Inward and outward effectiveness of cloth masks, a surgical 1
mask, and a face shield 2
Jin Pan, Charbel Harb, Weinan Leng, Linsey C. Marr* 3
Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA 24061 4
*Corresponding author: lmarr@vt.edu 5
Keywords: masks, aerosol, transmission, COVID-19, SARS-CoV-2, face coverings 6
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NOTE: This preprint reports new research that has not been certified by peer review and should not be used to guide clinical practice.
Abstract 7
We evaluated the effectiveness of 11 face coverings for material filtration efficiency, inward 8
protection efficiency on a manikin, and outward protection efficiency on a manikin. At the most 9
penetrating particle size, the vacuum bag, microfiber cloth, and surgical mask had material 10
filtration efficiencies >50%, while the other materials had much lower filtration efficiencies. 11
However, these efficiencies increased rapidly with particle size, and many materials had 12
efficiencies >50% at 2 µm and >75% at 5 µm. The vacuum bag performed best, with efficiencies 13
of 54-96% for all three metrics, depending on particle size. The thin acrylic and face shield 14
performed worst. Inward protection efficiency and outward protection efficiency were similar for 15
many masks; the two efficiencies diverged for stiffer materials and those worn more loosely (e.g., 16
bandana) or more tightly (e.g., wrapped around the head) compared to a standard earloop mask. 17
Discrepancies between material filtration efficiency and inward/outward protection efficiency 18
indicated that the fit of the mask was important. We calculated that the particle size most likely to 19
deposit in the respiratory tract when wearing a mask is ~2 µm. Based on these findings, we 20
recommend a three-layer mask consisting of outer layers of a flexible, tightly woven fabric and an 21
inner layer consisting of a material designed to filter out particles. This combination should 22
produce an overall efficiency of >70% at the most penetrating particle size and >90% for particles 23
1 µm and larger if the mask fits well. 24
25
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Introduction 26
Amid mounting evidence that COVID-19 is transmitted via inhalation of virus-laden aerosols 27
(Allen and Marr 2020; Asadi et al. 2020; Hadei et al. 2020; Morawska et al. 2020; Prather, Wang 28
and Schooley 2020), universal masking has emerged as one of a suite of intervention strategies for 29
reducing community transmission of the disease. There is a correlation between widespread mask 30
wearing (The Economist 2020), or at least interest in masks (Wong et al. 2020), and lower 31
incidence of COVID-19 by country and between mask mandates and county-level COVID-19 32
growth rates in the US (Lyu and Wehby 2020), but a causal relationship has not been confirmed. 33
Due to a shortage of medical masks and respirators, some public health agencies have 34
recommended the use of cloth face coverings. While there have been numerous studies on the 35
ability of surgical masks and N95 respirators to filter out particles, far less is known about the 36
ability of cloth masks to provide both inward protection to reduce the wearer’s exposure and 37
outward protection for source control. Ideally, a randomized controlled trial would be conducted, 38
but in the absence of such evidence, we can evaluate the ability of masks to block particles under 39
controlled conditions. 40
Reviews on the use of masks in both healthcare and non-healthcare settings to reduce transmission 41
of other respiratory diseases mostly show a protective effect. A systematic review and meta-42
analysis of interventions against respiratory viruses found that wearing simple masks was highly 43
effective at reducing transmission of severe acute respiratory syndrome (SARS) in five case 44
control studies (Jefferson et al. 2008). In contrast, a review of 10 randomized controlled trials of 45
mask wearing in non-healthcare settings concluded that there was not a substantial effect on 46
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The copyright holder for thisthis version posted November 20, 2020. ; https://doi.org/10.1101/2020.11.18.20233353doi: medRxiv preprint
influenza transmission in terms of risk ratio, although most of the studies were underpowered and 47
