Face mask performance related to potentially infectious aerosol particles, breathing mode and facial leakage, International Journal of Hygiene and Environmental Health

Abstract

During the COVID 19 pandemic, wearing certified Respiratory Protective Devices (RPDs) provided important means of protection against direct and indirect infections caused by virus-laden aerosols. Assessing the RPD performance associated with infection prevention in standardised certification tests, however, faces drawbacks, such as the representativeness of the test aerosols used, the protection of third parties during exhalation or the effect of facial leaks. To address these drawbacks, we designed a novel test bench to measure RPD performance, namely the number based total efficiency, size-segregated fractional filtration efficiency and net pressure loss, for 11 types of certified surgical masks and Filtering Face Pieces dependent on breathing mode and facial fit. To be representative for the context of potentially infectious particles, we use a test aerosol based on artificial saliva that is in its size distribution similar to exhaled aerosols. In inhalation mode excluding facial leaks, all investigated samples deposit by count more than 85% of artificial saliva particles, which suggests a high efficiency of certified RPD filter media related to these particles. In exhalation mode most RPDs tend to have similar efficiencies but lower pressure losses. This deviation tends to be significant primarily for the RPDs with thin filter layers like surgical masks or Filtering Face Pieces containing nanofibers and may depend on the RPDs shape. Both the filtration efficiency and pressure loss are strongly inter-dependent and significantly lower when RPDs are naturally fitted including facial leaks, leading to a wide efficiency range of approximately 30–85%. The results indicate a much greater influence of the facial fit than the filter material itself. Furthermore, RPDs tend be more effective in self-protection than in third-party protection, which is inversely correlated to pressure loss. Comparing different types of RPDs, the pressure loss partially differs at similar filtration efficiencies, which points out the influence of the material and the filter area on pressure loss.

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International Journal of Hygiene and Environmental Health 248 (2023) 114103
Available online 14 December 2022
1438-4639/© 2022 The Authors. Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Face mask performance related to potentially infectious aerosol particles,
breathing mode and facial leakage
Simon Berger
*
, Marvin Mattern, Jennifer Niessner
Institute of Flow in Additively Manufactured Porous Media (ISAPS), Heilbronn University of Applied Sciences, Max-Planck-Str. 39, 74081, Heilbronn, Germany
ARTICLE INFO
Keywords:
Respiratory protective device
Face mask
Performance measurement
Filtration efciency
Pressure loss
Respiratory aerosol
ABSTRACT
During the COVID 19 pandemic, wearing certied Respiratory Protective Devices (RPDs) provided important
means of protection against direct and indirect infections caused by virus-laden aerosols. Assessing the RPD
performance associated with infection prevention in standardised certication tests, however, faces drawbacks,
such as the representativeness of the test aerosols used, the protection of third parties during exhalation or the
effect of facial leaks. To address these drawbacks, we designed a novel test bench to measure RPD performance,
namely the number based total efciency, size-segregated fractional ltration efciency and net pressure loss, for
11 types of certied surgical masks and Filtering Face Pieces dependent on breathing mode and facial t. To be
representative for the context of potentially infectious particles, we use a test aerosol based on articial saliva
that is in its size distribution similar to exhaled aerosols. In inhalation mode excluding facial leaks, all investi-
gated samples deposit by count more than 85% of articial saliva particles, which suggests a high efciency of
certied RPD lter media related to these particles. In exhalation mode most RPDs tend to have similar ef-
ciencies but lower pressure losses. This deviation tends to be signicant primarily for the RPDs with thin lter
layers like surgical masks or Filtering Face Pieces containing nanobers and may depend on the RPDs shape.
Both the ltration efciency and pressure loss are strongly inter-dependent and signicantly lower when RPDs
are naturally tted including facial leaks, leading to a wide efciency range of approximately 30–85%. The
results indicate a much greater inuence of the facial t than the lter material itself. Furthermore, RPDs tend be
more effective in self-protection than in third-party protection, which is inversely correlated to pressure loss.
Comparing different types of RPDs, the pressure loss partially differs at similar ltration efciencies, which
points out the inuence of the material and the lter area on pressure loss.
