Issues Affecting Respirator Selection for Workers Exposed to Infectious Aerosols: Emphasis on Healthcare Settings

Abstract

The goal of occupational health practice is to protect the health of workers by preventing diseases and injuries from occurring. When work activities are anticipated, recognized, or found during an investigation to involve risks to workers' health, preventive measures should be taken to control hazardous exposures in the workplace. Respirators are often used to control inhalational exposures to hazardous airborne contaminants, including infectious agents. Of the three methods available for selecting a respirator, the expert opinion method is used most frequently to recommend respirators for controlling exposures to infectious agents. The size of the particles comprising an infectious aerosol has received particular attention relating to the selection of respiratory protection for healthcare workers. Conflicting meanings of the term “droplet” are central to this issue and may be partly responsible for confusion concerning the particle sizes that surgical masks are unlikely to protect against. Although workers caring for patients with contagious respiratory infections are at risk of exposure to large-particle droplets greater than 100 micrometers in diameter, their risks of inhalational exposure to infectious particles are likely to be predominantly to an aerosol consisting of a mixture of evaporating droplets and droplet nuclei that remain suspended in room air for prolonged periods. Because surgical masks are intended to be used only as barriers against large-particle droplets, only respirators certified by the National Institute for Occupational Safety and Health should be used as part of a strategy for protecting workers against inhalational exposures to infectious aerosols. The issues outlined in this paper are focused on workers in healthcare settings, but also apply in other settings where workers may be exposed to infectious aerosols.

Full text

20
Abstract
The goal of occupational health practice is to pro-
tect the health of workers by preventing diseases and
injuries from occurring. When work activities are an-
ticipated, recognized, or found during an investigation
to involve risks to workers’ health, preventive measures
should be taken to control hazardous exposures in the
workplace. Respirators are often used to control inha-
lational exposures to hazardous airborne contaminants,
including infectious agents. Of the three methods avail-
able for selecting a respirator, the expert opinion
method is used most frequently to recommend respira-
tors for controlling exposures to infectious agents. The
size of the particles comprising an infectious aerosol
has received particular attention relating to the selection
of respiratory protection for healthcare workers. Con-
flicting meanings of the term “droplet” are central to
this issue and may be partly responsible for confusion
concerning the particle sizes that surgical masks are
unlikely to protect against. Although workers caring
for patients with contagious respiratory infections are
at risk of exposure to large-particle droplets greater
than 100 micrometers in diameter, their risks of inhala-
tional exposure to infectious particles are likely to be
predominantly to an aerosol consisting of a mixture of
evaporating droplets and droplet nuclei that remain sus-
pended in room air for prolonged periods. Because sur-
gical masks are intended to be used only as barriers
against large-particle droplets, only respirators certified
by the National Institute for Occupational Safety and
Health should be used as part of a strategy for protect-
ing workers against inhalational exposures to infectious
aerosols. The issues outlined in this paper are focused
on workers in healthcare settings, but also apply in
other settings where workers may be exposed to infec-
tious aerosols.
Introduction
The primary goal of occupational health practice
is to protect the health of workers by preventing dis-
eases and injuries from occurring (International
Commission on Occupational Health, 2002). When
work activities are anticipated, recognized, or found
during an investigation (e.g., an outbreak of an infec-
tious disease) to involve risks to workers’ health, pre-
ventive measures should be taken to control hazard-
ous exposures. Respirators are often selected as a
means of reducing workers’ inhalational risks when
engineering controls or administrative measures are
insufficient or unavailable for controlling exposures
to hazardous airborne contaminants, including infec-
tious agents. Several different types of respirators
provide varying levels of protection; each type has
different characteristics, advantages, and disadvan-
tages. Knowledge of how to select a respirator is es-
sential for ensuring that a worker’s health is pro-
tected.
Applied Biosafety, 9(1) pp. 20-36 © ABSA 2004
Article
Issues Affecting Respirator Selection for
Workers Exposed to Infectious Aerosols:
Emphasis on Healthcare Settings
Steven W. Lenhart1, Teresa Seitz1, Douglas Trout1, and Nancy Bollinger2
1Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Cincinnati, Ohio;
and 2Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, Morgantown,
West Virginia

21
The purpose of this article is to review issues
affecting the selection of respirators for reducing
workers’ exposures to infectious aerosols. Aerosols
are dispersions of liquid or solid particles suspended
in air (Baron & Willeke, 2001, p. 1065), and infec-
tious aerosols are defined as dispersions of airborne
particles capable of causing infection. Methods that
can be used to make a respirator selection are ad-
dressed first; three selection methods are described.
Next, characteristics of infectious particles and the
potential for airborne spread of infectious agents in
healthcare settings are discussed. These issues are
important to understanding how to determine the
potential effectiveness of respirators in minimizing
exposures to infectious aerosols. Finally, information
is provided demonstrating why surgical masks
should not be worn as protection against infectious
aerosols.
Respirator Selection Methods
Once a decision has been made that respirators
are needed to protect the health of workers, the
process of selecting an appropriate respirator re-
quires an understanding of the work activities associ-
ated with potential exposures, the health effects of
overexposure, properties of the air contaminant
(e.g., in healthcare settings, the characteristics and
behavior of an aerosolized infectious disease agent),
and worker and workplace factors that may affect
how effective a respirator will be in protecting work-
ers (Johnson, 2001; McCullough & Brosseau, 1999).
In addition, consideration must be given to whether
wearing a certain type of respirator will adversely
affect a worker’s ability to perform his or her tasks or
will create a risk to the safety of the worker or others
(e.g., the trailing hose of a supplied air respirator can
be a tripping hazard). These latter issues should be
addressed by considering the advantages and disad-
vantages of the various types of respirators from
which a choice must be made. Table 1 provides ex-
amples of the advantages and disadvantages of differ-
ent types of respirators as they relate to potential
infectious aerosol exposures in healthcare settings.
Because different types of respirators provide
varying levels of protection, having information that
compares their relative protective capabilities is es-
sential when making a selection. This is one of the
functions of an assigned protection factor (APF), a
unitless value that historically has been defined as
representing the minimum level of protection that a
respirator class can be anticipated to provide for a
substantial proportion (usually 95%) of properly fit-
ted and trained respirator users (Guy, 1985; Myers,
Lenhart, Campbell, & Provost, 1983). For example,
an assigned protection factor of 50 means that a res-
pirator having this value will reduce most wearers’
exposures to a contaminant to 2% [(1/APF) x 100%
= 2%] of what they would have been exposed to if
they had not been wearing a respirator—a 98% expo-
sure reduction.
In 2003, the Occupational Safety and Health
Administration (OSHA) proposed the following
definition of assigned protection factor: the work-
place level of respiratory protection that a respirator
or class of respirators is expected to provide to em-
ployees when the employer implements a continu-
ing, effective respiratory protection program as speci-
fied by Title 29 CFR 1901.134 (Occupational Safety
and Health Administration [OSHA], 2003). As-
signed protection factors range from 10 for fit-tested,
air-purifying, half-facepiece respirators to 10,000 for
pressure demand self-contained breathing appara-
tuses (American National Standards Institute
[ANSI], 1992; McCullough & Brosseau, 1999; Na-
tional Institute for Occupational Safety and Health
[NIOSH], 1987; OSHA, 2003).
Choosing a selection method is the first impor-
tant decision in a respirator selection process. The
choices are the hazard ratio method, the risk analysis
method, and the expert opinion method.
Hazard Ratio Method
The hazard ratio method, or the industrial hy-
giene method, is a quantitative method used most
commonly to select respirators for noninfectious
aerosols, gases, and vapors. Using this method re-
quires estimates of the air concentrations of a con-
taminant measured during a person’s work activities
and knowledge of the established (or recommended)
occupational exposure limits of that contaminant. A
minimum level of respiratory protection is calculated
by dividing the highest air concentration measure-
ment by the most protective occupational exposure
Lenhart / Seitz / Trout / Bollinger

