Filtration Efficiencies of Nanoscale Aerosol by Cloth Mask Materials Used to Slow the Spread of SARS Cov-2 Christopher D

Subscriber access provided by the NIST Virtual Library (NVL) Article Filtration Efficiencies of Nanoscale Aerosol by Cloth Mask Materials Used to Slow the Spread of SARS CoV-2 Christopher D. Zangmeister, James G. Radney, Edward P Vicenzi, and Jamie Lynn Weaver ACS Nano, Just Accepted Manuscript • Publication Date (Web): 25 Jun 2020 Downloaded from pubs.acs.org on June 25, 2020 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. 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However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 37 ACS Nano 1 2 3 Filtration Efficiencies of Nanoscale Aerosol by Cloth Mask Materials Used to 4 5 Slow the Spread of SARS CoV-2 6 7 Christopher D. Zangmeister,1* James G. Radney,1 Edward P. Vicenzi,2,1** and Jamie L. 8 Weaver1,2** 9 10 1Material Measurement Laboratory, National Institute of Standards and Technology, 11 12 Gaithersburg, Maryland, 20899, USA 13 14 2Museum Conservation Institute, Smithsonian Institution, Suitland, Maryland, 20746, USA 15 16 *Corresponding author Christopher Zangmeister, ph: (301) 975-8079, e-mail: [email protected] 17 ** Second affiliations are via non-funded, guest research appointments at listed institutions 18 19 20 21 22 Abstract 23 24 Filtration efficiency (FE), differential pressure (ΔP), quality factor (QF) and construction 25 parameters were measured for 32 cloth materials (14 cotton, 1 wool, 9 synthetic, 4 synthetic blends, 26 27 and 4 synthetic/cotton blends) used in cloth masks intended for protection from the SARS CoV-2 28 virus (diameter 100 ± 10 nm). Seven polypropylene-based fiber filter materials were also 29 measured, including surgical masks and N95 respirators. Additional measurements were 30 performed on both multi-layered and mixed-material samples of natural, synthetic, or natural- 31 synthetic blends to mimic cloth mask construction methods. Materials were micro-imaged and 32 tested against size selected NaCl aerosol with particle mobility diameters between 50 nm and 825 33 34 nm. Three of the top five best performing samples were woven 100% cotton with high to moderate 35 yarn counts and the other two were woven synthetics of moderate yarn counts. In contrast to 36 recently published studies, samples utilizing mixed materials did not exhibit a significant 37 difference in the measured FE when compared to the product of the individual FE for the 38 components. The FE and ΔP increased monotonically with the number of cloth layers for a 39 lightweight flannel, suggesting that multi-layered cloth masks may offer increased protection from 40 nanometer-sized aerosol with a maximum FE dictated by breathability (i.e. ΔP). 41 42 43 44 Keywords: SARS-CoV-2, COVID-19, cloth masks, face masks, personal protection, aerosols, 45 respiratory protection 46 47 48 49 50 51 52 53 54 55 56 57 58 1 59 60 ACS Paragon Plus Environment ACS Nano Page 2 of 37 1 2 3 It has been recognized that the global spread of the Severe Acute Respiratory Syndrome 4 5 6 Coronavirus 2 (SARS CoV-2) has limited the supply of medical masks and particulate filtering 7 8 facepiece respirators (e.g. N95), so it has been recommended that their use be restricted to 9 10 healthcare settings.1 As of this writing the World Health Organization (WHO) and the United 11 12 States Centers for Disease Control and Prevention (US CDC) suggest that cloth masks be used in 13 14 15 non-medical settings to reduce the transmission of SARS CoV-2.2,3 The majority of states in the 16 17 United States and more than 130 nations, corresponding to over 75 % of the global population, 18 19 have issued official guidelines requiring or recommending the wearing of masks in public locations 20 21 22 to mitigate the spread of COVID-19, the disease caused by SARS CoV-2. Thus, non-medical use 23 24 of facial coverings will likely consist of masks made from a variety of fibrous cloth materials 25 26 utilizing a range of designs and construction techniques. The WHO has stated the need for rapid 27 28 29 dissemination of research investigating the performance of relevant mask parameters – e.g. 30 31 material, design and construction – for reducing exposure to micrometer-sized