on the Interna
tional Space Station.
Dr. Joe Vipond has worked as an
emergency physician for over twenty
years, currently at the Rockyview
General Hospital. Outside of clinical
work, he has focused his eorts on
advocacy, both on climate and COVID
policy. He is Past President of the
national charity Canadian Association of
Physicians for the Environment. He is
also the co-founder and board member of the local charity the
Calgary Climate Hub, and over the last four years, the co-founder
of #masks4Canada, the Canadian COVID Society, and ProtectOur
ProvinceAB.
Review Clinical Microbiology Reviews
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... The existence of this type of debate shows that the problem of epistemology in mould-related complaints requires investigation. This type of consideration is by no means unique to the current topic (Greenhalgh et al. 2024), but we will address views on mould-attributed symptomatology first before moving on to comparisons to other areas of microbiology. ...
... Since this section is in part directed to addressing the epistemological issues involved in evaluating D/M reactions and related literature, the point above about the potential importance of direct inflammatory effects summons the deployment of a term from Greenhalgh et al.'s (2024) analysis of similar problems in medical virology: "bibliographic virus". This expression refers to the embedding of an incorrect, obstructive, and confirmation-bias-inducing idea in literature that is repeated by rote in reviews and textbooks and that acts as a filter against investigation of its own problematic assertion. ...
... Later studies have strongly corroborated the accuracy of that investigation (Tuhkuri Matvejeff et al. 2024). As the choir outbreak findings contradicted existing "droplet" dogma, as noted above under "bibliographic viruses", (Greenhalgh et al. 2024), they were siloed into the category of insufficiently evidenced assertions. This consideration falls within the scope of Bradford Hill criterion, "consistency of association"-a not unreasonable, on the surface, demand for supportive confirmatory studies for novel findings. ...
Chapter
In recent years, several types of novel fungal health problems have been emerging in parallel. Antifungal-drug-resistant opportunistic pathogens have emerged both in previously unknown lineages, notably Candida auris, and in traditional pathogens and opportunists such as the Trichophyton mentagrophytes species complex and Aspergillus fumigatus. Emergence of resistance in Aspergillus is clearly connected to agricultural use of fungicides related to medical antifungals, but is brought into further prominence by the simultaneous emergence of immunosuppressive viral effects connected to influenza and SARS CoV 2 infections. The sources of drug resistance phenotypes in Candida auris and Trichophyton indotineae are unclear, but neither climate change nor drug misuse can be clearly implicated. In Onygenalean endemic mycoses, however, climate change is under suspicion of causing range extension in Coccidioides immitis. Decimation of some North American bat species associated with Histoplasma hot spots has not had a perceptible effect on this fungus so far. Historical reading suggests that it may have survived a previous loss of conditioned habitat when the passenger pigeon became extinct. Emergence of Emergomyces and new Blastomyces species appears mainly to be related to enhanced recognition. Two separate sporotrichosis outbreaks in cats in South America and Southeast Asia suggest a little-discussed mechanism of pathogen emergence, opportunity trawling, i.e. ingress of new potential hosts into pathogen habitats leading to novel epizootics. In the investigations of non-pathogenic fungi connected to immunologically or toxicologically mediated symptomatology in indoor environments, the slow augmentation of difficult-to-obtain evidence has clarified that indoor dampness and mould can cause diverse health effects, ranging from novel advent of asthmatic conditions to eczema-like skin disturbances. The methodology used to evaluate indoor mould symptomatology shares some of the deficiencies that racked medical evaluations of causality during the SARS CoV 2 outbreak. Pertinent epidemiological connections may be obscured by an excessive positivistic demand for proof where the combination of valid evidence and judicious attention to the precautionary principle would better serve the community.
... The global shortage of personal protective equipment (PPE) during the COVID-19 pandemic motivated inquiry into the effectiveness of public use of cloth masks [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. In July 2024, current WHO guidance recommends, "Make wearing a mask a normal part of being around other people" [20]. ...
