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Household Materials Selection for Homemade Cloth Face Coverings and Their Filtration Efficiency Enhancement with Triboelectric Charging

  • Mervin Zhao
    Mervin Zhao
    4C Air, Inc., Sunnyvale, California 94089, United States
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    Orcidhttp://orcid.org/0000-0002-7313-7150
  • , 
  • Lei Liao
    Lei Liao
    4C Air, Inc., Sunnyvale, California 94089, United States
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  • , 
  • Wang Xiao
    Wang Xiao
    4C Air, Inc., Sunnyvale, California 94089, United States
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  • , 
  • Xuanze Yu
    Xuanze Yu
    4C Air, Inc., Sunnyvale, California 94089, United States
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  • , 
  • Haotian Wang
    Haotian Wang
    4C Air, Inc., Sunnyvale, California 94089, United States
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  • , 
  • Qiqi Wang
    Qiqi Wang
    4C Air, Inc., Sunnyvale, California 94089, United States
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  • , 
  • Ying Ling Lin
    Ying Ling Lin
    World Health Organization, Geneva, CH-1211, Switzerland
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  • F. Selcen Kilinc-Balci
    F. Selcen Kilinc-Balci
    Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, National Personal Protective Technology Laboratory, Washington, DC 20201, United States
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  • Amy Price
    Amy Price
    Stanford Anesthesia Informatics and Media (AIM) Lab, Department of Anesthesiology, Perioperative, and Pain Medicine, Stanford University School of Medicine, Stanford, California 94305, United States
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  • , 
  • Larry Chu
    Larry Chu
    Stanford Anesthesia Informatics and Media (AIM) Lab, Department of Anesthesiology, Perioperative, and Pain Medicine, Stanford University School of Medicine, Stanford, California 94305, United States
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  • May C. Chu
    May C. Chu
    Colorado School of Public Health, University of Colorado, Aurora, Colorado 80045, United States
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  • Steven Chu
    Steven Chu
    Department of Physics, Stanford University, Stanford, California 94305, United States
    Department of Molecular and Cellular Physiology, Stanford University, Stanford, California 94305, United States
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  • , and 
  • Yi Cui*
    Yi Cui
    Department of Materials Science and Engineering, Stanford University, Stanford California 94305, United States
    Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States
    *Email: [email protected]
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    Orcidhttp://orcid.org/0000-0002-6103-6352
Cite this: Nano Lett. 2020, 20, 7, 5544–5552
Publication Date (Web):June 2, 2020
https://doi.org/10.1021/acs.nanolett.0c02211
Copyright © 2020 American Chemical Society
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The COVID-19 pandemic is currently causing a severe disruption and shortage in the global supply chain of necessary personal protective equipment (e.g., N95 respirators). The U.S. CDC has recommended use of household cloth by the general public to make cloth face coverings as a method of source control. We evaluated the filtration properties of natural and synthetic materials using a modified procedure for N95 respirator approval. Common fabrics of cotton, polyester, nylon, and silk had filtration efficiency of 5–25%, polypropylene spunbond had filtration efficiency 6–10%, and paper-based products had filtration efficiency of 10–20%. An advantage of polypropylene spunbond is that it can be simply triboelectrically charged to enhance the filtration efficiency (from 6 to >10%) without any increase in pressure (stable overnight and in humid environments). Using the filtration quality factor, fabric microstructure, and charging ability, we are able to provide an assessment of suggested fabric materials for homemade facial coverings.

KEYWORDS:

  Note

This article is made available via the ACS COVID-19 subset for unrestricted RESEARCH re-use and analyses in any form or by any means with acknowledgement of the original source. These permissions are granted for the duration of the World Health Organization (WHO) declaration of COVID-19 as a global pandemic.

The coronavirus disease 2019 (COVID-19) pandemic has caused more than six million confirmed infections and major global disruptions to daily life. (1) The disease is caused by infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The virus appears to be highly infectious and a major mode of transmission is thought to be spread from an infected person releasing virus-filled fluid droplets that may shrink due to evaporation and thereby aerosolize. (2−5) Larger particles >5 μm in diameter typically settle due to gravity and usually reach only the upper respiratory tract if inhaled. Meanwhile, fine particles with diameter <5 μm can critically reach the lower respiratory tract. (3,4,6) A detailed discussion of the symptoms as well as transmission are discussed in the Supporting Information.

For airborne particulates, including viral aerosols, the U.S. Centers for Disease Control and Prevention (CDC) recommends the use of N95 filtering facepiece respirators (FFR) as respiratory protection. (7−9) The N95 FFR designation is determined by the CDC’s National Institute for Occupational Safety and Health (NIOSH) and indicates a minimum filtration efficiency of 95% for particle sizes 0.022–0.259 μm (count median diameter of 0.075 ± 0.02 μm), according to 42 Code of Federal Regulations (CFR) Part 84. (10) As the viral aerosols are larger than the ∼0.120 μm virus itself, (11) N95 respirators are expected to provide suitable protection. (12,13) Meanwhile, medical face masks are used by healthcare workers during medical procedures to protect both the patient and the healthcare workers from the transfer of infectious microorganisms, body fluids, and particulate material. These masks are not recommended by the World Health Organization or the CDC for aerosol generating procedures. (9,14) A more detailed discussion of the approval requirements and usage of these two types of masks is given in the Supporting Information.