22
Respirator Type Advantages Disadvantages
Filtering Facepiece Respirator
Assigned protection factor = 10;
better-performing models, if fit
tested and used properly, reduce
respiratory exposure to 10% of what
it would be without the respirator.
yLightweight.
yNo maintenance, cleaning, or
disinfection needed.
yNo effect on mobility.
yOnly respirator type with models
available without an exhalation
valve. A healthcare worker can wear
such a respirator to protect patients
and others from expired aerosols of
the healthcare worker.
yProvides no eye protection.
yCan add to heat burden.
yInward leakage at gaps in face seal.
yDifficult for a user to do a seal check.
yLevel of protection varies greatly among models.
yCommunication may be difficult.
yFit testing required to select proper facepiece size.
ySome eyewear may interfere with the fit.
Elastomeric Half-facepiece Respirator
Assigned protection factor = 10;
most models, if fit tested and used
properly, reduce respiratory
exposure to 10% or less of what it
would be without the respirator.
yLow maintenance.
yReusable facepiece and replaceable
filters and cartridges.
yNo effect on mobility.
yProvides no eye protection.
yCan add to heat burden.
yFacepiece must be cleaned and disinfected before reuse,
which may place workers at risk of contact exposure.
yInward leakage at gaps in face seal.
yCommunication may be difficult.
yFit testing required to select proper facepiece size.
ySome eyewear may interfere with the fit.
Elastomeric Full-facepiece Respirator
Assigned protection factor = 50;
most models, if fit tested and used
properly, reduce respiratory
exposure to 2% or less of what it
would be without the respirator.
yProvides eye protection.
yLow maintenance.
yReusable facepiece and replaceable
filters and cartridges.
yNo effect on mobility.
yMore effective face seal than that of
filtering facepiece or elastomeric
half-facepiece respirators.
yCan add to heat burden.
yDiminished field-of-vision compared to half-facepiece.
yFacepiece must be cleaned and disinfected before reuse,
which may place workers at risk of contact exposure.
yInward leakage at gaps in face seal.
yFit testing required to select proper facepiece size.
yCommunication may be difficult.
yMust be quantitatively fit tested to reduce exposures to 2%.
yFacepiece lens can fog without nose cup or lens treatment.
ySpectacle kit needed for people who wear corrective glasses.
Powered Air-purifying Respirator with Hood, Helmet, or Loose-fitting Facepiece
Assigned protection factor = 25;
most models, if used properly,
reduce respiratory exposure to 4%
or less of what it would be without
the respirator.
yProtection for people with beards,
missing dentures, or facial scars.
yProvides eye protection.
yLow breathing resistance.
yFlowing air creates cooling effect.
yFace seal leakage is generally outward.
yFit testing is not required.
yPrescription glasses can be worn.
yHoods completely cover head and
neck and may also cover shoulders
and torso, providing extensive
barrier protection.
yCommunication less difficult than
with elastomeric half-facepiece or
full-facepiece respirators.
yReusable components and
replaceable filters.
yAdded weight of battery and blower.
yAwkward for some tasks.
yComponents must be cleaned and disinfected before
reuse, which may place other workers at risk for contact
exposure.
yBattery requires charging.
yNoise from a device’s blower may make stethoscope use
difficult.
yAir flow must be tested with flow device before use.
Table 1
Advantages and disadvantages of different respirator types as they
relate to potential infectious aerosol exposures in healthcare settings.