1. Introduction
Pathogen dissemination through aerosol particles emitted by the
respiratory system best explains several super-spreading events during
the COVID-19 pandemic (Katelaris et al., 2021; Kutter et al., 2021; Lu
et al., 2020; Zhang et al., 2020) and is therefore in the focus of
SARS-CoV-2 transmission. Particles that are formed and expelled
through the respiratory system, for example when talking, coughing or
breathing, may differ in size and number based on several factors such as
the individual’s physiology, health condition or activity (Archer et al.,
2022; Morawska et al., 2009; Schwarz et al., 2010). In SARS-CoV-2
infected persons, these particles may act as vehicles for pathogens
(Gutmann et al., 2022; Ma et al., 2021) and thus are determinant for the
denition of protective measures. Present studies suggest that the mode
of the exhaled particle size distribution most likely is in the order of
0.1–0.5
μ
m (Scheuch, 2020) allowing for the particles to stay airborne
over several hours in indoor environments. Contrary to breathing,
talking or coughing produces larger particles from the submicronic and
small micrometre range to particles larger than 50
μ
m (Alsved et al.,
2020; Asadi et al., 2019). Exposure to these respiratory-emitted particles
leads to two possible routes of infection. On the one hand, particles may
be transported directly from an infected person to a susceptible host
(direct route of infection), whereby the probability of larger particles
reaching a susceptible host decreases with the distance due to the par-
ticles’ settling velocity. Airborne transmission, on the other hand, only
occurs indoors, where the smaller fraction of respiratory-emitted parti-
cles may accumulate in the indoor air with increasing durations of stay,
numbers of persons present and their activity. Since particle transport is
still possible after an infected person has left the room, airborne trans-
mission may also be referred to as an indirect infection route (Brlek
* Corresponding author.
E-mail address: simon.berger@hs-heilbronn.de (S. Berger).
Contents lists available at ScienceDirect
International Journal of Hygiene and Environmental Health
journal homepage: www.elsevier.com/locate/ijheh
https://doi.org/10.1016/j.ijheh.2022.114103
Received 27 September 2022; Received in revised form 24 November 2022; Accepted 8 December 2022

International Journal of Hygiene and Environmental Health 248 (2023) 114103
2
et al., 2020; Cai et al., 2020).
To reduce the risk of both direct and indirect infections, infection
control measures such as ventilation, air purifying technologies or the
wearing of Respiratory Protective Devices (RPDs) were discussed and
introduced during the COVID-19 pandemic in many areas of public life,
such as schools, kindergartens, ofces, public buildings, hospitals or the
transportation sector. While ventilation and air purifying technologies
may affect mostly the indirect route of infection (Nardell, 2021), the
wearing of RPDs counteracts both direct and indirect infections by
reducing the number of inhaled as well as exhaled particles and thus
potentially provides an effective means of protecting oneself (self--
protection) and others (third-party protection) (Asadi et al., 2020). Since
certied RPDs, in particular surgical masks (DIN EN 14683:2019-10)
and ltering face piece respirators such as FFP (DIN EN 149:2009-08),
N95 (NIOSH approved 42 CFR 84) or KN95 (GB 2626–2006) are sub-
jected to standardised test procedures, requirements for the separation
performance are dened. Filtering Face Pieces according to DIN EN
149:2009-08 are categorized into three classes, with class FFP2
requiring a mass-based total efciency of at least 94% and the total in-
ward leakage not exceeding 11%. Test procedures use aerosols con-
taining submicronic solid-phase sodium chloride or liquid-phase
parafn oil particles with a broad range allowed for the geometric
standard deviation (Zoller et al., 2021) that partly overlap the size range
of potentially infectious aerosols (Penner et al., 2022). As the test pro-
cedure originates from occupational health and safety, the focus is on
self-protection against occupational pollutants, with third-party pro-
tection not being considered. Surgical masks according to DIN EN
14683:2019-10, on the other hand, are designated to protect others from
infectious droplets during medical procedures. In certication, the
number-based total ltration efciency of the lter medium is deter-
mined by the use of infectious particles from a bacterial suspension with
a median diameter of 3
μ
m that are one order of magnitude larger than
exhaled virus-laden aerosol particles from the respiratory tract. As with
all ltration processes, however, the efciency of the separation mech-
anisms is highly dependent on the particle size and particle character-
istics (Hinds and Zhu, 2022; Lee and Liu, 1982). To effectively remove
respiratory particles from both the inhaled and exhaled air, RPD lter
media need to be highly efcient with respect to particles in the relevant
size range and with similar properties to infectious particles such as
shape, charge and density. Furthermore, the overall efciency is
dependent on the face-to-mask seal, whereby leakage ows can cause
unltered breathing air to be inhaled or exhaled that bypasses the lter
medium (Koh et al., 2021; Pan et al., 2021). As a result, the performance
related to potentially infectious particles considering the nature and size
of exhaled aerosol particles and also the ltration performance associ-
ated with facial leakages in both self-protection and third-party pro-
tection may be a drawback of certication procedures for evaluating the
RPD performance in the COVID-19 pandemic context.
