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Inward and outward effectiveness of cloth masks, a surgical mask, and a face shield

Jin Pan1, Charbel Harb1, Weinan Leng1, Linsey C. Marr1•
Virginia Tech1
10 Mar 2021-Aerosol Science and Technology (Taylor & Francis)-Vol. 55, Iss: 6, pp 718-733
TL;DR: A three-layer mask consisting of outer layers of a flexible, tightly woven fabric and an inner layer consisting of a material designed to filter out particles is recommended, which should produce an overall efficiency of >70% at the most penetrating particle size and >90% for particles 1 m and larger if the mask fits well.
Abstract: We evaluated the effectiveness of 11 face coverings for material filtration efficiency, inward protection efficiency on a manikin, and outward protection efficiency on a manikin. At the most penetr...

Summary (3 min read)

Jump to: [Introduction] – [Masks] – [Material filtration efficiency] – [Inward and outward protection efficiency at close distance] – [Droplet deposition analysis] – [Size of challenge particles] – [Inward and outward protection efficiency] – [Discussion] and [Conclusion]

Introduction

  • While there have been numerous studies on the ability of surgical masks and N95 respirators to filter out particles, far less is known about the ability of cloth masks to provide both inward protection to reduce the wearer's exposure and outward protection for source control.
  • In a follow-up study, surgical masks blocked the release of seasonal coronaviruses in coarse and fine aerosols to undetectable levels, while they blocked influenza virus in most but not all patients (Leung et al. 2020 ).
  • Cloth masks, sweatshirts, t-shirts, towels, and scarves evaluated in a TSI Automated Filter Tester had filtration efficiencies of 10-60% against polydisperse NaCl particles ranging in size from 0.02 to 1.0 µm; the towels performed best (Rengasamy, Eimer and Shaffer 2010) .
  • The smaller mode of respiratory particles produced during breathing and speaking is centered around 1 µm, and there are relatively few particles smaller than 0.5 µm (Johnson et al. 2011) .

Masks

  • The authors tested nine materials that were fashioned into masks, one surgical mask, and one face shield, shown in Figure 1 .
  • These included a sewn mask made of two layers of a 200-thread count cotton pillowcase and a non-sewn mask cut from a cotton t-shirt (Centers for Disease Control and Prevention 2020).
  • The surgical mask had a single layer and was advertised to meet ASTM level 1 specifications, which require ≥95% filtration efficiency of particles larger than 1 µm.
  • The authors characterized the texture and structure of the masks using a scanning electron microscope (FEI Quanta 600 FEG).
  • Because it is not possible to generate or characterize particles spanning a wide range of sizes with a single experimental setup, the authors designed several different protocols for testing masks, optimizing among different types of equipment and detection limits, as described below.

Material filtration efficiency

  • Evaluation of the materials for filtration efficiency followed a protocol based on National Institute of Occupational Safety and Health testing procedures.
  • The authors measured particle concentrations and size distributions using a scanning mobility particle sizer (SMPS 3936, TSI Inc., MN, USA), with the particle density set to 2.165 g/cm 3 (NaCl) to convert from mobility diameter to aerodynamic diameter.
  • The authors also measured the pressure drop of each material in the filter holder using a differential pressure gauge (Minihelic II 2-5005, Dwyer Instruments, IN, USA).
  • The fabrics fashioned into the CDC non-sewn and CDC sewn masks, bandana (1 ply), and thin acrylic had even lower efficiencies of 5-40% for submicron particles.
  • That fabric structure alone does not explain filtration efficiency also applies to the filter quality factors.

