1) In "Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1", authors show that ... (pick all correct answers) Select one or more: a) On plastic, no viable SARS-CoV-2 was measured a
1 Scientific RepoRtS | (2020) 10:15665| https://doi.org/10.1038/s41598-020-72798-7
\b f masks and face
coverings in controlling outward
aerosol particle emission
from expiratory activities
Sima
Asadi 1, Christopher D. Cappa 2, Santiago Barreda 3, Anthony S. Wexler 2,4,5,6 ,
Nicole M. Bouvier 7,8 & William D. Ristenpart 1*
The COV \f w
transmission. \f
homemade masks as acceptable alternatives to surgical masks and N95 respirators. Although mask
wearing is intended, in part, to protect others from exhaled, virus-containing particles, few studies
have examined particle emission by mask-wearers into the surrounding air. Here, we measured
outward emissions of micron-scale aerosol particles by healthy humans performing various expiratory
activities while wearing di erent types of medical-grade or homemade masks. Both surgical masks
and unvented KN95 respirators, even without t-testing, reduce the outward particle emission
rates by 90% and 74% on average during speaking and coughing, respectively, compared to wearing
no mask, corroborating their e ectiveness at reducing outward emission. These masks similarly
decreased the outward particle emission of a coughing superemitter, who for unclear reasons emitted
up to two orders of magnitude more expiratory particles via coughing than average. \f
shedding of non-expiratory micron-scale particulates from friable cellulosic bers in homemade
cotton-fabric masks confounded explicit determination of their e cacy at reducing expiratory particle
emission. Audio analysis of the speech and coughing intensity con rmed that people speak more
loudly, but do not cough more loudly, when wearing a mask. Further work is needed to establish the
e cacy of cloth masks at blocking expiratory particles for speech and coughing at varied intensity
and to assess whether virus-contaminated fabrics can generate aerosolized fomites, but the results
strongly corroborate the e cacy of medical-grade masks and highlight the importance of regular
washing of homemade masks.
Airborne transmission of infectious respiratory diseases involves the emission of microorganism-containing
aerosols and droplets during various expiratory activities (e.g., breathing, talking, coughing, and sneezing).
Transmission of viruses in emitted droplets and aerosols to susceptible individuals may occur via physical contact
aer deposition on surfaces, reaerosolization aer deposition, direct deposition of emitted droplets on mucosal
surfaces (e.g., mouth, eyes), or direct inhalation of virus-laden aerosols
1,2. Uncertainty remains regarding the
role and spatial scale of these dierent transmission modes (contact, droplet spray, or aerosol inhalation) for
specic respiratory diseases, including for COVID-19
3 – 7, in particular settings, but airborne transmission stems
from the initial expiratory emission of aerosols or droplets. Consequently, the wearing of masks—in addition to
open
1Department of Chemical \b University of California Davis, 1 Shields Ave, Davis, CA 95616,
USA. 2Department of Civil and \b \b University of California Davis, 1 Shields Ave, Davis,
CA 95616, USA. 3Department of Linguistics, University of California Davis, 1 Shields Ave, Davis, CA 95616,
USA. 4Department of Mechanical and Aerospace \b w
CA 95616, USA. 5Air Quality Research Center, University of California Davis, 1 Shields Ave, Davis, CA 95616,
USA. 6Department of Land, Air and Water Resources, University of California Davis, 1 Shields Ave, Davis, CA 95616,
USA. 7Department of Medicine, Division of \f \fcahn School of Medicine at Mount Sinai, 1 Gustave
L. Levy Place, New York, NY 10029, USA. 8Department of Microbiology, \f w
Gustave L. Levy Place, New York, NY 10029, USA. *email: [email protected]
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vigilant hand hygiene—has been put forth as a means to mitigate disease transmission, especially in healthcare
settings 8
– 11 . Much research has indicated that masks can provide signicant protection to the wearer, although
proper mask tting is critical to realizing such benets 12–15 . Alternatively, masks can potentially reduce outward
transmission by infected individuals, providing protection to others 7,16,17 . ere have been indications of asymp-
tomatic carriers of COVID-19 infecting others 18–20 , leading to increasing, albeit inconsistent 21–24 , calls for more
universal wearing of masks or face coverings by the general public to help control disease transmission during
pandemics. It is therefore important to understand the ecacy of masks and face coverings of dierent types in
reducing outward transmission of aerosols and droplets from expiratory activities. Results from epidemiological and clinical studies assessing the eectiveness of masks in reducing disease
transmission suggest that mask wearing can provide some benets
10,11 , especially with early interventions, but
oen the results lack statistical signicance 25–31 . Laboratory studies provide another means to assess or infer
mask eectiveness. Measurement of material ltration eciencies can provide initial guidance on potential mask
eectiveness for preventing outward transmission
15,32– 35 , but do not directly address mask performance when
worn. Early photographic evidence indicates masks can limit the spread of cough-generated particles 36. Meas-
urements using simulated breathing with an articial test head showed the concentration of particles between
0.02 m-1 m decreases across masks of dierent types
37. Also using simulated breathing, Green et al. 38 found
surgical masks eectively reduced outward transmission of endospores and vegetative cells, with seemingly
greater reduction of particles > 0.7 m compared to smaller particles. Using volunteers, Davies et al.