Several studies with a focus on RPD performance related to infection
protection have already been conducted. Studies involving submicronic
particle collectives to determine total ltration efciencies of certied
RPDs show that the certied lter media are highly effective even when
considered on a number basis. Rengasamy et al. (2014) reported pene-
tration rates of less than 1% for sealed respirators and less than 10% for
surgical masks at a ow rate of 40 l/min using a NaCl aerosol. Bagheri
et al. (2021) suggested similar penetration rates for FFP2 masks with
dolomite dust, which are all below 6%, but have found a higher variance
in the penetration rates of different surgical masks. This includes several
masks with penetration rates below 12%, as well as up to 75%. Other
work (Bałazy et al., 2006; Grinshpun et al., 2009; Zangmeister et al.,
2020) similarly shows that penetrations are distributed over a wider
area in surgical masks than in respirators. When looking at fractional
efciencies, the most penetrating particle size (MPPS) varies for certied
RPDs and is typically in the order of 30–300 nm, with the upper bound
being more relevant for surgical masks (Bagheri et al., 2021; Bałazy
et al., 2006; Grinshpun et al., 2009; Zangmeister et al., 2020). RPDs that
have not been sealed in the test procedure show that facial leakage
sharply decreases the overall efciency. Grinshpun et al. (2009) found
that the total inward leakage is particle size dependent from 7 to 20
times greater than the penetration through the lter medium for respi-
rators and size independent from 4.8 to 5.8 times greater for surgical
masks. Various studies point to facial leakages lead to similar ltration
efciencies for both respirators and surgical masks, independent of the
initial efciency of the lter medium (Grinshpun et al., 2009; Li et al.,
2006; Rengasamy et al., 2014). When looking at the total outward
leakage, which is relevant for the effectivity in third-party protection,
however, only a few studies were conducted. Koh et al. (2021) and Pan
et al. (2021) indicate that both inward and outward leakages are similar
in respirators. For surgical masks, however, they indicate that the out-
ward leakage exceeds the inward leakage.
Despite the clear evidence that the lter media used in certied RPDs
is efcient for submicronic particles, to date little is known about how
the ltration performance is modulated by facial leaks on both the self-
protection and third-party protection. Questions on how the ltration
performance is inuenced by the real use case in the context of infection
prevention remain unanswered. For example, how is the ltration ef-
ciency affected when using a test aerosol representative for exhaled
aerosols? Does the certication or characteristics of the RPD inuence
the ltration performance in the real use case considering facial leaks?
How is the pressure loss, as a measure for the breathing resistance,
effected by facial leaks or the ow direction? To answer these questions,
the aim of this work is to determine performance parameters, namely
the fractional ltration efciency, number based total efciency and net
pressure loss, for different RPD classes and characteristics under con-
ditions representative for infection prevention in the COVID-19
pandemic context. This includes the
−set-up of a test bench to determine the performance under repre-
sentative test conditions.
−selection of a suitable uid for particle generation related to
respiratory-emitted aerosols and the evaluation of the test aerosol by
comparison to human exhaled particles.
−determination of RPD performance as a result of fractional ltration
efciency, representative number based total efciency and net
pressure loss.
−comparison of performance parameters dependent on ow direction
in terms of self- and third-party protection, as well as dependent on
facial t considering facial leakages.
First, in Sec. 2, the basic transport mechanisms of ltration and the
equations of the performance parameters are presented. Then, in Sec. 3,
the experimental setup, the test procedure and the materials used are
described. In Sec. 4, the results are presented and discussed. This in-
cludes the validation of the test aerosol with human exhaled particles, as
well as the screening of different RPDs. Finally, in Sec. 5, a conclusion of
this work is drawn and an outlook on future work is given.
2. Theory
Aerosol particles may be removed from the gas phase by porous
media when they reach the inner surface of a lter through various
transport mechanisms, namely Brownian diffusion, direct interception,
inertial impaction and electrostatic attraction (see Fig. 1).
Transport mechanisms are strongly dependent on particle size and
ow velocity. Brownian diffusion is the dominating mechanism for small
particle sizes and low ow velocities, whereby the particle motion is
governed by a superordinated chaotic movement. Thus, particles do not
follow the streamlines exactly and may randomly hit lter bres.
Brownian diffusion may be signicant for the separation of the smaller
particle fraction in the size of the infectious SARS-CoV-2. Particles that
follow exactly the streamlines may be removed by direct interception, if
the streamline passes within the particle radius on a lter bre. Inertial
S. Berger et al.

International Journal of Hygiene and Environmental Health 248 (2023) 114103
3
impaction is mainly dominant on larger particles that, due to their
inertia, are deected of their streamline by its redirection around a bre.
With mask leakage, also the total leakage ow can be accelerated and
redirected at the mask-to-face seal, potentially allowing inertial
impaction to effect the deposition of larger particles in the case of un-
sealed RPDs (Hinds and Kraske, 1987). The interaction of the three
transport mechanisms typically results in a most penetrating particle
size (MPPS), which represents the least effectively separated particle
size and is thus a characteristic of the respective lter medium. In air
ltration, the MPPS is empirically around 0.3
μ
m and therefore in the
size range of exhaled aerosol particles. In order to increase the removal
probability of small particles, lter media, such as media based on
nanobres, aim to increase the efciency at the MPPS through small
pore sizes. However, materials based on synthetic melt blown bres are
most commonly used in RPDs. Meltblown bres are electrostatically
charged due to their manufacturing process and therefore able to attract
particles of the opposite charge by electrostatic attraction. This may be
advantageous in increasing the efciency at the MPPS without reducing
permeability and thus, increasing pressure loss.