Inward and outward protection efficiency at close distance

  • The "exhaling" manikin was connected to a medical nebulizer filled with 2% NaCl solution, that produced a flow rate of 10 L/min through 0.79 cm i.d. tubing.
  • The authors then switched the valve so that the APS sampled through the inhaling manikin's mouth and measured particles that penetrated the mask, denoted Cm.
  • The difference between Cc1 and Cc2 was less than 10% in all cases.
  • The authors calculated the inward protection efficiency based on equation (1), replacing the numerator with Cm(DP) and the denominator with Cc(DP).
  • In each test, the authors ran the medical nebulizer for 30 s and then allowed particle concentrations to decay, as in scenario 2 of the inward protection protocol, and they measured the chamber concentration (Cc1) using the APS at 1-s resolution.

Droplet deposition analysis

  • The authors evaluated the ability of the face coverings to block droplets larger than 20 μm, which is the upper limit of the APS, using a modified droplet deposition analysis (DDA) (Johnson et al.
  • The authors connected the air brush to HEPA-filtered air and a gas regulator set at 165.5 kPa, resulting in a total flow rate of 10 L/min, the same as the flow rate of the medical nebulizer.
  • The authors precleaned each slide using 70% isopropyl alcohol wipes.
  • The authors put the face covering on the exhaling manikin and repeated the same steps.
  • To identify droplets on the slides, the authors processed the images using ImageJ and then manually counted the stains and measured their size with a limit of detection of 12.3 μm/pixel.

Size of challenge particles

  • The authors used four different types of aerosol generators to cover a broad size range and to accommodate different setups.
  • The Collison nebulizer produced particles with a geometric mean diameter (GMD) of 0.12 μm and geometric standard deviation (GSD) of 1.4, and the FMAG a GMD of 4 μm and GSD of 1.21.
  • The figure also shows the size distribution measured downstream of a MERV 12 filter to illustrate the data used to calculate filtration and protection efficiencies.
  • The medical nebulizer produced particles ranging in size from 0.5 to 5 μm; the GMD was below the detection limit of the APS .

Inward and outward protection efficiency

  • The inward protection efficiency (IPE) quantifies the capability of a mask, as worn on a manikin, to protect the wearer by filtering out particles moving in the inward direction through the mask, from the surrounding air to the wearer's respiratory tract.
  • The IPE and OPE were also similar to the respective material filtration efficiency for the CDC-sewn and thin acrylic masks , though their performance was much worse than that of the vacuum bag.
  • In response to a study that suggested that neck gaiters offer very little protection (Fischer et al. 2020 ), the authors measured the OPE of two neck gaiters, one made of thin 100% polyester and another made of a double layer of microfiber fabric that was 87% polyester and 13% elastane.
  • Due to the late addition of these face coverings, the authors were not able to measure their material filtration efficiency or IPE.

Discussion

  • For most of the face coverings tested, those with a high material filtration efficiency also had a better OPE and IPE.
  • The material filtration efficiencies of these two masks was much higher than their OPEs and IPEs .
  • Hence, the increased pressure caused by the expiratory flow was not able to push the CDC non-sewn mask outwards to create gaps between masks and the manikin like other conventional masks do (Lei et al.
  • The combined effects of reduced gaps and reduced air velocity resulted in a uniquely high OPE for the CDC non-sewn mask.
  • The smallest particle size considered in this analysis was 0.5 µm, but the deposition efficiency of 0.3 µm particles in the respiratory tract is even lower, so it is possible that concerns about mask efficiency at this size are overstated.

Conclusion

  • The authors evaluated the material filtration efficiency, inward protection efficiency, and outward protection efficiency of 10 masks and a face shield on a manikin, using NaCl aerosols over the size range of 0.04 µm to >100 µm.
  • The thin acrylic performed worst, with a material filtration efficiency of <25% for particles at 0.1 μm and larger, and inward and outward protection efficiencies of <50%.
  • Factors including stiffness of the material, the way of wearing the mask (e.g., earloops vs. tied around the head), and material hydrophobicity affected the fit of the mask and thus its performance.
  • Flow Focusing Monodisperse Aerosol Generator 1520 to the Marr lab.
  • This work used shared facilities at the Virginia Tech National Center for Earth and Environmental Nanotechnology Infrastructure , a member of the National Nanotechnology Coordinated Infrastructure (NNCI), supported by NSF (ECCS 1542100 and ECCS 2025151) .