The widespread and intense response to caring for patients during the pandemic has led to disruptions of the global supply chain and shortage of appropriate personal protective equipment (PPE), especially a shortage of N95 FFRs for healthcare workers. (15,16) The WHO has recommended rationing the use of PPE and prioritization of PPE during severe shortages with FFRs reserved for healthcare professionals, leaving the general public without easy access to high-grade personal protective equipment. (17) During critical supply shortages, the possibility of disinfection and reuse of disposable FFRs has been proposed. (18) The CDC has recommended use of cloth face coverings (the WHO refers to these as “non-medical masks”) by the public to slow the spread of the virus, especially when social distancing measures are difficult to maintain. (19) These cloth face coverings can be fashioned from household items at a low cost and used as an additional control option to limit the release of larger infectious droplets from the wearer. (20,21) As some local governments are requiring the public to use cloth face coverings, it is reasonable to investigate what readily available and inexpensive materials may provide the public with some degree of protection against airborne viruses. The cloth mask material and construction would not be approved by NIOSH as an N95 FFR unless all applicable requirements of 42 CFR Part 84 were met.

We evaluated the filtration efficiency and pressure drop of common household materials of natural and synthetic origin using a modified version of the NIOSH standard test procedure with 0.075 ± 0.02 μm (count median diameter) NaCl aerosols (fabric samples were not preconditioned in any way and the flow rate was substantially reduced). The testing here did not account for real-world scenarios where the leakage around the edges of the face cover may significantly impact the actual effectiveness of these coverings. Hence, having a tight seal of the cloth around the face is imperative for these results to align with real usage conditions. All tests were conducted on an Automated Filter Tester 8130A (TSI, Inc.) with a flow rate of 32 L/min (unless otherwise specified). While FFR testing uses a flow of 85 L/min to simulate high intensity, a flow rate of 32 L/min was chosen which is similar to that in typical human breathing. (22) The filtration efficiency is the percentage of NaCl particles filtered by the material and the pressure drop is the air resistance across the filter material. Lower pressures indicate higher breathability. Additional information may be found in the Methods of the Supporting Information.

A commonly used filtration quality factor (Q) to determine the filter’s performance is defined (23)

where α (penetration) = 1 – E/100, E is the filtration efficiency (in %), and ΔP is the pressure drop across the filter (in kilopascals). A maximum Q results from a high filtration efficiency (low penetration) with low pressure drop, which is sensible for facial coverings. In addition, Q is not theoretically altered if multilayers of a singular type of filter material are considered, as penetration is multiplicative and pressure is additive.

Common household materials’ filtration properties are given in Table 1, optical images in Figure S1, and scanning electron microscope (SEM) images are given in Figure 1. The sources of the materials are given in the Methods. For reference as to how common household materials compare to PPE materials, one respirator media (polypropylene 1) and two medical face mask media (polypropylene 2 and 3) were also tested. The quality factor of the respirator grade polypropylene 1 is ∼160 kPa−1 and far exceeds any of the other materials. While previous reports show that surgical mask filtration efficiency can vary from 10 to 96% (85 L/min air flow), (24) we were only able to obtain two medical face mask brands, both of which had a filtration efficiency of ∼20–30% (Q ~ 5 kPa−1).

Table 1. Evaluation of Reference and Common Materials’ Filtration Propertiesa
material source structure basis weight (g·m–2) bulk density (basis weight/thickness) (g·m–2·μm–1) initial filtration efficiency (%) initial pressure drop (Pa) filter quality factor, Q (kPa–1)
Personal Protection Materials
polypropylene 1 particulate FFR meltblown (nonwoven) 25 0.17 95.94 ± 2.00 9.0 ± 2.0 162.7 ± 21.3
polypropylene 2 medical face mask meltblown (nonwoven) 26 0.21 33.06 ± 0.95 34.3 ± 0.5 5.0 ± 0.1
polypropylene 3 medical face mask meltblown (nonwoven) 20 0.20 18.81 ± 0.50 16.3 ± 0.5 5.5 ± 0.1
Household Materials
polypropylene 4 (PP-4) interfacing material, purchased as-is spunbond (nonwoven) 30 0.26 6.15 ± 2.18 1.6 ± 0.5 16.9 ± 3.4
cotton 1b pillow cover woven 116 0.57 5.04 ± 0.64 4.5 ± 2.1 5.4 ± 1.9
cotton 2b clothing (t-shirt) knit 157 0.37 21.62 ± 1.84 14.5 ± 2.1 7.4 ± 1.7
cotton 3b clothing (sweater) knit 360 0.45 25.88 ± 1.41 17.0 ± 0.0 7.6 ± 0.4
polyester clothing (toddler wrap) knit 200 0.38 17.50 ± 5.10 12.3 ± 0.5 6.8 ± 2.4
silk napkin woven 84 0.54 4.77 ± 1.47 7.3 ± 1.5 2.8 ± 0.4
nylon clothing (exercise pants) woven 164 0.70 23.33 ± 1.18 244.0 ± 5.5 0.4 ± 0.0
cellulose 1 paper towel bonded 42.9 0.33 10.41 ± 0.28 11.0 ± 0.0 4.3 ± 2.8
cellulose 2 tissue paper bonded 32.8 0.39 20.2 ± 0.32 19.0 ± 1.0 5.1 ± 3.2
cellulose 3 copy paper bonded 72.8 0.76 99.85 ± 0.02 1883.6 ± 39.3 1.5 ± 0.2
a