To evaluate ltration performance dependent on particle size as well
as to determine the MPPS, the fractional ltration efciency is an
elementary parameter. The fractional ltration efciency is dened
according to Eq. 1
ECn(xi) = dCn,upstream(xi) − dCn,downstream(xi)
dCn,upstream(xi)(1)
and represents the measurable difference in particle concentration of
discretised particle size intervals in the raw gas (upstream of the lter)
and clean gas (downstream of the lter) related to the raw gas particle
concentration.
When only considering total particle concentrations, the total
ltration efciency is obtained according to Eq. (2).
ECn=Cn,upstream −Cn,downstream
Cn,upstream
(2)
Compared to the fractional ltration efciency, the total ltration
efciency depends on the particle size distribution and the metric with
which particle concentrations are measured (Zoller et al., 2021).
Therefore, test aerosols with a size distribution similar to that of
potentially infectious aerosol particles are a pre-requisite to evaluate the
protective effect of RPDs. With mass concentrations, larger particles
have a higher relative importance for the overall efciency than with
number concentrations. However, since particularly the small particles
are relevant in the context of disease transmission, we only use ef-
ciencies based on number concentrations.
To evaluate the wearing comfort of RPDs, the second performance
parameter is the pressure loss, which is given in porous media by the
Darcy equation if the ow is creeping, i.e. if the Reynolds number is
smaller than 1:
Δp
H=
η
•vf
B(3)
The pressure loss Δp related to the layer thickness H depends on the
dynamic viscosity
η
, the lter velocity vf and the permeabilityB, which is
a material constant depending on the ber diameter and porosity. The
net differential pressure Δpnet of RPDs is determined similar to the
procedure described in DIN EN 13274-3:2002-03
Δpnet =ΔpF−ΔpH(4)
where ΔpF is the measured differential pressure with the RPD mounted
to a test head. ΔpH takes into account pipe friction losses, changes in
cross-section and diversions due to the measuring apparatus, which is
determined from a second measurement.
3. Material and methods
In this section, the materials and methods used for testing RPDs are
described. First, the mask test bench is presented in detail with focus on
both the experimental set-up and the test procedure. Subsequently, the
materials used and the considered RPDs are described.
3.1. Mask test bench
Fig. 2 rst illustrates the experimental test set-up to determine
ltration-specic performance parameters of RPDs. Essential compo-
nents are an aerosol generator (1), a test head (2), a volume ow-
controlled fan (3) and measuring devices for the fractional particle
number concentration (4a) and the differential pressure (4 b).
The primary function of the aerosol generator (AGK 2000, Palas
GmbH) (1) is to produce test aerosol particles from a feeding liquid by
the use of a two-substance nozzle and compressed air in order to mimic
infectious particles from the respiratory system. Test aerosol particles
are injected at the beginning of the test bench tubing and thereby diluted
with ambient air to obtain a dry aerosol in measurable concentration.
The total volume ow is controlled and generated by a radial fan
mounted on the suction side. For ow control, an ultrasonic owmeter is
used to contactless measure the pressure- and temperature-compensated
volumetric ow without inuencing the ow prole and thus interfering
particle sampling. The different RPDs under consideration are mounted
to an additively manufactured head within a measuring cell. The
measuring cell allows the test head to be mounted in such a way that it
can be either owed through from the outside to the inside (third-party
protection) or vice versa from the inside to the outside (self-protection).
In order to be representative towards facial leaks the dimensions of this
test head are similar to ISO/TS 16976-2:2015-04 and represent an
average Central European head size. Differential pressure is measured
by the use of static ring pressure taps upstream and downstream the
measuring cell with two differential pressure sensors in the range of 250
Pa and 1250 Pa, respectively. Particle concentrations are measured in
both the raw and clean gas using an optical particle counter (Promo
3000, Palas GmbH). Therefore, an intrument-specic sampling volume
ow of 5 l/min is taken isokinetically upstream and downstream the
measuring cell. The particle concentration is determined by scattered
light using an optical particle sensor with a measuring range of 10
6
P/cm
3
(WELAS 2070).
3.2. Test procedure
A total of four different test scenarios are considered. First, RPDs are
attached to the test head by their existing head or ear loops. This intends
to mimic the natural t with leakage ows through the mask-to-face seal
may inuence the RPD performance. Second, to exclude facial leakage,
RPDs are rmly attached to the test head by the use of a sealing com-
pound. This provides the performance of RPDs if they would perfectly t
to a wearer’s face, which partly is a comparable conguration to
standardised certication procedures. Both modes of attachment are
looked at separately for two ow directions, inhalation and exhalation,
Fig. 1. Schematic illustration of the transport mechanisms in depth ltration.