32 found
that surgical and home-made cotton masks substantially reduce emission of culturable microorganisms from
coughing by healthy volunteers, with similar reduction observed over a range of particle sizes (from 0.65 m
to > 7 m). Milton et al.
16 found that surgical masks substantially reduced viral copy numbers in exhaled “ne”
aerosol (μ 5 m) and “coarse” droplets (> 5 m) from volunteers having in≤uenza, with greater reduction in the
coarse fraction. is result diers somewhat from very recent measurements by Leung et al.
13, who showed a
statistically signicant reduction in shedding of in≤uenza from breathing in coarse but not ne particles with
participants wearing surgical masks. ey did, however, nd that masks reduced shedding of seasonal corona -
virus from breathing for both coarse and ne particles, although viral RNA was observed in less than half of the
samples even with no mask, complicating the assessment. e above studies all indicate a strong potential for masks to help reduce transmission of respiratory illnesses.
To date, however, none have investigated the eectiveness of masks across a range of expiratory activities, and
limited consideration has been given to dierent mask types. Furthermore, no studies to date have considered
the masks themselves as potential sources of aerosol particles. It is well established that brous cellulosic mate -
rials, like cotton and paper, can release large quantities of micron-scale particles (i.e., dust) into the air
39–42 .
Traditionally, these particles have not been considered a potential concern for respiratory viral diseases like
in≤uenza or now COVID-19, since these diseases have been thought to be transmitted via expiratory particles
emitted directly from the respiratory tract of infected individuals
43. Early work in the 1940s indicated, however,
that infectious in≤uenza virus could be collected from the air aer vigorously shaking a contaminated blanket 44.
Despite this nding, over the next 70 years little attention focused on the possibility of respiratory virus trans -
mission via environmental dust; one exception was a study by Khare and Marr, who investigated a theoretical
model for resuspension of contaminated dust from a ≤oor by walking
45. Most recently, work by Asadi et al.
with in≤uenza virus experimentally established that “aerosolized fomites,” non-respiratory particles aerosolized
from virus-contaminated surfaces such as animal fur or paper tissues, can also carry in≤uenza virus and infect
susceptible animals
46. is observation raises the possibility that masks or other personal protective equip-
ment (PPE), which have a higher likelihood of becoming contaminated with virus, might serve as sources of
aerosolized fomites. Indeed, recent work by Liu et al. demonstrated that some of the highest counts of airborne
SARS-CoV-2 (the virus responsible for COVID-19) occurred in hospital rooms where health care workers doed
their PPE, suggesting that virus was potentially being aerosolized from virus-contaminated clothing or PPE, or
resuspended from virus-contaminated dust on the ≤oor
47. It remains unknown what role aerosolized fomites
play in transmission of infectious respiratory disease between humans, and it is unclear whether certain types
of masks are simultaneously eective at blocking emission of respiratory particles while minimizing emission
of non-expiratory (cellulosic) particles. Here, we report on experiments assessing the ecacy of unvented KN95 respirators, vented N95 respirators,
surgical masks, and homemade paper and cloth masks at reducing aerosol particle emission rates from breath-
ing, speaking, and coughing by healthy individuals. Two key ndings are that (i) the surgical masks, unvented
KN95 respirators, and, likely, vented N95 respirators all substantially reduce the number of emitted particles,
but that (ii) particle emission from homemade cloth masks—likely from shed ber fragments—can substantially
exceed emission when no mask is worn, a result that confounds assessment of their ecacy at blocking expiratory
particle emission. Although no direct measurements of virus emission or infectivity were performed here, the
results raise the possibility that shed ber particulates from contaminated cotton masks might serve as sources
of aerosolized fomites.
MethodsHuman subjects. We recruited 10 volunteers (6 male and 4 female), ranging in age from 18 to 45 years
old. e University of California Davis Institutional Review Board approved this study (IRB# 844,369–4), and
all research was performed in accordance with relevant guidelines and regulations of the Institutional Review
Board. Written informed consent was obtained from all participants prior to the tests, and all participants were
asked to provide their age, weight, height, general health status, and smoking history. Only participants who
self-reported as healthy non-smokers were included in the study.
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\b setup. e general experimental setup used was similar to that in previous
work 48,49 . In
brief, an aerodynamic particle sizer (APS, TSI model 3321) was used to count the number of particles between
0.3 to 20 m in aerodynamic diameter; the APS counting eciency falls o below ~ 0.5 µm, and thus the particles
counted between 0.3 and 0.5 µm likely underestimate the true number. e APS was placed inside a HEPA-
ltered laminar ≤ow hood that minimizes background particle concentration (Fig. 1a). Study participants were
asked to sit so that their mouth was positioned in front of a funnel attached to the APS inlet via a conductive sili-
cone tube. ey then performed dierent expiratory activities while wearing no mask or one of the masks shown
in Fig. 1b and described in more detail below. A microphone was placed immediately on the side of the funnel
to record the duration and intensity of talking and coughing activities (Fig. 1c). e participants were positioned
with their mouth approximately 1 cm away from the funnel entrance; the nose rest used in our previous setup
48,49
was removed to prevent additional particle generation via rubbing of the mask fabric on the nose rest surface.