... With increased manufacturing of masks and PPE, a variety of masks and respirators are available, including home-sewn and commercial cloth masks, certified medical masks and non-certified disposable masks, KN95/KF94s and N95/CaN99s. Aerosol transmission of COVID-19 is now widely accepted [17,[21][22][23][24][25][26]. Systematic evaluation of the filtration of aerosols, from the perspective of the wearer, across the whole range of masks and non-fit-tested respirators is highly relevant to public health advice. ...
... Cloth masks and medical masks, widely used in the pandemic, filter aerosols, accounting for their partial effectiveness in reducing transmission in an airborne pandemic [9,13,[76][77][78][79]; this should inform public health messaging and advice. Medical masks are no substitute for respirators in the personal protection of health care workers, in keeping with meta-analysis of trials comparing continuous mask use with continuous respirator use [17]. ...
Article
Full-text available
Importance Masks reduce transmission of SARS-CoV2 and other respiratory pathogens. Comparative studies of the fitted filtration efficiency of different types of masks are scarce. Objective To describe the fitted filtration efficiency against small aerosols (0.02–1 µm) of medical and non-medical masks and respirators when worn, and how this is affected by user modifications (hacks) and by overmasking with a cloth mask. Design We tested a 2-layer woven-cotton cloth mask of a consensus design, ASTM-certified level 1 and level 3 masks, a non-certified mask, KF94s, KN95s, an N95 and a CaN99. Setting Closed rooms with ambient particles supplemented by salt particles. Participants 12 total participants; 21–55 years, 68% female, 77% white, NIOSH 1–10. Main Outcome and Measure Using standard methods and a PortaCount 8038, we counted 0.02–1 µm particles inside and outside masks and respirators, expressing results as the percentage filtered by each mask. We also studied level 1 and level 3 masks with earguards, scrub caps, the knot-and-tuck method, and the effects of braces or overmasking with a cloth mask. Results Filtration efficiency for the cloth mask was 47–55%, for level 1 masks 52–60%, for level 3 masks 60–77%. A non-certified KN95 look-alike, two KF94s, and three KN95s filtered 57–77%, and the N95 and CaN99 97–98% without fit testing. External braces and overmasking with a well-fitting cloth mask increased filtration, but earguards, scrub caps, and the knot-and-tuck method did not. Limitations Limited number of masks of each type sampled; no adjustment for multiple comparisons. Conclusions and Relevance Well-fitting 2-layer cotton masks filter in the same range as level 1 masks when worn: around 50%. Level 3 masks and KN95s/KF94s filter around 70%. Over a level 1 mask, external braces or overmasking with a cloth-mask-on-ties produced filtration around 90%. Only N95s and CaN99s, both of which have overhead elastic, performed close to the occupational health and safety standards for fit tested PPE (>99%), filtering at 97–99% when worn, without formal fit testing. These findings inform public health messaging about relative protection from aerosols afforded by different mask types and explain the effectiveness of cloth masks observed in numerous epidemiologic studies conducted in the first year of the pandemic. A plain language summary of these findings is available at https://maskevidence.org/masks-compared.
... In the context of the COVID-19 pandemic, our study reports on the case of influential medical professionals rarely specifing the exact models of N95s, such as those without exhalation valves, which are critical for preventing airborne transmission. This oversight contributes to misunderstanding about the effectiveness of different mask types, an issue that could similarly arise in future public health crises requiring precise messaging whether related to mask-wearing during flu outbreaks or recommendations for air quality masks in wildfire seasons (see Greenhalgh et al., 2024). ...
... To minimize the spread of COVID-19, global health authorities like the World Health Organization (WHO), and the Centers for Disease Control and Prevention (CDC) emphasized the importance of mask-wearing as a critical public health measure (Croucher et al., 2023). Particularly during early 2021, as vaccines were not yet widely available, N95 respirators emerged as a vital component in reducing transmission risks (Greenhalgh et al., 2024), especially during a peak in cases across the US and UK. However, the public understanding of the appropriate use of N95s was often inconsistent, influenced significantly by messaging from health professionals and non-professionals verification status -offers a replicable framework that can help policymakers and health agencies design more effective digital communication strategies. ...