S. Berger et al.

International Journal of Hygiene and Environmental Health 248 (2023) 114103
4
by varying the position of the test head in the measuring cell. As a result,
the mask performance for perfect and imperfect tting RPDs can be
ascertained distinctive for the ow directions of inhalation and exha-
lation in self-protection and third-party protection. Here, third-party
protection only represents the efciency of particle removal in the
expiratory volume ow, while self-protection represents particle
reduction in the inspiratory volume ow.
To prepare a measurement, rst the test head is tted with an RPD
and then installed in the measuring cell according to the considered test
scenario. A constant volume ow of 95 l/min is then applied to represent
most unfavourable conditions. Thus, 95 l/min is an unrealistically high
ow rate for breathing, it is also used in DIN EN 149:2009-08 DIN EN
149 as an inspiratory ow rate and intends to mimic the peak condition
during sinusoidal breathing at 30 l/min according to DIN EN 13274–3.
As a result, the determined mask performance is representative only for
the peak condition of the breathing cycle. After preparation, tests are
carried out under room air conditions (p =10
5
Pa, T =25 ◦C, φ =
30–45%). Absolute pressure and temperature are measured online and
used for volume ow compensation with regard to small uctuations in
ambient conditions. After equilibration, the pressure loss of the unloa-
ded RPD is measured over a time interval of 30 s. The net pressure loss is
then determined according to Eq. (4), subtracting the reference pressure
loss of the test head and measuring cell in this conguration. After the
pressure loss measurement is nished, test aerosol is injected into the
tesdslbt tubing. In the rst 300 s, the raw gas concentration is deter-
mined. Here, the loading time of 300 s is necessary to equilibrate the
particle concentration in the measuring cell. The total number concen-
tration in the raw gas is approx. 50,000 P/l, thus background particle
concentration of 20 P/l is three orders of magnitude lower and is
therefore neglected. After a steady-state particle concentration has been
established, the clean gas concentration is determined during the next
300 s. To determine the fractional efciency according to Eq. (1), the last
60 s of the raw gas measurement and the rst 60 s of the clean gas
measurement are used.
3.3. Articial saliva
Aiming on a representative test aerosol to respiratory-emitted par-
ticles, a saliva substitute solution (apomix® Speichelersatzl¨
osung SR) is
used as a feeding liquid for aerosol generation. Saliva substitutes are
often used to moisten the oral mucosa in patients with xerostomia and
therefore intended to imitate certain properties of human saliva, such as
viscosity (Łysik et al., 2019). In saliva substitutes, the viscosity is mainly
inuenced by either the additive carboxymethylcellulose (CMC) or
mucin (Foglio-Bonda et al., 2022). Other components include electro-
lytes such as sodium chloride, potassium chloride, calcium chloride and
magnesium chloride. Moreover, water, sorbitol and substances that
serve as pH buffers and for preservation are contained.
3.4. RPDs
Certied surgical masks and ltering face pieces were selected for a
screening in four different test scenarios, as described. The selected
RPDs are listed in Fig. 3 and are categorized into ve groups based on in
their shape and characteristics. Four sh-shaped masks were considered,
with two based on meltblown lter media (FFP2_3; FFP3_1) and two
based on nanobres (FFP2_1*; FFP2_2*). Further, duckbill-shaped
(FFP2_4; FFP2_5) and classical axe-shaped ltering face pieces
(FFP2_6; FFP2_7) all based on meltblown lter media were selected. In
addition, two medical masks (SM_1; SM_2) as well as a reusable fabric
mask with a nanolter insert (FFP2_8* (R)) were screened.
4. Results and discussion
Prior to the actual measurements, the particle size distribution of the
test aerosol was investigated and compared to exhaled aerosols (Sec.
4.1) in order to evaluate its representativeness for respiratory-emitted
aerosols. Thereafter, the actual RPD screening was done on ve new
mask samples in each conguration with the test bench and test pro-
cedure described in Section 3. Performance parameters, namely the net
pressure loss, the number based total efciency and the fractional
ltration efciency were determined, aiming at a differentiated
distinction between ow direction (third-party/self-protection) and
tting (including/excluding facial leakage) (Sec. 4.2).
4.1. Test aerosol
Test aerosols for determining the ltration performance of RPDs can
be generated from various liquids such as those used in certication, for
example sodium chloride solutions and liquid parafn oil (DIN EN
13274-7:2019-09; DIN EN 149:2009-08), or biogenic solutions con-
taining viable bacteria (DIN EN 14683:2019-10). Unlike the norms, the
focus of the RPD screening is to determine the mask performance based
on a representative test aerosol that mimics respiratory emitted parti-
cles. Representative in this context means that the characteristic of
droplets and the size distribution are similar between exhaled and
technically generated particles. Therefore, we use a saliva substitute
solution as a feeding liquid for technical aerosol generation (see Sec.
3.3). To evaluate representativeness, the particle size distribution of the
technically generated aerosol from articial saliva is compared to an
exhaled aerosol optically measured in Penner et al. (2022) and
compared in Fig. 4. Since the total number of exhaled particles is several
orders of magnitude lower than of technically generated aerosols, a
different optical sensor with a lower measuring range was used for the
Fig. 2. Experimental test set-up for the determination of mask performance parameters (fractional ltration efciency and net pressure loss).