droplets and 32 33 nanometer-sized aerosol particles at size ranges relevant to the virus (90 nm diameter core with 34 35 ≈ 20 nm spike proteins).1,3,4 Particle measurements from hosptials in Wuhan, China demonstrated 36 37 38 that viable Covid-19 virus likely exists in particles with diameters between 250 nm and 500 nm.5 39 40 Facemasks can provide two modes of protection:6 1) by protecting the localized population 41 42 from an infected mask wearer by trapping expelled virus-laden atomized material (droplets and/or 43 44 45 aerosol), and 2) by protecting the mask wearer from ambient virus-laden atomized material by 46 47 filtering inhaled air.7 This reduces the risk of both direct and indirect viral exposure, respectively, 48 49 thereby decreasing the probability of infection.6 50 51 52 Prior research into cloth masks follows the history of infectious diseases spread by droplets 53 54 and/or aerosol and dates back to the 1918 - 1919 influenza pandemic.8,9 After that time the 55 56 57 58 2 59 60 ACS Paragon Plus Environment Page 3 of 37 ACS Nano 1 2 3 publication record on the utility of cloth masks is sparse and most research has focused on the 4 5 6 effectiveness of single use masks available in many developed countries for healthcare or other 7 8 occupational settings.10-16 The prior research shows that cloth materials offer limited protection 9 10 from particles in the size regime of SARS-COV2.10-12 11 12 When particles interact with a filter fiber, it is generally accepted that they are “collected” 13 14 15 by a fiber and retained through van Der Waals forces.17 For small particles, Brownian motion 16 17 increases the probability a particle will interact with a filter fiber. At larger sizes, particles can be 18 19 intercepted by a fiber when they are within one particle radius. When particle inertia becomes 20 21 22 sufficiently high, the particle may no longer follow a flowing gas streamline resulting in a higher 23 24 probability of inertially impacting a fiber. Electrostatic deposition, occurring due to a charge 25 26 difference between a fiber and a particle, can also be important in some materials. Collectively, 27 28 29 the sum of these efficiencies – diffusion (ED), interception (ER), impaction (EI) and electrostatic 30 31 deposition (EB) – yield the single fiber efficiency (EF = ED + ER + EI + EB). The reader is directed 32 33 to the seminal works by Brown,18 Emi,19 Fuchs,20,21 and Liu22 for detailed descriptions of filtration 34 35 theory. 36 37 38 The filtration efficiency (FE), is a common metric for reporting particle capture efficiency 39 18-23 40 of material and is related to EF through 41 42 ―4퐸퐹훼퐿 푁퐷 퐹퐸 = 1 ― exp = 1 ― (1) 43 ( 휋퐷푓 ) 푁푈 44 45 where α, L and D are the material porosity (i.e. packing density), filter thickness and fiber or yarn 46 f 47 48 diameter, respectively. The FE can be quantified by measuring the upstream and downstream (NU 49 -3 50 and ND, respectively) number density of particles per volume of air (particles cm ) where particles 51 52 ND passed through the filter and particles NU were incident on the filter (see Figure S1 of the 53 54 55 Supporting Information), and the difference between NU and ND represent the particles captured 56 57 58 3 59 60 ACS Paragon Plus Environment ACS Nano Page 4 of 37 1 2 3 by the filter. FE is reported as a percentage (by multiplying Eq. 1 by 100) and is a function of the 4 5 10-16, 24,25 6 particle diameter (Dm), the flow rate through the filter and the filter medium. 7 8 The variability of test methods (mask materials vs. fitted mask on human-scale forms vs. 9 10 standard mask testbed measurements) and the broad range of materials tested makes direct 11 12 quantitative comparability between published studies highly challenging.10-16 In studies before 13 14 15 2020, the published FE of cotton-based fabrics were 5 % to 60 % when tested against an NaCl 16 17 aerosol with 20 nm ≤ Dm ≤ 1000 nm, 10 % for spherical 100 nm latex particles, and < 10 % for 18 19 100 nm diesel soot.10-12 Generally, the published FE of cloth filter media are much lower when 20 21 22 compared to facepiece respirators that are regulated for use in healthcare and other professional 23 24 settings; e.g.

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