e air was pulled in by the APS at 5 L/min, with 1 L/min (20%) focused into the detector to count and size the
cumulative number of particles at 1-s intervals (Fig. 1d). Note that the funnel is a semi-conned environment,
and not all expired particles were necessarily captured by the APS. e wearing of masks may redirect some
of the expired air≤ow in non-outward directions (e.g., out the top or sides of the mask
50). Accordingly, we use
the terminology “outward emission” when referring the to the particle emissions measured here. erefore, the
measurements reported here do not represent the absolute number of emitted particles and may underestimate
contributions from particles that escape out the sides of the masks, but do allow relative comparisons between
dierent conditions. e particle emission rates reported here from the APS are likely smaller than the total
expiratory particle emission rates by, approximately, the ratio of the exhaled volumetric ≤owrate that enters the
funnel to the APS sample rate. All experiments were performed with ambient temperature between 22 to 24 °C. e relative humidity ranged
from 30 to 35% for most experiments; a second round of testing, comparing washed vs. unwashed homemade
masks, was performed at 53% relative humidity. Given the approximately 3-s delay between entering the funnel
and reaching the detector within the APS, under all these conditions the aqueous components of micron-scale
respiratory droplets had more than sucient time (i.e., more than ~ 100 ms) to evaporate fully to their dried
Figure 1. (a ) Schematic of the experimental setup showing a participant wearing a mask in front of the funnel
connected to the APS. (b ) Photographs of the masks used for the experiments. (c ) Microphone recording for a
participant (F3) coughing into the funnel while wearing no mask. (d ) e instantaneous particle emission rate
of all detected particles between 0.3 and 20 µm in diameter. Surg.: surgical; KN95: unvented KN95 respirator;
SL-P: single-layer paper towel; SL-T: single-layer cotton t-shirt; DL-T: double-layer cotton t-shirt; N95: vented
N95 respirator. e subject gave her written informed consent for publication of the images in (b ).
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residual (so-called “droplet nuclei” 51); see gure S3 of Asadi et al. 48 for direct experimental evidence of complete
drying under these conditions. Although large droplets (> 20 µm) can require substantially more than 1 s to
evaporate
52, as shown here the vast majority of particles are less than 5 µm and thus unlikely to have originated
at sizes larger than 20 µm. e size distributions presented here are based on the diameter as observed at the
APS detector.
\b Participants were asked to complete four distinct activities for each mask or respira-
tor type:
(i) Breathing : gentle breathing in through the nose and out through the mouth, for 2 min at a pace comfortable
for the participant. e particle emission rate was calculated as the total number of particles emitted over the
entire 2-min period, divided by two minutes to obtain the average particles per second.
(ii) Talking : reading aloud the Rainbow Passage (Fairbanks
53 and Supplementary Text S1), a standard 330-
word long linguistic text with a wide range of phonemes. Participants read this passage aloud at an interme -
diate, comfortable voice loudness. Since participants naturally read at a slightly dierent volume and pace,
the microphone recording was used to calculate the root mean square (RMS) amplitude (as a measure of
loudness) and duration of vocalization (excluding the pauses between the words). e particle emission rate
was calculated as the total number of particles emitted over the entire reading (approximately 100 to 150 s),
divided by the cumulative duration of vocalization excluding pauses. Excluding the pauses accounts for
person-to-person dierences in the fraction of time spent actively vocalizing while speaking (approximately
82% ± 5%) so that individuals who simply pause longer between words are not characterized with an articially
low emission rate due to vocalization.
(iii) Coughing : Successive, forced coughing for 30 s at a comfortable rate and intensity for the participant.
Similar to the talking experiment, the microphone data was used to determine the RMS amplitude of each
cough, the number of coughs, and cumulative duration of coughing (excluding the pauses between the
coughs). e particle emission rate was calculated as the total number of particles emitted during the 30 s
of measurement, divided either by the number of coughs (to obtain particles/cough) or by the cumulative
duration of the coughs (to obtain particles/s).
(iv) Jaw movement: moving the jaw as if chewing gum, without opening the mouth, for 1 min, while nose
breathing, to test whether facial motion in the absence of more extreme expiration caused signicant particle
emission. is activity technically counts as an expiratory activity since the participant was nose breath-
ing, but the main intent was to assess whether facial motion appreciably alters particle emission, due either
to gentle friction between the skin and the facemask yielding enhanced particle emission, or variable gap
distances between the mask and skin allowing more or less particles to escape. e particle emission rate
was calculated as the total number of particles emitted over the 1-min period, divided by 60 s to obtain the
average particles per second.
Mask types. Participants completed each of the four expiratory activities when they wore no mask or one of
the 6 dierent mask or respirator types:
(i) A surgical mask (ValuMax 5130E-SB) denoted as “Surg.”, tested by 10 participants.
(ii) An unvented KN95 respirator (GB2626-2006, manufacturer Nine Five Protection Technology, Dongguan,
China), tested by 10 participants.
(iii) A homemade single-layer paper towel mask (Kirkland, 2-PLY sheet, 27.9 cm × 17.7 cm) denoted as “SL-
P” and tested by 10 participants.
(iv) A homemade single-layer t-shirt mask, “SL-T”, made from a new cotton t-shirt (Calvin Klein Men’s Liquid
Cotton Polo, 100% cotton, item #1341469), tested by 10 participants.
(v) A homemade double-layer t-shirt mask, “DL-T”, made from the same t-shirt material as the SL-T mask,
and tested by 10 participants.