S. Berger et al.

International Journal of Hygiene and Environmental Health 248 (2023) 114103
5
exhalation measurement. To allow for comparison, the particle number
concentration of each particle size interval (dC
n
) is normalized to the
total particle number concentration (C
n
) as well as the logarithmic bin
size (Δlog (x
i
)) of the optical particle counter.
The size distribution of exhalation measurements is presented as the
mean of 21 measurements of 13 test persons and compared to a single
measurement of the saliva substitute solution that is technically
dispersed with the aerosol generator. The results show that both aerosols
contain particles in a similar size range that are mainly smaller than 2
μ
m. However, in the technically generated aerosol, the mode of the size
distribution is close to the metrological boundary of the optical particle
counter in the range of 0.2
μ
m. Here, counting errors may occur, which
suggests that the actual concentration at the mode may be even higher.
The mode for exhaled particles, on the other hand, is in the range of 0.4
μ
m and thus the exhaled size distribution contains relatively larger
particles. Thus, the technical generation principle is based on a two-
substance nozzle with larger particles partly being removed by a
cyclone, the used aerosol generator does not mimic the generation
mechanism of particles in the human lungs, which may explain the slight
differences in both distributions. Another reason may be the test con-
ditions for both set-ups, with the exhaled particle size distribution
determined undiluted at an air humidity of approx. 90% due to the low
particle concentration. Although the time required for exhaled particles
to evaporate is very short due to their small size and the associated high
surface tension (Gregson et al., 2022; Walker et al., 2021), incomplete
evaporation cannot be ruled out in this set-up due to the high humidity.
Technical aerosol, on the other hand, is diluted to a total ow of 95
l/min, which may result in a faster evaporation of the water content and
thus to a smaller particle size. On the whole, the overall differences are
minor; moreover, saliva substitute solution represents a more compa-
rable uid in terms of its composition and properties in the context of
infection protection and is therefore used for the RPD screening.
4.2. Screening of RPDs in new condition
RPDs act as particle sinks for the inhaled and exhaled air and thus
potentially provide an effective means of protecting oneself and others
from direct and indirect infections. The ltration performance, however,
may differ in self-protection and third-party protection for both perfect
and natural tted RPDs that partly allow for unltered breathing air to
pass at the mask-to-face seal. The RPD screening aims to provide the
performance related to this dependency on ow direction and facial
leakage by the use of a representative test aerosol (Sec. 4.1) and a newly
conceived test bench (Sec. 3.1). Therefore, 11 surgical masks and
Filtering Face Pieces (Sec. 3.3) are tested at a steady-state volume ow
of 95 l/min, which aims to represent the peak volume ow occurring
during sinusoidal breathing at 30 l/min. For each type of RPD, the
fractional ltration efciency, number based total efciency and the net
pressure loss (Sec. 2) are determined using ve new RPD samples. Fig. 5
illustrates the averaged fractional ltration efciencies.
The diagrams aligned vertically differ in whether RPDs were sealed
or naturally tted to the test head. When the RPDs were sealed, thus
were “perfectly tted”, 7 out of 11 masks exceed an efciency of 95% at
each particle size, which indicates a good ltration performance related
to aerosol particles from saliva substitute. Surgical mask SM_1 is similar
efcient compared to meltblown based Filtering Face Pieces, while the
second surgical mask (SM_2) shows a lower efciency that is still above
85% at the MPPS. RPDs containing nanobres (FFP2_1*, FFP2_2*,
FFP2_8*(R)) appear to have lower efciencies of approx. 75% (dispos-
able) and 85% (reusable) at the MPPS. For sealed nanobre-based RPDs,
however, the fractional ltration efciency curves deviate signicantly
in self-protection and third-party protection, thus the differences cannot
be explained solely by a lower efciency of nanobre-based lter media
but may also be the result of a more complicated sealing of these ma-
terials to the test head with the sealing compound used.
When RPDs were naturally tted and facial leakage is expected to
occur, the fractional efciency curve of each RPD type is signicantly
lower than in its sealed installation variant. This conrms the expecta-
tion in general. Moreover, it can be observed that the efciency curves
deviate over a wider range of approx. 20%–90%, which suggests a sig-
nicant inuence of facial leakage on ltration performance dependent
on the RPD t. To take a closer look, diagrams aligned horizontally differ
only in inhalation and exhalation mode, which is intended to represent
self-protection and third-party protection. Naturally tted RPDs in self-
protection, generally, tend to have a higher efciency than their
equivalent in third-party protection. This difference is most pronounced
in the case of surgical masks, nanobre-based RPDs and two of the
meltblown-based FFP masks, with these masks depositing partly twice as
the amount in self-protection as in third-party protection. The RPD
models FFP2_3, FFP2_7 and FFP3_1, however, deviate in their ltration
performance in both modes only slightly. As a result, this indicates that
the efciency of an RPD may strongly differ between inhalation and
exhalation dependent on its properties to minimize facial leakage, which
is discussed in the context of Fig. 7 in more detail.