23
Powered Air-purifying Respirator with Tight-fitting Half-facepiece or Full-facepiece
Assigned protection factor = 50;
most models, if used properly,
reduce respiratory exposure to 2%
or less of what it would be without
the respirator.
yProvides eye protection with full-
facepiece.
yLow breathing resistance.
yFlowing air creates cooling effect.
yFace seal leakage is generally
outward.
yReusable components and
replaceable filters.
yAdded weight of battery and blower.
yAwkward for some tasks.
yNo eye protection with half-facepiece.
yComponents must be cleaned and disinfected before
reuse, which may place other workers at risk for contact
exposure.
yFit testing required to select proper facepiece size.
yBattery requires charging.
yNoise from a device’s blower may make stethoscope use
difficult.
yCommunication may be difficult.
ySpectacle kit needed for people who wear corrective
glasses with full-facepiece respirators.
yAir flow must be tested with flow device before use.
Supplied Air Respirator
The three modes of operation of
supplied air respirators are demand
(negative pressure), continuous
flow, and pressure demand.
Pressure demand, full-facepiece
models (assigned protection factor =
2,000), if fit tested and used
properly, reduce respiratory
exposure to 0.05% or less of what it
would be without the respirator.
yProvides eye protection with full-
facepiece or hood.
yDoes not depend on filters to purify
ambient air.
yLow breathing resistance.
yFace seal leakage is outward.
yFlowing air creates cooling effect.
yMobility limited to air-supply hose length and proximity
of the air supply.
yTrailing hose may be a tripping hazard and may get in
the way of gurneys and other medical equipment on
wheels.
yFit testing required to select proper facepiece size.
yComponents must be cleaned and disinfected before
reuse, which may place other workers at risk for contact
exposure.
yCommunication may be difficult.
ySource of pressure-regulated Grade D breathing air
needed.
ySource of breathing air must be tested to ensure quality.
Self-contained Breathing Apparatus (SCBA)
Assigned protection factor =
10,000; pressure demand, full-
facepiece models, if fit tested and
used properly, reduce respiratory
exposure to 0.01% or less of what it
would be without the respirator.
yProvides eye protection.
yFace seal leakage is outward.
yDoes not depend on filters or
cartridges to purify ambient air.
yFlowing air creates cooling effect.
yDuration of use limited by service life of air cylinders.
yFrequent work stoppages needed to change air
cylinders.
yFit testing required to select proper facepiece size.
ySCBA weigh as much as 40 pounds.
yComponents must be cleaned and disinfected before
reuse, which may place other workers at risk for contact
exposure.
yCommunication may be difficult.
ySupply of replacement air cylinders needed.
yFacility needed to recharge empty air bottles.
ySource of breathing air must be tested to ensure quality.
ySCBA must be returned annually or every 3 years
depending on manufacturer for inspection and repair.
Table 1 (Continued)
Advantages and disadvantages of different respirator types as they
relate to potential infectious aerosol exposures in healthcare settings.
Respirator Type Advantages Disadvantages
Lenhart / Seitz / Trout / Bollinger

24
Issues Affecting Respirator Selection for Workers Exposed to Infectious Aerosols
limit of the contaminant. A respirator from the res-
pirator class having an assigned protection factor
equal to or exceeding this value would then be se-
lected. However, applying this method to respirator
selection decisions for infectious aerosols is difficult
and often impossible.
Two major obstacles limit application of the haz-
ard ratio method to worker exposures to infectious
agents. The first involves uncertainties about the air
concentrations of infectious agents (due in part to
difficulties in sampling air for viable infectious
agents). Second, neither occupational exposure lim-
its nor guidelines for infectious agents are generally
available. This is because the infectious inhalation
dose of most disease agents is poorly characterized
and may extend over a considerable range because of
variations in host susceptibility and related factors.
According to the OSHA respiratory protection stan-
dard (Title 29 CFR 1910.134), when exposures can-
not be reasonably estimated, an employer is required
to consider the work environment as immediately
dangerous to life or health and to provide to employ-
ees either a self-contained breathing apparatus
(SCBA) or a combination supplied-air respirator
with an auxiliary SCBA (OSHA, 1998b). Both of
these types of respirators would be impractical for
use in many work settings, including healthcare.
(Advantages and disadvantages of these respirator
types are described in Table 1.) However, OSHA did
not intend that only these two respirator types could
be selected when workplace-specific exposure meas-
urements did not exist. Rather, OSHA intended that
professional judgment also be used to evaluate all
factors associated with making a respirator selection
and that the most protective respirators needed to be
selected only when a less protective respirator could
not confidently be presumed safe (OSHA, 1998a).
Risk Analysis Method
The risk analysis method of making a respirator
selection is a quantitative modeling approach in
which cumulative risk is calculated. This method has
been applied to respirator selections for protecting
against inhalational exposures to Bacillus anthracis
spores (as a possible bioterrorism agent [Nicas &
Hubbard, 2003; Nicas, Neuhaus, & Spear, 2000]),
Mycobacterium tuberculosis (Nicas, 1995, 1996), and
Coccidioides immitis (Nicas & Hubbard, 2002).
The data used to compute the cumulative risk of
an infectious aerosol are estimates of the infectious
inhalational dose of an infectious agent, the air con-
centration of infectious particles, the respirator
user’s breathing rate, the fractional penetration value
of the user’s respirator, the duration of a respirator
use period, and the number of respirator use periods
(Nicas & Hubbard, 2002, 2003; Nicas et al., 2000).
Limitations of this method are that complete infor-
mation is seldom available, and, as is true when ap-
plying the hazard ratio method to infectious aero-
sols, that important data—the infectious inhalational
dose and the air concentrations of infectious parti-
cles to which workers may be exposed—are usually
quite uncertain. However, advantages of the method
are that all assumptions and data are identified, the
dose-response model is given, and an acceptable level
of risk is specified (Nicas & Hubbard, 2003).
Expert Opinion Method
The expert opinion method is a qualitative ap-
proach to making decisions about respirators based
on the subjective professional judgment of one or
more experts. This approach has been used when
the important data needed for quantitative respira-
tor selection methods are either uncertain or un-
available. Respirator selection is made after consider-
ing the characteristics of job activities that are recog-
nized or anticipated to involve risks of exposure to
airborne contaminants; consideration of the specific
agent involved; and knowledge of the assigned pro-
tection factors, advantages, and disadvantages of vari-
ous respirators. This approach has been criticized
because, in some cases, details of the method are ill-
defined, and a rationale for the final decision is typi-
cally not provided with respirator recommendations
to explain how decisions were made (Nicas & Hub-
bard, 2003; Nicas et al., 2000).
The expert opinion method has been used to
recommend respirators for protection against expo-
sures to M. tuberculosis (Centers for Disease Control
and Prevention [CDC], 1994), Histoplasma capsula-
tum (Lenhart, Schafer, Singal, & Hajjeh, 1997), B.
anthracis (CDC, 2001, November 6), hantavirus pul-
monary syndrome (CDC, 2002), and biological
agents of bioterrorism events (CDC, 2001, October