(vi)A vented N95 respirator (NIOSH N95, Safety Plus, TC-84A-7448)) tested by 2 participants; shortages at
the time of testing precluded a larger sample size. e primary dierence between an N95 and KN95 respira-
tor is where the mask is certied, in the US. (N95) or China (KN95).
e homemade cloth masks (SL-T, and DL-T) were made according to the CDC do-it-yourself instructions
for single- and double-layer t-shirt masks
54. e homemade paper towel masks were made according to do-it-
yourself instructions 55. Photographs of all mask types are shown in Fig. 1b.
Prior to wearing each mask, participants were verbally given general guidance on how to put on each mask.
In the case of surgical masks and KN95 respirators they were instructed to pinch the metal bar to conform
the mask to the nose. No t-testing of respirators, as mandated by OSHA standard (29 CFR Part 1910)
56, was
performed, with the intent of obtaining representative particle emission rates for untrained individuals without
access to professional tting assistance.
Mask washing. To test whether washing of the homemade cloth masks had any eect on the particle emis-
sion rate, a subset of 4 participants were asked to bring their double-layer t-shirt mask home and to hand-wash
it with water and soap, rinse it thoroughly, and let it air-dry. ese participants then returned and repeated the
four activities with a brand-new DL-T mask and their washed DL-T mask to provide a direct comparison of
washed versus unwashed fabric.
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Particle emission via hand-rubbing. Besides the above experiments to measure the particle emission
associated with dierent mask fabrics, we also performed a qualitative test of the friability of the masks by rub-
bing each mask by hand in front of the APS, using a procedure similar to that performed previously with paper
tissues (cf. Figure 4 of Asadi et al.
46). Specically, the mask was folded over on itself between thumb and index
nger, and the mask material was rubbed against itself. A sample of each mask type was rubbed by hand by the
same individual for 10 s in front of the APS, using to the best of their ability the same amount of force each time.
e test was repeated 3 times for each mask type. e particle emission rate was calculated as the total number of
particles emitted divided by the duration of rubbing (10 s). Note that this procedure does not preclude possible
particle shedding from the skin of the experimentalist
57; the observed particle emission rates for dierent mask
materials therefore represent only qualitative indications of the relative friability.
Statistical analysis. Box-and-whisker plots show the median (red line), interquartile range (blue box),
and range (black whiskers). Stata/IC 15 was used to perform Shapiro–Wilk normality test on the particle emis-
sion rates for each activity. Aer log-transformation of the data, mixed-eects linear regression was performed
to account for person-level correlations. Considering that we had only one primary random eect (person-
to-person variability), all variances were set equal with zero covariances. Post hoc pairwise comparisons were
performed and adjusted for multiple comparisons using Schee’s method. Schee groups are indicated with
green letters below each box plot; groups with no common letter are considered signicantly dierent (p < 0.05).
ResultsParticle emission rates for the four expiratory activities are shown in Fig. 2. Focusing rst on breathing (Fig. 2a),
when participants wore no mask, the median particle emission rate was 0.31 particles/s, with one participant
(M6) as high as 0.57 particles/s, and another participant (F3) as low as 0.05 particles/s. is median rate and
person-to-person variability are both broadly consistent with previous studies
48,51 . In contrast, wearing a sur -
gical mask or a KN95 respirator signicantly reduced the outward number of particles emitted per second of
breathing. e median outward emission rates for these masks were 0.06 and 0.07 particles/s, respectively,
representing an approximately sixfold decrease compared to no mask. Wearing a homemade single layer paper
towel (SL-P) mask yielded a similar decrease in outward emission rate, although not as statistically signicant
as the medical-grade masks. Surprisingly, wearing an unwashed single layer t-shirt (U-SL-T) mask while breathing yielded a signicant
increase in measured particle emission rates compared to no mask, increasing to a median of 0.61 particles/s.
e rates for some participants (F1 and F4) exceeded 1 particle/s, representing a 384% increase from the median
no-mask value. Wearing a double-layer cotton t-shirt (U-DL-T) mask had no statistically signicant eect on
the particle emission rate, with comparable median and range to that observed with no mask. Turning to speech (Fig. 2b), the overarching trend observed is that vocalization at an intermediate, comfort -
able voice loudness (Figure S1a and Table S1) yielded an order of magnitude more particles than breathing.
When participants wore no mask and spoke, the median rate was 2.77 particles/s (compared to 0.31 for breath-
ing). e general trend of the mask type eect on the particle emission was qualitatively similar to that observed
for breathing. Wearing surgical masks and KN95 respirators while talking signicantly decreased the outward
emission by an order of magnitude, to median rates of 0.18 and 0.36 particles/s, respectively. Likewise, wearing
the paper towel mask reduced the outward speech particle emission rate to 1.21 particles/s, lower than no mask
but representing a less pronounced decrease compared to surgical masks and KN95 respirators. In contrast, the
homemade cloth masks again yielded either no change or a signicant increase in emission rate during speech
compared to no mask. e outward particle emissions when participants wore U-SL-T masks exceeded the no-
mask condition by an order of magnitude with a median value of 16.37 particles/s. Wearing the U-DL-T mask
had no signicant eect. e third expiratory activity – coughing – again yielded qualitatively similar trends with respect to mask
type (Fig. 2c). We emphasize that participants coughed at dierent paces, and therefore the number of coughs,
cumulative cough duration, and acoustic power varied between participants (Figure S1b, Figure S2, and Table S2).