Fig. 3. Selected meltblown based and nanobre* based RPDs for performance screening.
Fig. 4. Comparison of the particle size distributions of the technically gener-
ated aerosol from saliva substitute solution and human exhaled aerosols of 13
subjects normalized to the total particle concentration and logarithmic bin size
(Penner et al., 2022).
S. Berger et al.

International Journal of Hygiene and Environmental Health 248 (2023) 114103
6
With the sealed installation, there are fewer differences between the
two ow directions compared to naturally attached RPDs. Deviating
ltration efciency curves can be seen in the surgical masks and
nanobre-based RPDs, which, in addition to the directionality of the
facial leakage, also indicates a directionality of the lter material on
ltration performance. Since these mask materials are generally thinner
and less rigid, they may be more easily drawn to the test head in the
inhalation mode, thus reducing the effective lter area and increasing
the specic load. As described in Section 2, the transport mechanisms of
particles to the inner surface of the lter material are dependent on the
ow velocity, which would well explain the observed differences here.
Assuming further that the distance between an RPD and a wearers face is
very small, so that the time required for complete evaporation of the
water content of the particles during exhalation is insufcient, larger
particle sizes could be relevant for third-party protection. In this case,
RPDs with increasing efciency over particle size, such as the SM_1 and
SM_2 surgical masks and the FFP2_1* and FFP2_2* nanobre-based
masks, would be more efcient in a real application.
Pressure loss, as the second key performance parameter, is an indi-
cator of breathing resistance and thus crucial for the wearing comfort.
RPDs with a low pressure loss impair breathing less and are thus
desirable especially for vulnerable individuals with pre-existing condi-
tions of the respiratory system or low tidal volumes. As with the
ltration efciency, also the pressure loss may depend on facial leakage
and the direction of ow for different mask characteristics. In order to
view both performance parameters side-by-side, the number-based total
ltration efciencies determined from the fractional efciencies are
illustrated above the net pressure loss in Fig. 6. Each point is the mean of
ve measurements on ve new RPD samples, with the error bars rep-
resenting the standard deviation. The total ltration efciency in sealed
installation shows for most RPDs again that the requirement of DIN EN
149 for a lower penetration than 6% is fullled, if the total efciency is
determined on a number basis and with a representative test aerosol of
saliva substitute solution, both in third-party and self-protection.
Naturally tted masks, on the other hand, vary in the range of 30%
and 85%, thus the efciency is signicantly decreased due to facial
leakage.
A comparison of the different RPDs in sealed installation shows that
the pressure loss varies over a wide range, with the surgical masks at the
lower bound of approximately 30 Pa–90 Pa. FFP masks, for example
FFP2_5 and FFP2_7, tend to highly differ in pressure loss although the
ltration efciency is similar. In general, these observed differences can
simply be explained by different effective lter areas, material thick-
nesses and permeabilities. When comparing naturally tted RPDs, on
the contrary, the pressure loss is strongly reduced, resulting from the
effect of facial leakage. RPDs with a sharp decrease in pressure loss,
Fig. 5. Fractional separation efciencies of selected RPDs at 95 l/min using articial saliva in sealed installation excluding leakage, as well as in natural installation
including leakage, differentiated in self-protection and third-party protection. Each fractional ltration efciency curve is the mean of ve measurements on ve new
RPD samples.
S. Berger et al.

International Journal of Hygiene and Environmental Health 248 (2023) 114103
7
compared to its sealed t, also show a sharp decrease in total ltration
efciency, suggesting that both performance parameters are affected in
a mutually dependent manner. Nevertheless, when comparing different
RPD types in the natural t, such as FFP2_3 and FFP2_7, for example,
then similar efciencies but different pressure losses can be observed.
Despite the strong inuence of facial leakage, this demonstrates the still
existing dependency on lter area and lter material. By looking at the
dependency of pressure loss on ow direction in Fig. 6, with open
symbols representing self-protection and lled symbols representing
third-party protection, RPDs in the inhalation mode exhibit the highest
pressure losses in both sealed and non-sealed installation. This suggests
a greater resistance when inhaling than when exhaling. Comparing the
surgical mask SM_2 and the Filtering Face Piece FFP3_1 in both
breathing modes, it is also evident that this difference in pressure loss
but also ltration efciency between inhalation and exhalation is
signicantly greater with the surgical mask.
In order to take a closer look at how breathing mode-based differ-
ences result for different mask types, Fig. 7 aims at a relative compari-
son. Here, the performance parameters of self-protection are related to
those of third-party protection, subdivided into the mask groups
described in Section 3.