25
25). A method using expert opinion and a modifica-
tion of the hazard ratio method was published to
address respirator selection for healthcare workers
exposed to infectious aerosols (McCullough &
Brosseau, 1999). This approach, which could be
termed a qualitative ranking method, uses qualita-
tive rankings of airborne concentrations (instead of
contaminant measurements themselves) and rank-
ings of “toxicity” or risk (as surrogates for occupa-
tional exposure limits) to identify graphically the
level of respiratory protection to which assigned pro-
tection factors could be compared.
In some applications of the expert opinion
method, categorical risk estimates are developed
with the levels of recommended respiratory protec-
tion increasing as the levels of perceived risk in-
crease. An example of an application of this ap-
proach to an infectious aerosol is the CDC guidance
document for protecting workers at risk of exposure
to H. capsulatum spores (Lenhart et al., 1997). Respi-
rators are described in that guide that should be
worn during work activities associated with expo-
sures to spore-contaminated airborne dust. The rec-
ommended respirators range from disposable, filter-
ing facepiece respirators for low-risk situations (e.g.,
site surveys of bird roosts) to full-facepiece, powered
air-purifying respirators for extremely dusty work
(e.g., removing accumulated bird or bat manure
from an enclosed area such as an attic).
Another example of an application of the expert
opinion method is the rationale used to select full-
facepiece powered air-purifying respirators for CDC
investigators performing environmental sampling for
B. anthracis in post offices and other environments
(CDC, 2001, November 6). This application differs
from the previous example in that, instead of rank-
ing respirator options according to perceived in-
creases in levels of exposure, acceptable respirators
were described as those respirators that met specific
criteria. Factors considered important in that appli-
cation of the expert opinion method included the
following:
y The infective dose (the potency) of the B. an-
thracis spores was unknown. To be conservative, the
spore-containing material in contaminated letters
was considered to have been bioengineered to make
it highly infectious. Also, no reliable estimates of
possible exposure levels were available, and it was
likely that they would have varied considerably by
location, time, and the investigators’ activities. For
these reasons, a higher level of protection than the
90 percent exposure reduction generally associated
with negative-pressure, half-facepiece respirators was
considered essential. Consequently, they were elimi-
nated from further consideration.
y Metropolitan mail processing and distribution
centers are large facilities, and the investigators do-
ing environmental sampling needed a respirator that
allowed mobility and the ability to wear the respira-
tor comfortably for an hour or more. These factors
eliminated supplied air respirators (because hose
lines limit mobility) and self-contained breathing
apparatuses (because of their limited service-life and
weight). The options remaining were air-purifying,
full-facepiece respirators and powered air-purifying
respirators with half- or full-facepieces, hoods, or
loose-fitting facepieces or helmets.
y The final step was selecting the respirator type
having the highest assigned protection factor from
the remaining options. Thus, a NIOSH-certified,
powered air-purifying respirator with a full facepiece
was selected.
Characteristics of Infectious Particles
In certain situations, healthcare workers have
risks of exposure to infectious aerosols that may re-
sult in the transmission of infection. Infection con-
trol precautions to prevent this method of agent
transmission have been termed airborne precautions
(Garner & Hospital Infection Control Practices Ad-
visory Committee, 1996). Precautions for preventing
the spread of infectious disease agents by other
routes of exposure include contact precautions (to
prevent spread by direct and indirect contact) and
droplet precautions (to prevent spread associated
with deposition of projected droplets, splatter, and
sprays onto conjunctivae, nasal mucosa, and the
mouth) (Garner & Hospital Infection Control Prac-
tices Advisory Committee, 1996).
Risks of person-to-person transmission of infec-
tious aerosols in healthcare settings have been associ-
ated with actions such as speaking, sneezing, or
spontaneous coughing by patients with contagious
Lenhart / Seitz / Trout / Bollinger

26
Issues Affecting Respirator Selection for Workers Exposed to Infectious Aerosols
respiratory infections. Person-to-person transmission
is also associated with cough-inducing or aerosol-
generating procedures such as aerosolized medica-
tion administration, diagnostic sputum induction,
bronchoscopy, airway suctioning, and endotracheal
intubation performed on patients. In some cases,
spread of an infectious agent may occur indirectly
from handling contaminated fomites (e.g., smallpox
virus transmission from handling infected bed linens
and clothing contaminated with scabs and vesicle
fluid from skin lesions [Downie et al., 1965; Tho-
mas, 1974]). Methods of protecting healthcare work-
ers from airborne transmission of infectious agents
include engineering and administrative controls,
respirators, disease prevention interventions such as
active immunization or antibiotic prophylaxis, or
combinations of these measures.
Size Distributions of Particles in Aerosols
from Possibly Infectious Persons
The factors influencing whether an infectious
agent will be transmitted by airborne spread to an-
other person include the size of the particles pro-
duced by contagious persons, the airborne concen-
trations and inhalational dose of the microorganism,
characteristics of the microorganism (e.g., infectivity,
pathogenicity, and viability after exposure to envi-
ronmental stresses), environmental factors (e.g., air
movement, temperature, relative humidity, and
sunlight), and host factors (e.g., susceptibility and
immunization status) (Cole & Cook, 1998; McCul-
lough & Brosseau, 1999). Of these factors, one that
has received particular attention relating to the selec-
tion of respirators for healthcare workers concerns
the sizes of the particles comprising an infectious
aerosol.
Conflicting meanings applied to the term
“droplet” are central to the issue of particle sizes pro-
duced by persons with contagious respiratory infec-
tions and are a source of continuing confusion. For
example, OSHA addressed issues related to infec-
tious aerosols, droplets, and the role of surgical
masks in protecting healthcare workers in the pream-
ble of its bloodborne pathogens standard, Title 29
CFR 1910.1030 (OSHA, 1991). Regarding infec-
tious aerosols and whether a potential for airborne
transmission existed, OSHA wrote that conflicting
opinions and a lack of information prevented it
from forming an opinion on the matter, and conse-
quently, the agency did not believe it was justified in
pursuing regulation of aerosols.
Regarding protection against exposure to drop-
lets, the OSHA bloodborne pathogens standard re-
quires that masks in combination with eye protec-
tion devices, such as goggles or glasses with solid side
shields, or chin-length face shields, shall be worn
whenever splashes, spray, spatter, or droplets of
blood or other infectious materials may be gener-
ated, and eye, nose, or mouth contamination can be
reasonably anticipated. In the preamble, OSHA
clarified further that protection of mucous mem-
branes of the face and upper respiratory tract against
large-droplet spattering could be provided by glasses,
face shields, and surgical masks, alone or in combi-
nation as appropriate to the task being performed
(OSHA, 1991).
OSHA did not cite the specific sizes of droplets
against which the use of barrier precautions would
be protective. However, by stating that the blood-
borne pathogens standard did not apply to infec-
tious aerosols and by using words such as sprays,
splashes, and spatter, the agency implied that the
droplets addressed in the standard are those particles
sometimes referred to as projectile particles. Projec-
tile particles are large enough to be visible to the na-
ked eye, are essentially unaffected by room air cur-
rents, and remain airborne only briefly. They have
ballistic trajectories that do not deviate from their
courses until they collide head-on with or impact a
surface. In physiological terms, droplets created by
sprays, splashes, and spatters are distinguished from
aerosol particles in that they are much too large to
be inhaled; for this reason, they have also been
called nonrespirable particles.
The position of OSHA was that barrier devices
such as glasses, face shields, and surgical masks
would protect healthcare workers from infectious
agents generated as large-particle droplets of sprays,
splashes, or spatters. However, a conflict arises when
comparing the term “droplet” to describe particles
that settle out quickly (as used by OSHA and sum-
marized above) and the term “droplet” as used in
infection control documents. Part II of the Guideline
for Isolation Precautions in Hospitals states that the