Nonetheless, we observe that coughing with no mask produced a median of 10.1 particles/s, with most partici -
pants in the range of 3 to 42 particles/s. For comparison, given a coughing rate of 6 times per minute, the median
outward particle count due specically to coughing over that minute is slightly smaller than that from breathing,
and an order of magnitude smaller than talking over a minute (see Fig. S3 for equivalent numbers of particles
per cough). Similar general trends as for breathing and speaking were observed for coughing when wearing the
dierent mask types. e surgical mask decreased the median outward emission rate to 2.44 particles/s (75%
decrease), while the KN95 yielded an apparent but not statistically signicant decrease to 6.15 particles/s (39%
decrease). e SL-P mask yielded no statistically signicant dierence compared to no mask. In contrast, the
homemade U-SL-T and U-DL-T masks however yielded a signicant increase in outward particle emission per
second (or per cough) compared to no mask, with median emission rates of 49.2 and 36.1 particles/s, respectively. Notably, one individual, M6, emitted up to two orders of magnitude more aerosol particles while cough-
ing than the others, emitting 567 particles/s with no mask. Even when M6 wore a surgical mask he emitted
19.5 particles/s while coughing, substantially above the median value for no mask, although still a substantial
decrease compared to no mask for this individual. Acoustic analysis of the coughing, both in terms of the root
mean square amplitude (Figure S1b) and the ltered power density, indicate that the coughs by M6 were not
particularly louder nor more energetic than the others (see Figure S2 and Table S2). It is unclear what caused
this individual to emit a factor of 100 more aerosol particles than average while coughing, although qualitatively,
the coughs of M6 appeared to originate more from the chest, compared to other participants for whom coughs
generally appeared to originate more from the throat; notably, this individual emitted a much closer to average
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amount of particles while speaking and breathing. Furthermore, the signicantly higher aerosol particle emission
compared to average during coughing for M6 persisted regardless of the mask type.
Finally, Fig. 2d shows the particle emission rate when participants moved their jaw, similar to chewing gum
with their mouth closed, while only breathing through their nose. In general, jaw movement with nose breathing
and no mask produced slightly fewer particles per second than the breathing activity (breathing in through nose
and out through mouth), with a median rate of 0.12 particles/s for no mask. As participants were still breath-
ing with closed mouth during the jaw movement, the lower particle production likely results from participants
exhaling through the nose rather than through the mouth
48,51 . Wearing a surgical mask or KN95 respirator had
no statistically signicant eect on particle emission from jaw movement compared to no mask. In contrast,
wearing all other types of homemade masks (SL-P, U-SL-T, and U-DL-T) substantially increased the particle
emission rate, with the single-layer mask yielding the most at 1.72 particles/s. All of the above experiments were also repeated with vented N95 respirators, albeit with only 2 participants
(due to shortages at the time of testing). e small sample size precludes signicance testing, but in general the
particle emission rates of the two tested were comparable to the surgical mask and unvented KN95 in terms of
reduction in the overall emission rates. e emission rates presented in Fig. 2 represent the total for all particles in the size range 0.3 to 20 µm. We also
measured the corresponding size distributions in terms of overall fraction for all trials (Fig. 3). In general, all size
distributions observed here were lognormal, with a peak somewhere near 0.5 µm and decaying rapidly to negli-
gible fractions above 5 µm. Breathing while wearing no mask emitted particles with a geometric mean diameter
of 0.65 µm (Fig. 3a), with 35% of the particles in the smallest size range of 0.3 to 0.5 µm. Regardless of the mask
Figure 2. Particle emission rates associated with (a ) breathing, (b) talking, (c) coughing, and (d ) jaw movement
when participants wore no mask or when they wore one of the six mask types considered. Schee groups are
indicated with green letters; groups with no common letter are considered signicantly dierent (p < 0.05). Surg.:
surgical; KN95: unvented KN95; SL-P: single-layer paper towel; U-SL-T: unwashed single-layer cotton t-shirt;
U-DL-T: unwashed double-layer cotton t-shirt; N95: vented N95. Note that the scales are logarithmic and the
orders of magnitude dier in each subplot.
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type, wearing masks while breathing signicantly increased this fraction of particles in the smallest size range
(e.g., to as high as 60% for KN95 respirator), shiing the geometric mean diameter toward smaller sizes. Talking
with no mask yielded slightly larger particles compared to breathing, with mean diameter of 0.75 µm (Fig.
3b).
Wearing a mask while talking aected the size distribution in a qualitatively similar manner to that observed
with breathing, in that a higher fraction of particles were in the smallest size range. Unlike breathing however,
the U-SL-T and U-DL-T masks released the highest fractions of small particles (47% and 51%, respectively). e eect of wearing a mask was more pronounced on the size distribution of the particles produced by
coughing (Fig. 3c). For no mask, the mean diameter of cough-generated particles was 0.6 µm. e majority of
particles emitted were in the smallest size range (up to 57%) during coughing while wearing homemade masks
(SL-P, U-SL-T, and U-DL-T). We also note that for coughing, which produced the highest rates of particle emis -
sion for of all expiratory activities tested, wearing homemade masks considerably reduced the fraction of large
particles (> 0.8 µm). Finally, for jaw movement the overall size distributions for no-mask and with-mask cases
were similar except that the fraction of smallest particles was lowest for no-mask and the surgical mask (Fig. 3d).