... Pulmonary ventilators play a crucial role in modern healthcare, particularly for patients recovering from severe respiratory conditions such as COVID-19 [1][2][3]. The global pandemic has underscored the necessity of reliable and high-quality ventilators to support patients with respiratory failure [4]. For post-COVID patients, lung function can remain compromised for an extended period, making ventilatory assistance essential for recovery [5]. ...
Article
Full-text available
Optimisation of medical devices is crucial for the safety and efficiency of healthcare treatments. This study applies the Taguchi method for the parametric optimisation of a lung ventilator, focusing on the identification and analysis of critical variables affecting its performance. The research aims to improve the stability and efficiency of the device while minimising operational variability. The study employs an orthogonal Taguchi matrix to systematically analyse the effects of control variables such as air pressure, sensor signal and ventilation speed. The research has two phases, the first analysing the relationship between potentiometer, duty cycle and fan cycle, the second optimising the fan speed in relation to the airflow sensor readings. The results indicate that a duty cycle between 40% and 60% ensures adequate airflow, while a PA range set between 10 and 20 provides the best performance in terms of stability. Taguchi improves reliability and efficiency in real medical applications by reducing device variability. This study confirms the importance of statistical optimisation techniques in biomedical engineering, highlighting how methodical experimentation can contribute to the development of more robust and reliable medical devices.
... In addition, efforts to prevent transmission of airborne respiratory viruses can reduce the number of acute cases, and thus long-term sequelae. Layered mitigation strategies such as provision of flexible, nonpunitive paid sick leave, ventilation and filtration, and the use of masks and respirators remain useful strategies for reducing workplace transmission [51][52][53]. The American Academy of Physical Medicine and Rehabilitation guidance on Long COVID symptoms and conditions provides strategies for managing Long COVID; many of these are applicable to the workplace [54]. ...
Article
Introduction Prior research has observed increased risks for numerous chronic conditions among individuals with Long COVID. Chronic conditions have been associated with employment limitations and increased economic hardships. Data from the Behavioral Risk Factor Surveillance System (BRFSS) present an opportunity to examine changes by employment status in the prevalence of a range of chronic conditions between 2019 (pre‐pandemic) and, in 2022, by self‐reported COVID‐19 or Long COVID. Methods We assessed the prevalence of chronic conditions in 2022 by employment status and self‐reported COVID‐19 and Long COVID history using data from BRFSS for adults of prime working age (25–54 years) who were employed for wages, self‐employed, unemployed less than 1 year, unemployed 1 year or more, or unable to work. For each chronic condition (coronary heart disease and myocardial infarction [combined], stroke, ever and current asthma, chronic obstructive pulmonary disease, kidney disease, diabetes, and arthritis), we generated adjusted prevalence ratios (aPRs) comparing 2022 prevalence by COVID‐19/Long COVID category to prevalences among respondents in that employment status before the pandemic (2019). Results The prevalence of both asthma and diabetes increased significantly between 2019 and 2022 among respondents in all included employment categories and COVID‐19/Long COVID histories combined. Among employed respondents with Long COVID in 2022, aPRs using 2019 prevalence figures for all employed respondents as a baseline for comparison had statistically significant elevations for every chronic condition assessed. Conclusions The increased prevalence of a range of chronic conditions between 2019 and 2022 among adults with Long COVID may present a burden for individuals, the workplace, the healthcare system, and the economy. Additional research in a longitudinal context could better quantify these associations. Efforts to prevent, identify, and treat Long COVID can reduce this burden.
... These evolving dynamics highlight the urgent need for robust surveillance systems, cross-border collaboration, and innovative research to anticipate, detect, and mitigate the impact of respiratory viral emergencies. As masking is effective in reducing contamination, the situations where it should be recommended or mandated as well as the optimal filtration characteristics should be better defined [137]. ...