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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
. CC-BY-NC-ND 4.0 International licenseIt is made available under a
is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.(which was not certified by peer review)preprint
The copyright holder for thisthis version posted November 20, 2020. ; https://doi.org/10.1101/2020.11.18.20233353doi: medRxiv preprint
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
. CC-BY-NC-ND 4.0 International licenseIt is made available under a
is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.(which was not certified by peer review)preprint
The copyright holder for thisthis version posted November 20, 2020. ; https://doi.org/10.1101/2020.11.18.20233353doi: medRxiv preprint
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
. CC-BY-NC-ND 4.0 International licenseIt is made available under a
is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.(which was not certified by peer review)preprint
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
. CC-BY-NC-ND 4.0 International licenseIt is made available under a
is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.(which was not certified by peer review)preprint
The copyright holder for thisthis version posted November 20, 2020. ; https://doi.org/10.1101/2020.11.18.20233353doi: medRxiv preprint
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
. CC-BY-NC-ND 4.0 International licenseIt is made available under a
is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity.(which was not certified by peer review)preprint
The copyright holder for thisthis version posted November 20, 2020. ; https://doi.org/10.1101/2020.11.18.20233353doi: medRxiv preprint
Citations
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A guideline to limit indoor airborne transmission of COVID-19.

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Martin Z. Bazant1, John W. M. Bush1•
Massachusetts Institute of Technology1
27 Apr 2021-Proceedings of the National Academy of Sciences of the United States of America
TL;DR: In this paper, the authors derived an indoor safety guideline that would impose an upper bound on the cumulative exposure time, the product of the number of occupants and their time in an enclosed space, and demonstrate how this bound depends on the rates of ventilation and air filtration, dimensions of the room, breathing rate, respiratory activity and face mask use of its occupants.
Abstract: The current revival of the American economy is being predicated on social distancing, specifically the Six-Foot Rule, a guideline that offers little protection from pathogen-bearing aerosol droplets sufficiently small to be continuously mixed through an indoor space. The importance of airborne transmission of COVID-19 is now widely recognized. While tools for risk assessment have recently been developed, no safety guideline has been proposed to protect against it. We here build on models of airborne disease transmission in order to derive an indoor safety guideline that would impose an upper bound on the "cumulative exposure time," the product of the number of occupants and their time in an enclosed space. We demonstrate how this bound depends on the rates of ventilation and air filtration, dimensions of the room, breathing rate, respiratory activity and face mask use of its occupants, and infectiousness of the respiratory aerosols. By synthesizing available data from the best-characterized indoor spreading events with respiratory drop size distributions, we estimate an infectious dose on the order of 10 aerosol-borne virions. The new virus (severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2]) is thus inferred to be an order of magnitude more infectious than its forerunner (SARS-CoV), consistent with the pandemic status achieved by COVID-19. Case studies are presented for classrooms and nursing homes, and a spreadsheet and online app are provided to facilitate use of our guideline. Implications for contact tracing and quarantining are considered, and appropriate caveats enumerated. Particular consideration is given to respiratory jets, which may substantially elevate risk when face masks are not worn.

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Rajat Mittal, Rui Ni, Jung Hee Seo
20 Apr 2020
TL;DR: In this article, the authors summarized what we know, and more importantly, what we need to learn about the science underlying these issues so that we are better prepared to tackle the next outbreak of COVID-19 or a similar disease.
Abstract: Flow physics plays a key role in nearly every facet of the COVID-19 pandemic. This includes the generation and aerosolization of virus-laden respiratory droplets from a host, its airborne dispersion and deposition on surfaces, as well as the subsequent inhalation of these bioaerosols by unsuspecting recipients. Fluid dynamics is also key to preventative measures such as the use of face masks, hand-washing, ventilation of indoor environments, and even social distancing. This article summarizes what we know, and more importantly, what we need to learn about the science underlying these issues so that we are better prepared to tackle the next outbreak of COVID-19 or a similar disease.