As already seen on the basis of the fractional ltration efciencies in
sealed installation, the ltration efciency is similar in both breathing
modes when avoiding facial leakage. However, axe-shaped FFP and
surgical masks have a higher pressure loss during inhalation, which may
be due to a deformation of the mask caused by the direction of ow. In
axe-shaped masks, both halves of the mask may contact each other due
to the negative pressure during inhalation, while medical masks may
touch the test head due to their exible material. Both would lead to a
reduction in lter area, increasing the ow velocity at constant volume
ow, which in turn leads to a higher pressure loss based on Equation (3).
As discussed above, relatively higher ltration efciencies but also
pressure losses are determined in self-protection in the natural t
including facial leaks. Subdivided into the different RPD shapes, this
difference is found to be most pronounced for surgical masks. Fish-
shaped FFP masks show the smallest differences, while duckbill-
shaped masks and axe-shaped masks are in between. A closer look at
the measured pressure loss of sh-shaped RPDs shows that the pressure
loss increase in self-protection related to third-party protection is also
differently pronounced within this subgroup. FFP2_1 and FFP2_2 with
nanobre materials show a higher relative pressure loss increase on
inhalation than FFP2_3 and FFP3_1 based on meltblown lter media,
highlighting the still existing inuence of the mask material. Mask shape
and lter material most likely affect the extent to which RPDs are being
drawn to the test head on inhalation due to the negative pressure. The
reduction of leakage areas may be advantageous for self-protection, but
the minimization of leakage areas also increases the pressure loss.
5. Conclusions
This work focused on a screening of certied surgical and FFP masks
used in the COVID-19 pandemic context with respect to respiratory-
emitted particles. To this end, we presented a novel experimental set-
up that allows the determination of mask performance parameters,
namely the fractional ltration efciency and the net pressure loss, as a
function of the ow direction (self and third-party protection) and of the
facial t (sealed and natural t) using a test aerosol based on articial
saliva. The particle size distributions of exhaled breath and the test
aerosol were compared in exhalation mode. Measurements show that
they are in a similar size range up to 0.4
μ
m with most particles smaller
than 2
μ
m. The results of the mask screening in sealed tting show that
both the FFP and surgical masks examined feature a high ltration ef-
ciency with regard to articial saliva and, with a few exceptions, would
meet total number-based efciencies of 94% related to the requirements
in DIN EN 149 using articial saliva. The ltration efciencies of the
Fig. 6. Comparison of the number-based total separation efciency with the net pressure loss @ 95 l/min. Open symbols represent measuring points in self-
protection, lled symbols represent the third-party protection. Each point is the mean of ve measurements on ve new mask samples, with the error bars repre-
senting the standard deviation of the vefold determination.
Fig. 7. Illustration of the relative change of the performance parameters in self-
protection in relation to third-party protection. A relative value of 100% would
mean that twice the value was measured in self-protection as in third-
party protection.
S. Berger et al.

International Journal of Hygiene and Environmental Health 248 (2023) 114103
8
sealed t are similar in both ow directions, but higher pressure losses
are found in self-protection. One reason for this might be a reduction in
lter area during inhalation, which presumably results from the drawing
of masks with less stiff and thinner materials against the test head or,
especially in the case of axe-shaped masks, might be the result of two
mask surfaces being merged. In natural tting, facial leakage signi-
cantly decreases both the ltration efciency and pressure loss for each
mask model tested. Here, the total efciencies between different masks
are in the order of 30%–85%, whereas the pressure loss appears to
decrease in an inter-dependent manner with ltration efciency. As a
result, we conclude that the mask performance is more inuenced by the
mask t and sealing material qualities than the ltration-specic prop-
erties of the lter media. As far as the ow direction is concerned, the
ltration efciency and pressure loss tend to be lower in third-party
protection than in self-protection. This can similarly be caused by a
drawing to the test head during inhalation, which might reduce the size
of leakage areas between test face and RPD. Here, the relative change of
the performance parameters may be inuenced by thin and less stiff
materials that favour such drawing to the test head, but may also be
inuenced by different RPD shapes (sh-, duckbill-, axe-shape).
One can conclude from our study that, considering naturally tted
masks, in addition to ltration-specic material properties, the ow
direction, the dimensional stability, the mask shape as well as the sealing
material properties inuence the RPD performance signicantly. How-
ever, these properties may be inuenced also through humid and
particle-laden breath, which is why future work needs to focus on the
inuence of wearing time on RPD performance. In addition, the inu-
ence of RPD shape indicates potential for optimisation, especially for the
development of well-separating masks with reduced pressure losses that
are suitable for infection prevention even for high-risk patients with
restricted tidal volume.
Data availability statement
Data are available from the corresponding author upon reasonable
request.
Declaration of competing interest
The authors declare no conict of interest.
Acknowledgements
This work was funded by the German Federal Ministry of Education
and Research (BMBF), grant no. 01KI20241B. The authors are thankful
to National Instruments for providing a data acquisition device and to
Oliver Wachno for its set-up and commissioning.
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