Article
Full-text available
Respiratory viruses are widespread in the community, affecting both the upper and lower respiratory tract. This review provides an updated synthesis of the epidemiology, pathophysiology, clinical impact, and management of severe respiratory viral infections in critically ill patients, with a focus on immunocompetent adults. The clinical presentation is typically nonspecific, making etiological diagnosis challenging. This limitation has been mitigated by the advent of molecular diagnostics—particularly multiplex PCR (mPCR)—which has not only improved pathogen identification at the bedside but also significantly reshaped our understanding of the epidemiology of respiratory viral infections. Routine mPCR testing has revealed that respiratory viruses are implicated in 30–40% of community-acquired pneumonia hospitalizations and are a frequent trigger of acute decompensations in patients with chronic comorbidities. While some viruses follow seasonal patterns, others circulate year-round. Influenza viruses and Pneumoviridae, including respiratory syncytial virus and human metapneumovirus, remain the principal viral pathogens associated with severe outcomes, particularly acute respiratory failure and mortality. Bacterial co-infections are also common and substantially increase both morbidity and mortality. Despite the growing contribution of respiratory viruses to the burden of critical illness, effective antiviral therapies remain limited. Neuraminidase inhibitors remain the cornerstone of treatment for severe influenza, whereas therapeutic options for other respiratory viruses are largely lacking. Optimizing early diagnosis, refining antiviral strategies, and systematically addressing bacterial co-infections are critical to improving outcomes in patients with severe viral pneumonia.
Article
Full-text available
Since winter 2019, SARS-CoV-2 has emerged, spread, and evolved all around the globe. We explore 4 y of evolutionary epidemiology of this virus, ranging from the applied public health challenges to the more conceptual evolutionary biology perspectives. Through this review, we first present the spread and lethality of the infections it causes, starting from its emergence in Wuhan (China) from the initial epidemics all around the world, compare the virus to other betacoronaviruses, focus on its airborne transmission, compare containment strategies (“zero-COVID” vs. “herd immunity”), explain its phylogeographical tracking, underline the importance of natural selection on the epidemics, mention its within-host population dynamics. Finally, we discuss how the pandemic has transformed (or should transform) the surveillance and prevention of viral respiratory infections and identify perspectives for the research on epidemiology of COVID-19.
Article
Full-text available
Introduction Acute infections sharply rose in the post-COVID-19 era but declined during the COVID-19 pandemic. Epidemics of common and rare diseases have been observed both in season and out of season, and the importance of NPI cannot be ignored. Objectives This systematic review aims to assess the role of NPIs in controlling infectious diseases in the post-COVID-19 era, focusing on their applicability, limitations, and future directions. Methods We conducted a systematic review using primary sources, scholarly articles, and secondary bibliographic indexes, and databases from January 2020 to September 2024. The research method was an in-depth and targeted review of research articles on COVID-19, infectious diseases, the post-corona era, and NPI. The main search engines used in this research were PubMed, SciELO, and Google Scholar. Results Infectious pathogens emerged as a result of the discontinuation of NPI, the absence of specialized international communication, the lack of financial budgeting, the allocation of facilities, and the long-term planning of nations for viral epidemics. The COVID-19 pandemic underscored the critical role of NPIs in mitigating disease transmission and reducing strain on healthcare systems. NPIs, including physical distancing, mask-wearing, hand hygiene, and quarantine measures, were widely implemented and provided valuable lessons for managing infectious disease outbreaks. Conclusion The post-COVID-19 era has seen the resurgence of infectious diseases and the return of circulating viruses. Therefore, the development and global culture of paying attention to NPI is more necessary than ever to save the world from the next pandemic.