153 citations

Journal Article•DOI•
Effectiveness of Face Mask or Respirator Use in Indoor Public Settings for Prevention of SARS-CoV-2 Infection — California, February–December 2021

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Kristin L. Andrejko, Jake Pry, Jennifer F. Myers, Nozomi Fukui, J. DeGuzman, John J. Openshaw, James P. Watt, Joseph A Lewnard, Seema Jain, Yasmine Abdulrahim, C. M. Barbaduomo, Miriam I Bermejo, Julia Cheunkarndee, Adrian F Cornejo, Savannah Corredor, Najla Dabbagh, Zheng N Dong, A. P. Dyke, Anna T Fang, Diana Felipe, Paulina M Frost, Timothy Ho, M Javadi, Amandeep Kaur, Ileana Castaño Lam, Sophia S Li, Monique Miller, Jessica Ni, Hyemin Park, Diana J Poindexter, Helia Samani, Shrey Saretha, Maya Spencer, Michelle M Spinosa, Vi Ha Tuong Tran, Nikolina Walas, Christine Wan, Erin Xavier 
04 Feb 2022-Morbidity and Mortality Weekly Report
TL;DR: It is reinforced that in addition to being up to date with recommended COVID-19 vaccinations, consistently wearing a face mask or respirator in indoor public settings reduces the risk of acquiring SARS-CoV-2 infection.
Abstract: The use of face masks or respirators (N95/KN95) is recommended to reduce transmission of SARS-CoV-2, the virus that causes COVID-19 (1). Well-fitting face masks and respirators effectively filter virus-sized particles in laboratory conditions (2,3), though few studies have assessed their real-world effectiveness in preventing acquisition of SARS-CoV-2 infection (4). A test-negative design case-control study enrolled randomly selected California residents who had received a test result for SARS-CoV-2 during February 18-December 1, 2021. Face mask or respirator use was assessed among 652 case-participants (residents who had received positive test results for SARS-CoV-2) and 1,176 matched control-participants (residents who had received negative test results for SARS-CoV-2) who self-reported being in indoor public settings during the 2 weeks preceding testing and who reported no known contact with anyone with confirmed or suspected SARS-CoV-2 infection during this time. Always using a face mask or respirator in indoor public settings was associated with lower adjusted odds of a positive test result compared with never wearing a face mask or respirator in these settings (adjusted odds ratio [aOR] = 0.44; 95% CI = 0.24-0.82). Among 534 participants who specified the type of face covering they typically used, wearing N95/KN95 respirators (aOR = 0.17; 95% CI = 0.05-0.64) or surgical masks (aOR = 0.34; 95% CI = 0.13-0.90) was associated with significantly lower adjusted odds of a positive test result compared with not wearing any face mask or respirator. These findings reinforce that in addition to being up to date with recommended COVID-19 vaccinations, consistently wearing a face mask or respirator in indoor public settings reduces the risk of acquiring SARS-CoV-2 infection. Using a respirator offers the highest level of personal protection against acquiring infection, although it is most important to wear a mask or respirator that is comfortable and can be used consistently.

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COVID-19 false dichotomies and a comprehensive review of the evidence regarding public health, COVID-19 symptomatology, SARS-CoV-2 transmission, mask wearing, and reinfection.