To provide a direct comparison of the ecacy of medical-grade and homemade masks in mitigating the
emission of particles of dierent sizes, we divided the entire size range measured by APS (0.3 – 20 µm) into
three sub-ranges (smallest, 0.3 – 0.5 µm; intermediate, 0.5 – 1 µm; and largest, 1 – 20 µm), and calculated the
corresponding percent change in the median particle emission rate of each sub-range during breathing, talking,
and coughing compared to no mask (Fig. 4). For the smallest particles, Fig. 4a shows that up to a 92% reduction
in 0.3 – 0.5 µm particle emission rate occurred while wearing surgical and KN95 masks for breathing, talking,
and coughing, with the KN95 yielding a smaller decrease of 20.5% in this size range. e SL-P mask caused a 60%
reduction in 0.3 – 0.5 µm particle emission for talking and breathing, but yielded a 77% increase for coughing.
e least eective masks in terms of minimizing emissions of the smallest particles were the U-SL-T and U-DL-T
masks, with U-SL-T substantially increasing the emission of 0.3 – 0.5 µm particles by almost 600% for speech,
and the U-DL-T mask yielding very slight changes for talking and breathing and an almost 300% increase for
coughing. Qualitatively similar trends were observed for intermediate size particles in the range of 0.5 – 1 µm
(Fig. 4b), with the medical-grade masks yielding signicant reductions. e main dierence for this size range is
Figure 3. Observed particle size distributions, normalized by particles/s per bin, associated with (a ) breathing,
( b ) talking, (c ) coughing, and (d ) jaw movement when participants wore no mask or one of the ve mask types
considered. Each curve is the average over all 10 participants. e solid lines represent the data using a 5 -point
smoothing function. Data points with horizontal error bars show the small particles ranging from 0.3 to 0.5 m
in diameter detected by APS with no further information about their size distribution in this range. Surg.:
surgical; KN95: unvented KN95; SL-P: single-layer paper towel; U-SL-T: unwashed single-layer cotton t-shirt;
U-DL-T: unwashed double-layer cotton t-shirt; N95: vented N95.
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that the SL-P mask yielded a 15.7% decrease in particle emissions for coughing, and the U-DL-T mask provided
up to 34.1% reduction in particle emissions for breathing and talking.
As for the largest particle sizes (1 – 20 µm), the observed trends were again qualitatively similar to the
intermediate particles (Fig. 4c), with the medical-grade masks yielding large reductions. Notably, the U-DL-T
mask emitted much fewer large particles for breathing and talking with approximately 60% reductions, but still
a sizable 160% increase for coughing. e percent change in median particle emission over the entire size range
of 0.3 – 20 µm is presented in Fig. 4d, which shows that the homemade masks in general yielded more particles
in total for coughing, and had mixed ecacy in reducing particle emissions for breathing and talking. e key
point is that the surgical and KN95 masks eectively decreased the particle emission for all expiratory activities
tested here over the entire range of particle sizes measured by the APS. To help interpret our ndings we also quantied the particles emitted from manual rubbing of mask fabrics.
e results (Fig. 5a) show that, in the absence of any expiratory activity, rubbing a surgical mask fabric generated
on average 1.5 particles per second, while KN95 and N95 respirators produced fewer than 1 particle per second.
In contrast, rubbing the homemade paper and cotton masks aerosolized signicant number of particles, with
the highest values for SL-P (8.0 particles/s) and U-SL-T (7.2 particles/s) masks. Intriguingly, we found that the
size distribution of the particles aerosolized from homemade mask fabrics via manual rubbing (Fig. 5b) was
qualitatively dierent from when participants wore the same masks to perform expiratory activities. An extra
Figure 4. Percent change in median particle emission rate (N) for 10 participants compared to no-mask
median, while wearing dierent mask types and while breathing (blue points), talking (red points), or coughing
(green points), for particles in the following size ranges: (a ) smallest, 0.3–0.5 µm; (b) intermediate, 0.5–1 µm;
( c ) largest, 1–20 µm; and (d ) all sizes, 0.3–20 µm. e dashed lines are to guide the eye. Surg.: surgical; KN95:
unvented KN95; SL-P: single-layer paper towel; U-SL-T: unwashed single-layer cotton t-shirt; U-DL-T:
unwashed double-layer cotton t-shirt.
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peak appeared at approximately 6 µm and the fraction of small particles dropped to below 27%, suggesting that
the frictional forces of bers against bers helped fragment and dislodge larger particulates into the air. Impor
-
tantly, however, manual rubbing produced a sizeable number of particulates in the size range of 0.3 to 2 µm,
commensurate with the range observed while the masks were worn during expiratory activities. Note that the
coarse skin particulates (> 2 µm) released from hand during the mask fabric rubbing experiments could have
contributed to the observed particle counts
57. However, since this factor was the same in all the manual rubbing
experiments, and only facemask fabrics diered, it is dicult to explain the observed trends solely in terms of
friction between skin and mask fabrics. Moreover, although in these experiments the applied tribological force
was not strictly controlled or quantied, the presented results strongly suggest that cotton fabric masks have
much more friable material, consistent with our observation that more particles are emitted when participants
perform expiratory activities in those cotton fabric masks. Since the cotton masks were all prepared from fabric that was brand new and unwashed, as a nal test we
hypothesized that perhaps washing the masks would remove surface-bound dust and otherwise friable material
and decrease the emission rate. Our experiments do not corroborate this hypothesis. Handwashing the double-
layer t-shirt mask with soap and water followed by air-drying yielded no signicant change in the particle
emission rate as compared to the original unwashed masks (Fig. 6). Moreover, manual rubbing of a washed
double-layer cotton mask aerosolized slightly more particles than the unwashed mask. ese results suggest that
a single washing has little impact on the presence of aerosolizable particulate matter in standard cotton fabrics.