Article
BACKGROUND The VIPS (Vascular Effects of Infection in Pediatric Stroke) II prospective cohort study aimed to better understand published findings that common acute infections, particularly respiratory viruses, can trigger childhood arterial ischemic stroke (AIS). The COVID-19 pandemic developed midway through enrollment, creating an opportunity to assess its impact. METHODS Twenty-two sites (North America, Australia) prospectively enrolled 205 children (aged 28 days to 18 years) with AIS from December 2016 to January 2022, including 100 cases during the COVID-19 pandemic epoch, defined here as January 2020 to January 2022. To assess background rates of subclinical infection, we enrolled 100 stroke-free well children, including 39 during the pandemic. We measured serum SARS-CoV-2 nucleocapsid total antibodies (present after infection, not vaccination; half-life of 3–6 months). We assessed clinical infection via parental interview. RESULTS The monthly rate of eligible AIS cases declined from spring through fall 2020, recovering in early 2021 and peaking in the spring. The prepandemic and pandemic cases were similar except pandemic cases had fewer clinical infections in the prior month (17% versus 30%; P =0.02) and more focal cerebral arteriopathy (20% versus 11%; P =0.09). Among pandemic cases, 26 of 100 (26%) had positive antibodies, versus 4 of 39 (10%) of pandemic-era well children ( P =0.04). The first SARS-CoV-2 positive case occurred in July 2020. Ten of the 26 (38%) positive cases had a recent infection by parental report, and 7 of those 10 had received a diagnosis of COVID-19. Only 1 had multisystem inflammatory syndrome in children. Median (interquartile range) nucleocapsid IgG total levels were 50.1 S/CO (specimen to calibrator absorbance ratio; 26.9–95.3) in the positive cases and 18.8 (12.0–101) in the positive well children ( P =0.33). CONCLUSIONS The COVID-19 pandemic may have had dual effects on childhood AIS: an indirect protective effect related to public health measures reducing infectious exposure in general, and a deleterious effect as COVID-19 emerged as another respiratory virus that can trigger childhood AIS.
Article
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Chronic respiratory diseases impact patients’ social, psychological, and physical well-being. Before COVID-19, asthma management focused on preventing lung damage and treating flare-ups. Despite more treatments, many patients struggle with medication adherence and experience isolation. The pandemic shifted asthma management toward preventive measures across the world. Practices such as wearing masks universally, improved hygiene, and more use of telemedicine were adopted globally. These helped to minimize hospital burden, with drop in asthma patients, proving that adhering to preventive practices can successfully treat long-term respiratory dis-eases. However, the pandemic also exacerbated the mental well-being of asthmatic patients, causing a decrease in their quality of life because of increased stress, anxiety, and loneliness. This formed a vicious cycle in which wors-ening physical and mental health worsened asthma management. Consequently, there is a requirement for a more patient-centered, multidisciplinary care approach for tackling social, mental, and physical health requirements. Disparities in healthcare continued, especially in rural and disadvantaged communities. Telemedicine and digital health technology, adopted so quickly during the pandemic, allowed for the continuity of care, increased medi-cation compliance, and fewer hospital visits. This article delves into asthma care before and after the COVID-19 pandemic, demonstrating the multifaceted impact of the crisis. It highlights the significance of individualized treatment and prevention as well as the value of incorporating mental healthcare in asthma management. In the future, new models of asthma care could leverage learnings from the pandemic, integrating telemedicine with improved preventive measures to enhance patient outcomes, equity, and access. The pandemic has triggered innovations in the treatment of asthma, such as the creation of digital tools for individualized care and novel biologic treatments. These advances need to be incorporated into the existing management practices to improve asthma control globally.
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Background Mask use for prevention of respiratory infectious disease transmission is not new, but has proven controversial during the SARS-CoV-2 pandemic. In Ontario, Canada, irregular regional introduction of community mask mandates in 2020 created a quasi-experiment useful for evaluating the impact of such mandates; however, Ontario SARS-CoV-2 case counts were likely biased by testing focused on long-term care facilities and healthcare workers. Methods We developed a regression-based method that allowed us to adjust cases for undertesting by age and gender. We evaluated mask mandate effects using count-based regression models with either unadjusted cases, or testing-adjusted case counts, as dependent variables. Models were used to estimate mask mandate effectiveness, and the fraction of SARS-CoV-2 cases, severe outcomes, and costs, averted by mask mandates. Results Models using unadjusted cases as dependent variables identified modest protective effects of mask mandates (Range 31%-42%), with variable statistical significance. Mask mandate effectiveness in models predicting test-adjusted case counts was higher, ranging from 49% (95% CI 44%-53%) to 76% (95% CI 57%-86%). The prevented fraction associated with mask mandates was 46% (95% CI 41%-51%), with 290,000 clinical cases, 3008 deaths, and loss of 29,038 QALY averted from June to December 2020, representing $CDN 610 million in economic wealth. Conclusions Under-testing in younger individuals biases estimates of SARS-CoV-2 infection risk, and obscures the impact of public health preventive measures. After adjustment for under-testing, mask mandates emerged as highly effective. Community masking saved substantial numbers of lives, and prevented economic costs, during the SARS-CoV-2 pandemic in Ontario, Canada.