27
OSHA bloodborne pathogens standard requires the
wearing of masks, eye protection, and face shields to
reduce the risk of exposures to bloodborne patho-
gens and that healthcare workers generally can wear
surgical masks as protection against the spread of
infectious large-particle droplets (Garner & Hospital
Infection Control Practices Advisory Committee,
1996). However, large-particle droplets were defined
in the guideline as particles larger than 5 micrometer
(m) and generated either by an infected person dur-
ing coughing, sneezing, or talking or during the per-
formance of procedures such as suctioning and bron-
choscopy. Furthermore, particles of 5 m or less
were defined as droplet nuclei (i.e., residues of
evaporated droplets [Wells, 1955]). The rationale
supporting this definition of large-particle droplets
as 5 m and larger and the view that surgical masks
protected against exposures to them are unstated
and, as will be demonstrated below, may be flawed.
Critical to this discussion is the distinction be-
tween the size of particles comprising an aerosol and
the size of particles that settle out quickly. From the
principles of aerosol physics, spherical particles set-
tling freely in still air are known to reach an equilib-
rium or terminal settling velocity. The terminal set-
tling velocity of a particle can be calculated and is a
function of the viscosity and density of air, the parti-
cle’s density and its diameter squared, and accelera-
tion due to gravity (Baron & Willeke, 2001).
The terminal settling velocities of particles can
be used to distinguish particles that tend to remain
airborne from those that settle out. For example, the
terminal settling velocity of a particle of unit density
(i.e., 1 gram per cubic centimeter) and a diameter of
100 m is approximately 30 centimeters per second
(cm/sec) (Baron & Willeke, 2001), which suggests
that particles of this size and larger will settle quickly
on surfaces near the point at which they were gener-
ated. By comparison, a particle with the same unit
density and a diameter of 5 m (which is the cut-off
used in the infection control guide) has a terminal
settling velocity of only 0.08 cm/sec (Baron &
Willeke, 2001), and thus, it tends to remain air-
borne for a relatively long time. Therefore, as a rule-
of-thumb, airborne particles having diameters of 100
m or less have been defined as comprising an aero-
sol, and those greater than 100 m are particles that
will settle out quickly (Baron & Willeke, 2001;
Hirshfeld & Laub, 1941; Wells, 1934). Conse-
quently, an assumption that all droplets greater than
5m are large-particle droplets that do not remain
suspended in the air and generally travel only short
distances, usually 3 feet or less, through the air
(Garner & Hospital Infection Control Practices Ad-
visory Committee, 1996) is inconsistent with well-
established understanding of how aerosol particles
behave.
Duguid (1946) and Papineni & Rosenthal
(1997) studied the sizes of droplets expelled during
talking, coughing, sneezing, nose breathing, and
mouth breathing. Duguid used a microscope to esti-
mate the size of respiratory droplets by measuring
stain marks on slides exposed directly to mouth
spray. He reported that droplets produced by talk-
ing, coughing, and sneezing ranged from 1 to 2,000
m; 95% of the droplets had diameters between 2
and 100 m; and most droplets had diameters be-
tween 4 and 8 m (Duguid, 1946). The composition
and bacterial or viral content of droplets and droplet
nuclei are likely to be highly variable, with large ag-
gregate droplets and strings of mucus possibly con-
taining many organisms and droplet nuclei contain-
ing one or two organisms at most and sometimes
containing none (Reponen, Willeke, & Nevalainen,
2001; Riley & O’Grady, 1961; Wells, 1955).
More recently, Papineni and Rosenthal (1997)
used an optical particle counter and transmission
electron microscope to characterize the size distribu-
tion of droplets exhaled by mouth breathing, nose
breathing, coughing, and talking. They reported that
the diameters of respiratory droplets produced by
healthy persons ranged from 0.3 m (the lower limit
of detection of the sampling method) to approxi-
mately 8 m. The findings of a study comparing the
elimination of inhaled 6-m Teflon particles from
the tracheobronchial tract of healthy persons and
patients with respiratory tract disease showed that
only patients with increased respiratory secretions
eliminated test particles from their lungs by cough-
ing (Camner, Mossberg, Philipson, & Strandberg,
1979). Thus, a conclusion was made that increased
respiratory secretions were necessary for coughing to
be an effective means of eliminating particles. In-
creased secretion of fluids on airway surfaces and
Lenhart / Seitz / Trout / Bollinger