Note also that the ranges observed here accord qualitatively with the prior measurements taken with the same
4 participants on a previous day (compare the results for each category in Fig. 6 versus the U-DL-T columns for
the respective expiratory activities in Fig. 2). is observation suggests that day-to-day variability for a given
individual is less than the person-to-person variability observed for all expiratory activities and mask types tested.
DiscussionOur results clearly indicate that wearing surgical masks or unvented KN95 respirators reduce the outward particle
emission rates by 90% and 74% on average during speaking and coughing, respectively, compared to wearing no
mask. However, for the homemade cotton masks, the measured particle emission rate either remained unchanged
(DL-T) or increased by as much as 492% (SL-T) compared to no mask for all of the expiratory activities. For jaw
movement, the particle emission rates for homemade paper and cloth masks were an order of magnitude larger
than that of no mask (Fig. 2d). ese observations, along with our results from manual mask rubbing experi-
ments (Fig. 5), provide strong evidence of substantial shedding of non-expiratory micron-scale particulates from
friable cellulosic bers of the paper and cloth masks owing to mechanical action
40. e higher particle emission
rate for jaw movement than for breathing is an indication of greater frictional shedding of the paper towel and
cotton masks during jaw movement compared to breathing, at least as tested here. Likewise, the dierence in
the size distributions of mask rubbing and with-mask expiratory activities is likely due to the vigorous frictional
force applied by hand on the masks. Regardless of the larger particles (> 5 µm), rubbing mask fabrics generates a
considerable number of particles in the range of 0.3–5 µm similar to that observed for the expiratory activities.
Figure 5. (a ) Number of particles emitted per second of manual rubbing for all masks tested. Each data point
is time-averaged particle emission rate over 10 s of rubbing. (b ) Corresponding size distribution for homemade
paper and cotton masks for a total of 30 s of manual rubbing in front of the APS. e solid lines represent the
data using a 5 -point smoothing function. Data points with horizontal whiskers show the small particles ranging
from 0.3 to 0.5 m in diameter detected by APS. Surg.: surgical; KN95: unvented KN95; SL-P: single-layer paper
towel; U-SL-T: unwashed single-layer cotton t-shirt; U-DL-T: unwashed double-layer cotton t-shirt; N95: vented
N95.
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is nding corroborates the interpretation that some proportion of the particulates observed during expiration
were particulates aerosolized from the masks themselves.
Another factor to consider is that masks can reduce the intelligibility of the speech signal
58, and can reduce
the intensity of sounds passed through them by a signicant amount (e.g., > 10 dB in Saedi et al.59). Likely as a
response to this, people will speak louder and otherwise adjust their speech when wearing masks. Mendel et al. 60
found that the measured intensity of speech was approximately the same for a group of speakers with and with-
out surgical masks, suggesting that speakers increased the actual intensity of their speech when wearing masks.
Fecher
61 found that speakers will actually produce louder output through some types of masks in cases where
they overestimate the dampening eects of the mask. It is also possible that speakers may produce Lombard
speech when wearing certain types of masks
62. Lombard speech is louder, has a higher fundamental frequency,
and tends to have longer vowel durations, all characteristics that may contribute to an increase in the emission
of aerosols
48, 49 . Our results showed that the root mean square amplitude of speech, as measured externally when
participants wore any type of mask, equaled or exceeded that of the no-mask condition (Figure S1a), suggesting
that participants were indeed talking louder with the mask. Although an increase in the intensity of the speech
signal when wearing masks would result in greater output of particles in these conditions
48, the dierence in the
intensity of speech across the dierent conditions was not very large (Figure S1a). As a result, this mechanism
alone cannot explain the increased particle output in some of the masked conditions. Intriguingly, the root mean
square amplitude of coughing decreased for most of the participants aer they wore a type of mask (Figure S1b),
suggesting that they do not cough louder when they wear a mask, i.e., there is no Lombard eect for coughing. e substantial particle shedding by the cloth masks confounds determination of the cloth mask ecacy
for reducing outward emission of particles produced from the expiratory activity. Measured material ltration
eciencies vary widely for dierent cloth materials
32,34,35,63 . e in≤uence of particle shedding on such deter -
minations has not been previously considered; our results raise the possibility that particle shedding has led to
underestimated material ltration eciencies for certain materials. While the material eciency of the cotton
masks was not determined here, we note that the use of the double-layer cotton masks reduced the emission of
larger particles (both on a normalized and absolute basis), indicating some reasonable ecacy towards reduction
of the expiratory particle emission. Further work dierentiating between expiratory and shed particles, possibly
based on composition, can help establish the specic ecacy of the cloth masks towards expiratory particles. at
the masks shed bers from mechanical stimulation indicates care must be taken when removing and cleaning
(for reusable masks) potentially contaminated masks so as to not dislodge deposited micro-organisms. We also note that the emission reduction due to surgical masks was greater than the corresponding reduction
due to KN95 respirators, although this dierence was only signicant for coughing (p < 0.05). at the surgical
masks appear to provide slightly greater reduction than the KN95 respirators is perhaps surprising, as KN95s
are commonly thought of as providing more protection than surgical masks for inhalation. Both surgical masks
and KN95 respirators typically have high material ltration eciencies (> 95%)
63, although the quality of surgi-
cal masks can vary substantially 64. e t of surgical and KN95 respirators diers substantially. Here, no t tests
were performed to ensure good seals of the KN95 respirators. It may be that imperfect tting of KN95 respirators
allows for greater escape of particles from the mask-covered environment compared to the more ≤exible surgical
Figure 6. Particle emission rate from breathing, talking, coughing and jaw movement for 4 participants
wearing unwashed or washed double-layer t-shirt masks (U-DL-T vs. W-DL-T). Last column shows the
particles emission rates for manual rubbing of washed and unwashed masks (three 10-s trials for each mask).