compliance was not perfect (Xiao et al. 2020). A systematic review of interventions against SARS-48
CoV-2 and the coronaviruses that cause SARS and Middle East respiratory syndrome found that 49
the use of face masks could result in a large reduction in the risk of infection (Chu et al. 2020). 50
Laboratory studies have demonstrated the ability of surgical masks to provide both inward and 51
outward protection against viruses. Testing of eight different surgical masks on a manikin with 52
influenza virus in droplets/aerosols of size 1–200 µm found that the amount of virus detected 53
behind the mask was reduced by an average of 83%, with a range of 9% to 98% (Makison Booth 54
et al. 2013). The ability of a mask to block influenza virus was correlated with its ability to block 55
droplets/aerosols containing only phosphate buffered saline (PBS) and bovine serum albumin 56
(BSA). Surgical masks used for source control on influenza patients during breathing and coughing 57
reduced the amount of virus released into the air in coarse (> 5 µm) and fine (≤ 5 µm) aerosols by 58
96% and 64%, respectively (Milton et al. 2013). In a follow-up study, surgical masks blocked the 59
release of seasonal coronaviruses in coarse and fine aerosols to undetectable levels, while they 60
blocked influenza virus in most but not all patients (Leung et al. 2020). 61
There have been some studies of cloth masks, which have been found to be less protective than 62
surgical masks in most, but not all, cases. A variety of cloth materials mounted in a filter holder 63
removed 49% to 86% of aerosolized bacteriophage MS2, compared to 89% removal by a surgical 64
mask (Makison Booth et al. 2013). According to fit tests on 21 adults in the same study, homemade, 65
100% cotton masks provided median inward filtration efficiencies of 50%, compared to 80% for 66
surgical masks. The filtration efficiencies of 44 materials and medical masks, challenged with 67
sodium chloride (NaCl) particles of diameter 0.03–0.25 µm, ranged from <10% for polyurethane 68
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foam to nearly 100% for a vacuum cleaner bag (Drewnick et al. 2020). Cloth masks, sweatshirts, 69
t-shirts, towels, and scarves evaluated in a TSI Automated Filter Tester had filtration efficiencies 70
of 10–60% against polydisperse NaCl particles ranging in size from 0.02 to 1.0 µm; the towels 71
performed best (Rengasamy, Eimer and Shaffer 2010). Homemade masks made from tea cloths 72
and worn by volunteers had a median inward filtration efficiency of 60%, compared to 76% for a 73
surgical mask (van der Sande, Teunis and Sabel 2008). Pieces of a bandana, veil, shawl, 74
handkerchief, and cotton t-shirt mounted in a filter holder and challenged with volcanic ash 75
particles were found to have filtration efficiencies of 18% to 43% in terms of mass concentration 76
(Mueller et al. 2018). 77
N95 respirators and cloth masks serve different purposes, so the testing procedure for N95s is not 78
necessarily well-suited for cloth masks. An N95 must be able to protect an individual worker in 79
high-risk situations. A critical component of its efficacy is the fit test to ensure that the respirator 80
seals completely to the face with no leaks. On the other hand, the overall goal of wearing cloth 81
masks during the COVID-19 pandemic is to reduce community transmission. Cloth masks provide 82
some degree of both source control and exposure reduction. While an N95 must block at least 95% 83
of NaCl particles of the most penetrating size, 0.3 µm, cloth masks can be effective if they remove 84
at least some particles, particularly those of the size that is most relevant for transmission. 85
Although we do not yet know which size particles are most important, we can make some 86
inferences from existing studies. SARS-CoV-2 and other viruses are carried by particles ranging 87
in size from <1 µm to >5 µm (Chia et al. 2020; Liu et al. 2020; Yan et al. 2018; Yang, Elankumaran 88
and Marr 2011). A SARS-CoV-2 virion is 0.1 µm in diameter, but it is carried in respiratory 89
droplets that also contain salts, proteins, and other components of respiratory fluid. Even if all the 90
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The copyright holder for thisthis version posted November 20, 2020. ; https://doi.org/10.1101/2020.11.18.20233353doi: medRxiv preprint