28
Issues Affecting Respirator Selection for Workers Exposed to Infectious Aerosols
greater respiratory actions such as coughing and
sneezing by persons with respiratory illnesses may
alter the size distribution of their exhaled droplets
and increase droplet concentrations (Papineni &
Rosenthal, 1997).
Droplets Versus Droplet Nuclei
If expelled droplets remained at their original
size, those having diameters greater than 100 m
would settle rapidly from the air, and only people
close to an infectious patient would be at risk for
exposure to these large aerosolized particles (Burge,
1995; Silverman, Billings, & First, 1971; Wells,
1934). However, droplets do not remain the same
size after being expelled, but rather they begin imme-
diately to evaporate and within seconds, or even frac-
tions of a second, become droplet nuclei (Burge,
1995; Silverman et al., 1971; Wells, 1934).
From a size distribution of the droplets emitted
during sneezing, one researcher concluded that prac-
tically all droplets would rapidly evaporate to droplet
nuclei (Riley & O’Grady, 1961). To demonstrate
how rapidly droplets evaporate, Wells (1955) calcu-
lated the drying times (total evaporation times) of
water droplets having diameters of 100 and 50 m
falling in unsaturated (50% relative humidity) air
and reported times of 1.3 and 0.3 second, respec-
tively. More recently, Ferron and Soderholm (1990)
calculated the drying time of a water droplet having
a diameter of 50 m in 50% relative humidity air to
be approximately 5 seconds. Despite this latter esti-
mate being more conservative than that of Wells, the
results still demonstrate that small water droplets
evaporate quickly; that is, water droplets having di-
ameters of 20 m and smaller were calculated to
evaporate in less than 1 second (Ferron & Soder-
holm, 1990). Wells, and Ferron and Soderholm
based their calculations on pure water particles.
Droplets containing dissolved substances, such as
salts and proteins, or containing a microorganism
would likely evaporate less rapidly than a water drop-
let (Riley & O’Grady, 1961).
Duguid (1946) measured the size of droplet nu-
clei collected on oiled slides exposed in a slit sampler
and reported that their diameters ranged from 0.25
to 42 m; 97% of the droplet nuclei were between
0.5 and 12 m; and most droplet nuclei had diame-
ters between 1 and 2 m. Duguid’s findings demon-
strated also that the most common expired droplets
were between 4 and 8 m, and therefore, for practi-
cal purposes, are so small that they may be consid-
ered to behave in air like droplet nuclei. Particles in
these size ranges and larger are affected by turbulent
air movement created by worker activities and the
ventilation in a room. The result is that aerosolized
particles smaller than 100 m can remain suspended
in air for prolonged periods of time because typical
room air velocities (10 to 30 cm/sec [Baldwin &
Maynard, 1998; Silverman et al., 1971]) exceed the
terminal settling velocities of the particles.
Applications Concerning Characteristics
of Infectious Aerosols
Recommendations for isolation precautions in
hospitals have defined the sizes of large-particle
droplets as greater than 5 m and the sizes of
droplet nuclei as 5 m or less (Garner & Hospital
Infection Control Practices Advisory Committee,
1996). Whether a healthcare worker is judged to be
exposed to infectious droplets or infectious droplet
nuclei have been controversial and are at the heart
of some debates concerning the level of protection
needed by healthcare workers exposed to infectious
agents. Among the subjects of these debates are two
high-priority infectious disease agents posing a risk
to national security—variola (smallpox) virus and
Yersina pestis (plague) bacteria (CDC, 2000). Because
a terrorist attack involving smallpox or plague is
likely to involve covert dissemination, healthcare
workers would likely be the first to identify exposed
individuals when they became ill. These workers
would be at risk for infection by person-to-person
transmission.
Person-to-person spread of pneumonic plague is
known to occur, and although very uncommon in
the United States, bioterrorism preparedness has
made this a topic of concern (Inglesby et al., 2000).
Guidelines addressing infection control for pneu-
monic plague state that there is no epidemiological
evidence suggesting person-to-person spread of pneu-
monic plague by droplet nuclei and that the
mechanism of transmission is via respiratory drop-
lets at close contact, within 6 feet (Garner & Hospi-
tal Infection Control Practices Advisory Committee,