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27 Jul 2021-BMC Infectious Diseases
TL;DR: In this paper, the authors deconstruct six common COVID-19 false dichotomies, address the evidence on these topics, identify insights relevant to effective pandemic responses, and highlight knowledge gaps and uncertainties.
Abstract: Scientists across disciplines, policymakers, and journalists have voiced frustration at the unprecedented polarization and misinformation around coronavirus disease 2019 (COVID-19) pandemic. Several false dichotomies have been used to polarize debates while oversimplifying complex issues. In this comprehensive narrative review, we deconstruct six common COVID-19 false dichotomies, address the evidence on these topics, identify insights relevant to effective pandemic responses, and highlight knowledge gaps and uncertainties. The topics of this review are: 1) Health and lives vs. economy and livelihoods, 2) Indefinite lockdown vs. unlimited reopening, 3) Symptomatic vs. asymptomatic severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, 4) Droplet vs. aerosol transmission of SARS-CoV-2, 5) Masks for all vs. no masking, and 6) SARS-CoV-2 reinfection vs. no reinfection. We discuss the importance of multidisciplinary integration (health, social, and physical sciences), multilayered approaches to reducing risk (“Emmentaler cheese model”), harm reduction, smart masking, relaxation of interventions, and context-sensitive policymaking for COVID-19 response plans. We also address the challenges in understanding the broad clinical presentation of COVID-19, SARS-CoV-2 transmission, and SARS-CoV-2 reinfection. These key issues of science and public health policy have been presented as false dichotomies during the pandemic. However, they are hardly binary, simple, or uniform, and therefore should not be framed as polar extremes. We urge a nuanced understanding of the science and caution against black-or-white messaging, all-or-nothing guidance, and one-size-fits-all approaches. There is a need for meaningful public health communication and science-informed policies that recognize shades of gray, uncertainties, local context, and social determinants of health.

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An upper bound on one-to-one exposure to infectious human respiratory particles.

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Gholamhossein Bagheri1, Birte Thiede1, Bardia Hejazi1, Oliver Schlenczek1, Eberhard Bodenschatz1 •
Max Planck Society1
07 Dec 2021-Proceedings of the National Academy of Sciences of the United States of America
TL;DR: In this article, the authors introduced the concept of an upper bound for one-to-one exposure to infectious human respiratory particles and applied it to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission.
Abstract: There is ample evidence that masking and social distancing are effective in reducing severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission. However, due to the complexity of airborne disease transmission, it is difficult to quantify their effectiveness, especially in the case of one-to-one exposure. Here, we introduce the concept of an upper bound for one-to-one exposure to infectious human respiratory particles and apply it to SARS-CoV-2. To calculate exposure and infection risk, we use a comprehensive database on respiratory particle size distribution; exhalation flow physics; leakage from face masks of various types and fits measured on human subjects; consideration of ambient particle shrinkage due to evaporation; and rehydration, inhalability, and deposition in the susceptible airways. We find, for a typical SARS-CoV-2 viral load and infectious dose, that social distancing alone, even at 3.0 m between two speaking individuals, leads to an upper bound of 90% for risk of infection after a few minutes. If only the susceptible wears a face mask with infectious speaking at a distance of 1.5 m, the upper bound drops very significantly; that is, with a surgical mask, the upper bound reaches 90% after 30 min, and, with an FFP2 mask, it remains at about 20% even after 1 h. When both wear a surgical mask, while the infectious is speaking, the very conservative upper bound remains below 30% after 1 h, but, when both wear a well-fitting FFP2 mask, it is 0.4%. We conclude that wearing appropriate masks in the community provides excellent protection for others and oneself, and makes social distancing less important.

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"Inward and outward effectiveness of..." refers background or methods in this paper

  • ...250 The efficiency curves exhibit the expected U shape with a minimum in most cases in the range 251 0.1–0.3 µm, where no collection mechanism is especially efficient (Hinds 1999)....

    [...]

  • ...We 286 further related the pressure drop to the material filtration efficiency using a filter quality factor (Q), 287 as defined by equation (2) (Hinds 1999; Podgórski, Bałazy and Gradoń 2006), where FE(DP) is 288 the material filtration efficiency at a particle size of DP, and ∆P is the pressure…...

    [...]