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masks. Regardless, all surgical masks, KN95 and N95 respirators tested here provided substantial reduction of
particle emission compared to no mask.
A particularly important observation was the existence of a coughing superemitter, who for unknown reasons
emitted two orders of magnitude more particles during coughing than average (Fig. 2c, red points for M6). is
huge dierence persisted regardless of mask type, with even the most eective mask, the surgical mask, only
reducing the rate to a value twice the median value for no mask at all. Although the underlying mechanism lead-
ing to such enhanced particle emission is unclear, these observations nonetheless conrm that some people act
as superemitters during coughing, similar to “speech superemitters”
48, and “breathing high producers” 65. is
observation raises the possibility that coughing superemitters could serve as superspreaders who are dispropor -
tionately responsible for outbreaks of airborne infectious disease. Notably, the coughing superemitter was not
a breathing superemitter or speaking superemitter, indicating that testing only one type of expiratory activity
might not necessarily identify superemitters for other expiratory activities. As a nal comment, we emphasize that here we only measured the physical dynamics of outward aerosol
particle emission for dierent expiratory activities and mask types. Redirected expiratory air≤ow, involving
exhaled air moving up past the nose or out the side of the mask, were not measured here but should be consid-
ered in future work. Likewise, more sophisticated biological techniques are necessary to gauge mask ecacy at
blocking emission of viable pathogens. Our work does raise the possibility, however, that virus-contaminated
masks could release aerosolized fomites into the air by shedding ber particulates from the mask fabric. Since
mask ecacy experiments are typically only conducted with fresh, not used, masks, future work assessing emis -
sion of viable pathogens should consider this possibility in more detail. Our work also raises questions about
whether homemade masks using other fabrics, such as polyester, might be more ecient than cotton in terms of
blocking expiratory particles while minimizing shedding of fabric particulates, and whether repeated washings
might aect homemade masks. Future experiments using controlled bursts of clean air through the masks will
help to resolve the source of these non-expiratory particles. Nonetheless, as a precaution, our results suggest
that individuals using homemade fabric masks should take care to wash or otherwise sterilize them on a regular
basis to minimize the possibility of emission of aerosolized fomites.
Conclusionsese observations directly demonstrate that wearing of surgical masks or KN95 respirators, even without t-
testing, substantially reduce the number of particles emitted from breathing, talking, and coughing. While the
ecacy of cloth and paper masks is not as clear and confounded by shedding of mask bers, the observations
indicate it is likely that they provide some reductions in emitted expiratory particles, in particular the larger
particles (> 0.5 m). We have not directly measured virus emission; nonetheless, our results strongly imply that
mask wearing will reduce emission of virus-laden aerosols and droplets associated with expiratory activities,
unless appreciable shedding of viable viruses on mask bers occurs. e majority of the particles emitted were
in the aerosol range (< 5 m). As inertial impaction should increase as particle size increases, it seems likely that
the emission reductions observed here provide a lower bound for the reduction of particles in the droplet range
(> 5 m). Our observations are consistent with suggestions that mask wearing can help in mitigating pandem -
ics associated with respiratory disease. Our results highlight the importance of regular changing of disposable
masks and washing of homemade masks, and suggests that special care must be taken when removing and
cleaning the masks.
Data availabilitye datasets generated during and/or analyzed during the current study are available in the Dryad Digital Reposi-
t o r y, https ://doi.org/10.25338 /B87C9 V.
Received: 9 June 2020; Accepted: 23 August 2020
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Acknowledgementsis research was supported by National Institute of Allergy and Infectious Diseases of the National Institutes
of Health (NIAID/NIH), Grant R01 AI110703.
Author contributionsS.A. performed the APS measurements of airborne particulates, fabricated the homemade masks, conducted the
statistical analyses, and prepared gures. S.A., C.D.C, and W.D.R. analyzed the APS data and wrote the manu -
script. S.A. and S.B. analyzed and interpreted the acoustic data. All authors reviewed and revised the manuscript
for accuracy and intellectual content.
Competing interests
e authors declare no competing interests.
Additional informationSupplementary information is available for this paper at https ://doi.org/10.1038/s4159 8-020-72798 -7.
Correspondence and requests for materials should be addressed to W.D.R.
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