29
1996; Inglesby et al., 2000; Inglesby, Henderson,
O’Toole, & Dennis, 2000). Because of this, some
researchers consider surgical masks sufficient for pro-
tecting healthcare workers from person-to-person
transmission of pneumonic plague and wearing a
respirator to be unwarranted. However, Y. pestis has
been found in oral secretions of infected animals
and humans (Chernin, 1989; Meyer, 1961; Speck &
Wolochow, 1957). As reviewed above, aerosols from
infected patients can, in the course of routine activi-
ties and procedures, produce small particles that will
remain airborne for long periods. In fact, some have
argued that the possibility of transmission of pneu-
monic plague by droplet nuclei should not be dis-
missed and have recommended that healthcare
workers at risk should wear a respirator (Hawley &
Eitzen, 2001; Levison, 2000).
Regarding smallpox, guidelines have stated that
the smallpox virus is transmitted predominantly by
droplets during close contact with an infectious per-
son. CDC has defined close contact as being within
6 to 7 feet of a smallpox patient (CDC, 2003). How-
ever, the findings in one experimental study have
shown that sedimentation plates placed 20 feet from
the bed of a smallpox patient were positive for vari-
ola virus (Thomas, 1974), and epidemiological evi-
dence (i.e., the findings of an outbreak investigation)
suggested that droplet-nuclei transmission was re-
sponsible for causing a smallpox outbreak in a Ger-
man hospital (Wehrle, Posch, Richter, & Hender-
son, 1970). These data suggested that smallpox virus
can be transmitted via an aerosol and are in part the
basis for recommendations that healthcare workers
should wear a respirator when caring for patients
with smallpox (Association for Professionals in Infec-
tion Control and Epidemiology, 1999; CDC, 2003).
Surgical Masks Versus Respirators
Despite confusion over what particle size distin-
guishes large-particle droplets from aerosol particles,
it may be reasonable to assume that a surgical mask
might provide an adequate barrier to large-particle
droplets. However, research has shown that surgical
masks should not be depended upon to protect
healthcare workers from infectious aerosols.
The original purpose of a surgical mask was to
prevent wound contamination by bacteria from the
mouth and upper respiratory tract of surgeons. Sur-
gical masks have also been recommended for pa-
tients who are suspected of having or known to have
infectious tuberculosis as a component of routine
infection control practice (CDC, 1994). A 1941
study evaluating surgical masks made of either gauze
or muslin concluded that they were inadequate for
protecting wounds because bacteria-containing parti-
cles passed through the filter material and around
the edges of the masks (Hirshfeld & Laube, 1941).
Subsequent studies, in which not only surgical
masks made of gauze and muslin but also ones made
of paper, foam, and synthetic materials were evalu-
ated, resulted in filter efficiencies ranging from the
teens to nearly 100% (Brosseau, McCullough, &
Vesley, 1997; Ford & Peterson, 1963; Ford, Peter-
son, & Mitchell, 1967; Miller, 1973, 1995; Rogers,
1980). The findings of other studies in which surgi-
cal masks were evaluated (with some reported to
have highly efficient filters) have emphasized that a
secure face seal is essential for preventing infectious
particles from escaping (as well as entering) at a
mask’s edges (Ha’eri & Wiley, 1980; Johnson, Mar-
tin, & Resnick, 1994; Pippin, Verderame, & Weber,
1987; Tuomi, 1985).
Researchers, who have studied the aerosols and
spatters produced during some dental procedures
and the blood aerosols and spatters generated during
surgeries, defined the size of spatter droplets to be
50 m and larger (Heinsohn & Jewett, 1993; Miller,
1973). However, research has been conducted to
measure the blood-containing particles generated by
common powered dental instruments and to evalu-
ate the effectiveness of surgical masks in protecting
against exposures to these particles (Miller, 1995).
The findings of the study showed that powered den-
tal instruments aerosolized mostly respirable-sized
particles smaller than 10 m in diameter, and the
efficiencies of the tested surgical masks ranged from
17% to 85%. From these findings, Miller (1995)
concluded that “the use of surgical masks for preven-
tion of occupational infection appears to be poorly
founded” (p. 675).
A draft guidance document of the U.S. Food
and Drug Administration (FDA) describes four labo-
ratory tests for measuring the filtration efficiencies
Lenhart / Seitz / Trout / Bollinger

30
Issues Affecting Respirator Selection for Workers Exposed to Infectious Aerosols
of surgical masks (Food and Drug Administration,
2003). In lieu of providing the results of one of these
tests to the FDA, the draft proposes that mask
manufacturers can submit the NIOSH certification
number of a model of surgical mask that has been
tested and certified by NIOSH as an N95 respirator.
However, the filtration efficiency tests recommended
by FDA should not be assumed to produce results
that are equivalent to the NIOSH certification tests
of an N95 respirator.
The results of a study comparing the abilities of
a surgical mask and a NIOSH-approved N95 respira-
tor to protect workers against exposures to airborne
latex allergenic particles provide evidence suggesting
that the FDA tests might overestimate the filter effi-
ciencies of surgical masks (Mitakakis et al., 2002).
Latex exposures of 20 healthcare workers were esti-
mated using nasal air samplers (Graham, Pavlicek,
Sercombe, Xavier, & Tovey, 2000) and Institute of
Occupational Medicine filter samplers (Mark & Vin-
cent, 1986). All samples were analyzed for particles
bearing the Hev b 5 latex allergen. The results of the
study showed that wearing a mask did not signifi-
cantly reduce the number of allergenic particles in-
haled and that wearing a respirator reduced the
number of inhaled particles by 17-fold. The mask
and the respirator were made by the same manufac-
turer and appeared to be identical; however, their
filter materials were different. The particle filtration
efficiencies and bacterial filtration efficiencies of the
mask and the respirator were both reported to be
greater than or equal to 99%, but their differential
pressures differed: less than 2.0 millimeters of water
per square centimeter (mm H2O/cm2) for the mask
and less than 5.0 mm H2O/cm2 for the respirator
(Shalfoon Dental Limited, 2002a, 2002b). Because
the mask and respirator had the same facepiece fit-
ting characteristics, the most likely explanation for
the difference in the levels of protection provided
between the mask and the respirator was penetration
of particles through the mask’s filter material rather
than face seal leakage (C. Solano, Kimberly-Clark
Corporation, N. Richland Hills, TX, personal com-
munication, September 3, 2003).
Conclusions and Recommendations
Air concentration measurements and exposure
limits applicable to infectious disease agents to
which workers may be exposed are essentially non-
existent, and the absence of these essential data im-
pedes the process of selecting appropriate respiratory
protection. Until particle-size distributions and the
viability and infectivity of particles comprising infec-
tious aerosols generated in healthcare settings can be
better characterized, the expert opinion method will
likely continue as the method used most frequently
to make respirator selections for healthcare workers.
Specifying the rationale and all data inputs used in a
respirator selection process is essential when using
this method. Important factors to consider include
the known limitations of data, historical experience
with infectious agents in epidemiological evaluations
of outbreak situations, availability of information on
infectious diseases, work tasks perceived to result in
potentially higher risk for aerosol exposure, and the
known properties of and experience with respirators
in healthcare settings and other workplaces.
Outbreaks of new and emerging infectious dis-
eases may present the most difficult challenges to the
selection and use of respirators in healthcare settings
where workers’ risks of exposure to an infectious
agent (e.g., the etiology of the problem, the source or
mode of transmission) are uncertain (Goodman,
Buehler, & Koplan, 1990; Reingold, 1998). Health-
care workers caring for patients in such settings may
be at risk of infection while the data of the outbreak
investigation are being collected and analyzed. The
importance of balancing the need for thorough as-
sessment of causality with the potentially conflicting
need to intervene quickly to protect the health of
workers means, in practice, that implementing con-
trol measures will oftentimes be appropriate at any
point in the outbreak investigation sequence
(Reingold, 1998). This public health approach is
consistent with guidance concerning occupational
health practice that states: “When doubts exist about
the severity of an occupational hazard, prudent pre-
cautionary action must be considered immediately
and taken as appropriate” (International Commis-
sion on Occupational Health, 2002).