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This paper addresses lessons learned from the COVID-19 pandemic from a UK Occupational Medicine perspective to permit comparison with other national accounts. In spite of good prior research and statute, the necessary resources to protect workers' health were seriously lacking when the pandemic struck. Weak public health guidance, which did not recognise dominant airborne transmission, was applied to workplaces, leaving workers and others unprotected, especially in respect to Respiratory Protective Equipment (RPE). The Health and Safety Executive (HSE) as regulator was lacking, for example, in not producing guidance to protect HealthCare Workers (HCW) who were amongst the most at risk. The UK COVID-19 Public Inquiry should address shortcomings such as these, but recommendations must be accompanied by robust means to ensure appropriate implementation. These should range from substantial measures to improve indoor air quality, to a permanent pandemic management organization with adequate resources. The enforcing authority has to be obliged to publish more specific workplace guidance than the public health authorities. Occupational medicine as a discipline needs to be better prepared, and hence to assert its responsibility towards high standards of workers' health protection. Future research has to include investigating the best means of mitigation against airborne infection and the management of post-acute covid sequelae.
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Rationale Masks have been widely used as a preventative tool during the COVID‐19 pandemic. However, the use of masks by children has been controversial, with international guidelines recommending a risk‐based approach to national policymakers. Aims and objectives We aimed to conduct a systematic review that explores children's experiences of mask‐wearing, drawing on an evidence base that describes mask‐wearing in different contexts including air pollution, and to prevent the spread of infectious disease. Methods We searched MEDLINE, Embase and PsycINFO in June 2021, with repeat searches in August 2022 and January 2024, for primary research studies exploring children's experiences of masks. Included studies reported on participants between 4 and 14 years (inclusive), with no restrictions on language where an English translation was available. Two reviewers independently screened titles and abstracts and reviewed full texts, with discrepancies resolved by a third reviewer. We used the Mixed Methods Appraisal Tool for quality appraisal and narrative synthesis to identify key findings. We also conducted stakeholder consultation (Patient and Public Involvement (PPI)) with nine children, where they submitted annotated drawings of their preferred masks to complement our review findings. Results We screened 982 titles and abstracts and reviewed 94 full texts. 45 studies were included in the synthesis. Children's experiences of mask‐wearing were influenced by their perceived necessity, social norms around their use and parental attitudes. Challenges related to mask‐wearing were described, including difficulty reading facial expressions and physical discomfort. Children found it easier to wear masks when sitting and in cooler environments, and they benefited from unmasking during outdoor break time at school. As part of the PPI consultation, children highlighted the importance of mask design and the environmental impact of masks. Conclusion Children's experiences of mask‐wearing were varied and context‐dependent, with several mask‐design challenges raised. Future policy on mask‐wearing needs to consider the context in which mask‐wearing would be most beneficial, and how local adaptations to policy can respond to children's needs.
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This Viewpoint discusses declining vaccination rates in the US, specifically against COVID-19, and the ways in which clinicians and the Food and Drug Administration can counter the current large volume of vaccine misinformation.