  • ...…the 442 masked deposition rate (MD) by combining the inward protection effectiveness (IPE) and the 443 International Commission and Radiological Protection (ICRP) model (Hinds 1999), as shown in 444 equation (3): 445 𝑀𝑀𝐷𝐷(𝐷𝐷𝑃𝑃) = (1− 𝐼𝐼𝑃𝑃𝐹𝐹(𝐷𝐷𝑃𝑃))�1 − 0.5 × 1 1 + 0.00076𝐷𝐷𝑃𝑃2....

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3,237 citations

Journal Article•DOI•
Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis.

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Derek K. Chu1, Elie A. Akl2, Elie A. Akl1, Stephanie Duda1, Karla Solo1, Sally Yaacoub2, Holger J. Schünemann1, Holger J. Schünemann3, Amena El-Harakeh, Antonio Bognanni, Tamara Lotfi, Mark Loeb1, Anisa Hajizadeh, Anna Bak, Ariel Izcovich, Carlos A. Cuello-Garcia, Chen Chen, David Harris, Ewa Borowiack, Fatimah Chamseddine, Finn Schünemann, Gian Paolo Morgano, Giovanna E.U. Muti Schünemann, Guang Chen, Hong Zhao, Ignacio Neumann, Jeffrey Chan, Joanne Khabsa, Layal Hneiny, Leila Harrison, Maureen A. Smith, Nesrine Rizk, Paolo Giorgi Rossi, Pierre AbiHanna, Rayane El-Khoury, Rosa Stalteri, Tejan Baldeh, Thomas Piggott, Yuan Zhang, Zahra Saad, Assem M. Khamis, Marge Reinap •
McMaster University1, American University of Beirut2, Cochrane Collaboration3
27 Jun 2020-The Lancet
TL;DR: The optimum distance for avoiding person-to-person virus transmission is investigated and the use of face masks and eye protection to prevent transmission of viruses is assessed to investigate the effects of physical distance, face masks, and eye Protection on virus transmission in health-care and non-health-care settings.

2,900 citations

"Inward and outward effectiveness of..." refers background in this paper

  • ...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)....

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Journal Article•DOI•
Respiratory virus shedding in exhaled breath and efficacy of face masks.

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Nancy H. L. Leung1, Daniel K.W. Chu1, Eunice Y.C. Shiu1, Kwok-Hung Chan1, James J. McDevitt2, Benien J.P. Hau1, Benien J.P. Hau3, Hui-Ling Yen1, Yuguo Li4, Dennis K. M. Ip1, J. S. Malik Peiris1, WH Seto1, WH Seto5, Gabriel M. Leung1, Donald K. Milton6, Benjamin J. Cowling1 •
Li Ka Shing Faculty of Medicine, University of Hong Kong1, Harvard University2, Queen Mary University of London3, University of Hong Kong4, Baptist Memorial Hospital-Memphis5, University of Maryland, College Park6
03 Apr 2020-Nature Medicine
TL;DR: A study of 246 individuals with seasonal respiratory virus infections randomized to wear or not wear a surgical face mask showed that masks can significantly reduce detection of coronavirus and influenza virus in exhaled breath and may help interrupt virus transmission.
Abstract: We identified seasonal human coronaviruses, influenza viruses and rhinoviruses in exhaled breath and coughs of children and adults with acute respiratory illness. Surgical face masks significantly reduced detection of influenza virus RNA in respiratory droplets and coronavirus RNA in aerosols, with a trend toward reduced detection of coronavirus RNA in respiratory droplets. Our results indicate that surgical face masks could prevent transmission of human coronaviruses and influenza viruses from symptomatic individuals.

1,827 citations

"Inward and outward effectiveness of..." refers background in this paper

  • ...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)....

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Journal Article•DOI•
Aerodynamic analysis of SARS-CoV-2 in two Wuhan hospitals.