31
Lenhart / Seitz / Trout / Bollinger
In response to outbreaks of new or emerging
infectious diseases, administrators of respirator pro-
grams should use all available data to make respira-
tor selection decisions. Whenever possible, data col-
lected during an outbreak investigation should
include descriptions of respirators (e.g., manufac-
turer, model number, NIOSH certification number)
worn by healthcare workers when caring for infec-
tious patients; whether respirators with tight-fitting
facepieces were assigned based on facepiece fit-
testing; whether respirators were worn correctly; the
nature of ventilation conditions in patients’ rooms
(e.g., ventilation effectiveness, air change rates); and
estimates of the air concentration, size, and infectiv-
ity of infectious particles generated by a patient or an
aerosol-generating procedure.
In cases where doubt remains about the level of
protection that should be recommended, a respira-
tor type having a higher assigned protection factor
can be selected until additional data are gathered
indicating that protection could be provided by a
respirator having a lower assigned protection factor
or even that respirator use could safely be stopped
entirely. (This approach was used in healthcare set-
tings during the 2003 outbreak of severe acute respi-
ratory syndrome [Twu et al., 2003]). Increased mone-
tary costs of maintaining a respirator program is a
factor to consider with this conservative approach.
Other potential factors to consider in healthcare set-
tings using this approach could include conse-
quences related to infection control (i.e., increased
potential for contact contamination) and interfer-
ence with patient care.
Evidence is presented in this paper supporting a
position that 100 m, and not 5 m, should be con-
sidered the particle size defining the boundary be-
tween large-particle droplets and aerosol particles.
Information is also presented demonstrating that,
although healthcare workers caring for patients with
contagious respiratory infections are at risk of expo-
sure to large-particle droplets greater than 100 m in
diameter, their risks of inhalational exposure to in-
fectious particles are likely to be predominantly to
an aerosol consisting of a mixture of rapidly evapo-
rating droplets and droplet nuclei that remain sus-
pended in room air for prolonged periods of time.
Applications of polymerase chain reaction-based
methods for analyzing air samples collected in
healthcare settings have shown promise for provid-
ing insight to the nosocomial spread of viral patho-
gens (Aintablain, Walpita, & Sawyer, 1998; Sawyer,
Chamberlin, Wu, Aintablain, & Wallace, 1994). For
example, contact with contaminated secretions and
large-particle droplets are thought to be the primary
route of transmission of both respiratory syncytial
virus and Bordetella pertussis. However, the possibility
that aerosol particles may contribute to the nosoco-
mial transmission of these agents has been suggested
by the detection of their nucleic acid material in air
at relatively large distances from patients’ beds
(Aintablain et al., 1998). Whether the quantities
detected are sufficient to transmit an infectious dose
or whether the material detected represents viable,
infectious organisms is unknown. These findings
suggest that defining a specific distance as the
boundary of a healthcare worker’s exposure to parti-
cles expired by a patient with a contagious respira-
tory infection may be inappropriate.
Dependence on the findings of outbreak investi-
gations to suggest indirectly whether large-particle
droplets (in which case wearing a surgical mask
would be indicated) or droplet nuclei (in which case
wearing a respirator would be indicated) are respon-
sible for transmission of an infectious agent does not
sufficiently account for other important characteris-
tics of infectious aerosols. Thus, when making a res-
pirator selection, factors in addition to the findings
of outbreak investigations and data concerning the
size distribution of the airborne infectious particles
are likely to be important. These other factors in-
clude estimates of the air concentrations of infec-
tious particles generated by different activities, esti-
mates of the amount of time that a healthcare
worker will be near an infectious patient or to proce-
dures likely to generate infectious aerosols, and the
characteristics of the infectious agent (e.g., its infec-
tivity and viability after exposure to evaporation and
other environmental stresses).
A finding that the highest air concentrations of
viable, respirable-size infectious particles most likely
occur during aerosol-generating procedures could
lead to a recommendation that respirators that pro-
vide higher levels of protection should be used by
nearby healthcare workers (Singh et al., 2003). For

32
Issues Affecting Respirator Selection for Workers Exposed to Infectious Aerosols
example, use of a powered air-purifying respirator
was recommended instead of a negative-pressure air-
purifying respirator for situations where healthcare
workers were likely to encounter high levels of infec-
tious aerosols during autopsy, orthopedic proce-
dures, and bronchoscopy (Johnson et al., 1994). The
rationale of this selection included the following
advantages of this respirator: face-seal leakage is es-
sentially prevented by the device’s air flow rate; the
presence of a face shield; and the ability to protect
people with beards. Similarly, wearing a powered
air-purifying respirator has been recommended for
healthcare workers during cough-inducing proce-
dures on patients suspected of having tuberculosis
or during autopsies on deceased persons suspected
of having had tuberculosis (Fennelly, 1997, 1998;
Fennelly & Nardell, 1998; McCullough & Brosseau,
1999; Nicas, 1995).
The filter media of surgical masks allow penetra-
tion of small particles, and the poor fitting character-
istics of their face seals allow the passage of particles
at the edges of the masks. Thus, only NIOSH-
certified respirators should be used as part of a strat-
egy for protecting workers from inhalational expo-
sures to infectious aerosols. Surgical masks may be
useful as barrier devices for protecting the mucous
membranes of a worker’s nose and mouth from in-
advertent exposures in situations where the only risk
is to large-particle droplets of splashes, sprays, or
spatters of blood or other potentially infectious ma-
terial (Mangram et al., 1999). However, surgical
masks cannot be considered respirators.
Finally, preventing the inhalational transmission
of infectious disease agents remains only one compo-
nent of infection control. Other mechanisms of in-
fectious agent transmission must also be addressed
comprehensively by the application of contact pre-
cautions and other preventive measures.
Acknowledgement
The authors gratefully acknowledge the always
outstanding editorial work of Ms. Priscilla Wopat of
the Spokane Research Laboratory of the National
Institute for Occupational Safety and Health.
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