Preprint
BackgroundA previously published study is cited as evidence that medical masks (MM) are noninferior to N95 respirators (N95) in the prevention of COVID infections. As COVID is transmitted via infectious aerosols generated during coughing as well as routine activities such as breathing and speaking, and N95s (in contrast to MM) are designed, validated and specified in national standards to provide protection against such hazards, we re-analysed the published data to resolve this unexpected result.Methods Study data was extracted from the publication, and analyses pre-specified in the original study protocol but omitted from the publication were carried out. Anomalies identified in the process were subject to additional analyses for statistical significance.ResultsPrespecified analyses reverse the reported outcome, which is the product of multiple alterations to the trial that were not introduced into the registry until after publication. Methodological shortcomings include compromised randomization, with statistically significant correlation between female sex and allocation to the higher-risk arm of the trial. Trial conditions and results at unregistered trial sites in Egypt were inconsistent with – but overwhelmed findings from – sites in the registered countries, which reflected the expected inferiority of medical masks. Substantial additional sources of bias were identified. Unexpected patterns were observed in the data.Conclusions The results of the study do not support the claim that medical masks are noninferior to N95s for the prevention of COVID-19.
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Introduction Facemasks are an important piece of personal protective equipment (PPE) to mitigate the spread of respiratory illnesses, but they can impede communication between patients and healthcare providers. The purpose of this scoping review is to identify effective communication practices while wearing facemasks. Design Scoping review using a systematic search of articles from the PubMed, CINAHL, and Embase databases. Methods The PEO (population, exposure, outcome) methodology was selected for this systematic scoping review. The population of interest (P) includes humans of all ages (children, adults, and older adults); the exposure of interest (E) is PPE that covers the mouth (i.e., facemasks); and the outcome of interest (O) is successful or unsuccessful communication practices. The Johns Hopkins Evidence‐Based Practice for Nurses and Healthcare Professionals appraisal guidelines were used to determine the level and quality of the research. Results Thirty‐nine articles met the inclusion criteria. Seventeen of these were high‐ or good‐quality research studies, and the remaining 22 were non‐research articles included with separate analysis as part of the scoping review. The 17 articles encompassed 2656 participants. The highest quality evidence indicated that standard surgical masks have the least impact on speech perception compared to other non‐transparent mask types, and that recognizing emotions is less accurate with facemasks, necessitating compensatory actions (i.e., reducing extraneous noise, using a microphone to amplify voice, and employing clear speech). Evidence was contradictory regarding the use of transparent masks. Evidence was of limited quality for other non‐verbal and verbal communication strategies. Conclusion Awareness of communication challenges is crucial when wearing facemasks. More high‐quality studies are needed to evaluate communication techniques when speakers are wearing facemasks. Basic strategies such as selecting an appropriate mask type, reducing extraneous noise, using microphones, verbalizing emotions, and employing clear speech appear to be beneficial. Clinical Relevance The findings of this scoping review highlight the importance of considering communication challenges while wearing facemasks in the healthcare settings. The review suggests that selecting an appropriate mask type, reducing extraneous noise, verbalizing emotions, and employing clear speech are some strategies that may be effective in mitigating the impact of facemasks on communication between patients and healthcare providers.
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Importance As demonstrated by the influenza virus and SARS-CoV-2, viruses spread by the respiratory route can cause deadly pandemics, and face masks can reduce the spread of these pathogens. The effectiveness of responses to future epidemics and pandemics will depend at least in part on whether evidence on masks, including from the COVID-19 pandemic, is utilized. Observations Well-designed observational studies have demonstrated the association of mask use with reduced transmission of SARS-CoV-2 in community settings, and rigorous evaluations of mask mandates have found substantial protection. Disagreement about whether face masks reduce the spread of SARS-CoV-2 has been exacerbated by a focus on randomized trials, which are limited in number, scope, and statistical power. Many effective public health policies have never been assessed in randomized clinical trials; such trials are not the gold standard of evidence for the efficacy of all interventions. Masking in the community to reduce the spread of SARS-CoV-2 is supported by robust evidence from diverse settings and populations. Data on the epidemiologic, environmental, and mask design parameters that influence the effectiveness of masking provide insights on when and how masks should be used to prevent transmission. Conclusions and Relevance During the next epidemic or pandemic caused by a respiratory pathogen, decision-makers will need to rely on existing evidence as they implement interventions. High-quality studies have shown that use of face masks in the community is associated with reduced transmission of SARS-CoV-2 and is likely to be an important component of an effective response to a future respiratory threat.