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Yuan Liu1, Zhi Ning2, Yu Chen1, Ming Guo1, Yingle Liu1, Nirmal Kumar Gali2, Li Sun2, Yusen Duan, Jing Cai3, Dane Westerdahl2, Xinjin Liu1, Ke Xu1, Kin Fai Ho4, Haidong Kan3, Qingyan Fu, Ke Lan1 •
Wuhan University1, Hong Kong University of Science and Technology2, Fudan University3, The Chinese University of Hong Kong4
27 Apr 2020-Nature
TL;DR: It is proposed that room ventilation, open space, sanitization of protective apparel, and proper use and disinfection of toilet areas can effectively limit the concentration of SARS-CoV-2 RNA in aerosols, although the infectivity of the virus RNA was not established in this study.
Abstract: The ongoing outbreak of coronavirus disease 2019 (COVID-19) has spread rapidly on a global scale. Although it is clear that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is transmitted through human respiratory droplets and direct contact, the potential for aerosol transmission is poorly understood1-3. Here we investigated the aerodynamic nature of SARS-CoV-2 by measuring viral RNA in aerosols in different areas of two Wuhan hospitals during the outbreak of COVID-19 in February and March 2020. The concentration of SARS-CoV-2 RNA in aerosols that was detected in isolation wards and ventilated patient rooms was very low, but it was higher in the toilet areas used by the patients. Levels of airborne SARS-CoV-2 RNA in the most public areas was undetectable, except in two areas that were prone to crowding; this increase was possibly due to individuals infected with SARS-CoV-2 in the crowd. We found that some medical staff areas initially had high concentrations of viral RNA with aerosol size distributions that showed peaks in the submicrometre and/or supermicrometre regions; however, these levels were reduced to undetectable levels after implementation of rigorous sanitization procedures. Although we have not established the infectivity of the virus detected in these hospital areas, we propose that SARS-CoV-2 may have the potential to be transmitted through aerosols. Our results indicate that room ventilation, open space, sanitization of protective apparel, and proper use and disinfection of toilet areas can effectively limit the concentration of SARS-CoV-2 RNA in aerosols. Future work should explore the infectivity of aerosolized virus.

1,526 citations

"Inward and outward effectiveness of..." refers background in this paper

  • ...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)....

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Related Papers (5)
Respiratory virus shedding in exhaled breath and efficacy of face masks.

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Size distribution and sites of origin of droplets expelled from the human respiratory tract during expiratory activities

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Lidia Morawska, Graham R. Johnson, Zoran Ristovski, Megan Hargreaves, Kerrie Mengersen, Stephen Corbett, Christopher Y.H. Chao, Yuguo Li, David Katoshevski 
Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1.

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17 Mar 2020-The New England Journal of Medicine
Neeltje van Doremalen, Trenton Bushmaker, Dylan H. Morris, Myndi G. Holbrook, Amandine Gamble, Brandi N. Williamson, Azaibi Tamin, Jennifer L Harcourt, Natalie J. Thornburg, Susan I. Gerber, James O. Lloyd-Smith, Emmie de Wit, Vincent J. Munster 
Aerosol emission and superemission during human speech increase with voice loudness

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20 Feb 2019-Scientific Reports
Sima Asadi, Anthony S. Wexler, Christopher D. Cappa, Santiago Barreda, Nicole M. Bouvier, William D. Ristenpart 
Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks.

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24 Apr 2020-ACS Nano
Abhiteja Konda, Abhinav Prakash, Abhinav Prakash, Gregory A. Moss, Michael Schmoldt, Gregory D. Grant, Supratik Guha, Supratik Guha 
Frequently Asked Questions (1)
Q1. What contributions have the authors mentioned in the paper "Inward and outward effectiveness of cloth masks, a surgical mask, and a face shield" ?

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. The vacuum bag performed best, with efficiencies 13 of 54-96 % for all three metrics, depending on particle size. The authors calculated that the particle size most likely to 19 deposit in the respiratory tract when wearing a mask is ~2 μm. 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. It is made available under a is the author/funder, who has granted medRxiv a license to display the preprint in perpetuity. 

Trending Questions (1)
Where is the copper fit mask made?

Discrepancies between material filtration efficiency and inward/outward protection efficiency indicated that the fit of the mask was important.

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