All materials were tested in samples of three unless denoted. Uncertainties denoted here represent the standard deviation between the samples. Materials in bold were further studied to investigate if the simple triboelectric charging can positively impact the filtration properties. For the particulate FFR sample, the meltblown is independently procured for usage in FFRs as it is difficult to obtain enough sample to test with the filter tester.

b

Indicates this sample did not have enough material and data is presented in samples of two.

Figure 1

Figure 1. SEM images of the microscopic structure of various household materials. All images are given in pairs. The first of the pairs has a scale bar (left, black bar in white background) corresponding to 300 μm. The second of the pairs has a scale bar (right, white bar in black background) corresponding to 75 μm. (a–c) Polypropylene samples from PPE (a,b) and common spunbond (c). (d–f) Cotton samples, as given in Table 1. (g–i) Polyester, silk, and nylon samples, respectively. (j–l) Other cellulose-based products, paper towel, tissue paper, and printing paper, as per given in Table 1.

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From the microscopic images in Figure 1a,b, the PPE meltblown nonwoven has microfibers of various diameters, typically around 1–10 μm with large distances between fibers (tens of microns). The structure is bulky and clearly three-dimensional with multilayers of fibers. In contrast, the polypropylene spunbond (PP-4) sample (Figure 1c), is composed of relatively uniform fibers of size ∼20 μm. The nonwoven structure makes for a random network of fibers with select spots that are bonded together (left corners in Figure 1c). The spunbond PP-4 has a large pore size reaching ∼100 μm. Both of these nonwoven structures with random fiber networks have a large porosity and lower pressure drops. Though PP-4 has a lower filtration efficiency due to the larger fiber diameter and pores compared to the other polypropylenes, its Q ∼ 16.9 kPa−1 is among the highest in Table 1. We note a large difference in Q and filtration efficiency between polypropylenes 2 and 3 (Figure 1b as representative) and polypropylene 1, even though all are produced from the meltblown process. This most likely resulted from the difference in electrostatic charge (discussed later in the text).

Previous reports show that cloth face coverings had 10–60% instantaneous penetration levels when challenged with polydisperse NaCl aerosols. (25,26) In this study, the cotton samples taken from common household materials all exhibited similar Q, though they had various construction and filtration efficiencies. Microscopically, we see that Cotton 1 has a finer fiber diameter (∼10 μm) compared to Cottons 2–3 (∼20 μm). All the fibers are bundled into yarns of similar size, ∼150 μm. However, in Cotton 1 (Figure 1d) clear pores of ∼100 μm can be observed, whereas in Cotton 2 and 3 (Figure 1e,f) there are no such clear pores and yarn-to-yarn gaps are not as apparent. The clear pores in Cotton 1 can leak both particles and air through, which explains why it has much lower filtration efficiency of ∼5% and lower pressure drop of ∼2.5 Pa, compared to Cotton 2 and 3 (20–26% filtration efficiency, 14–17 Pa pressure drop). Based on the data in Table 1, the basis weight and density are not clearly related to the efficiency, as Cotton 3 has nearly double the basis weight of Cotton 2, but the filtration efficiency increase is only moderate. We note that the Cotton 2 and 3 filtration properties were comparable to some grades of medical face masks. As cotton is a very common material for clothing, it would be beneficial to the public to select cotton construction with the highest filtration quality factor. The cotton should be woven/knit at a high density such that there are no visible pores under light. If a lower density cotton is used, it may be best to use multilayers.

Among polyester, silk, and nylon, the most apparent feature is the high thread count in the nylon sample, leading to the high pressure (>200 Pa, whereas most other materials are <20 Pa). The nylon fibers are approximately 10 μm in diameter and in bundles of 200 μm (Figure 1i). It may be possible to procure nylon with a lower thread count, resulting in a lower pressure drop and higher Q, and it may be more suitable for facial coverings. Comparatively, we can see that the polyester sample (fleece-like fabric source) is composed of more randomly oriented fibers of 10 μm on the surface (Figure 1g). The polyester Q is comparable to cotton’s and has similar filtration efficiency to some cotton fabrics (dependent on bulk density). Silk is composed of similar fiber sizes and yarns of 100 μm (Figure 1h). From the SEM images, the silk sample has gaps between the yarns of ∼50 μm, which led to the leaking of air/particles and thus the lower filtration efficiency and pressure drop.

Finally, among the paper-based products, we see moderate filtration performance with the paper towel or tissue paper, but unsuitable pressure drop in printing paper. In the paper towel and tissue paper, the Q is comparable to some of the previous fabrics with a slightly higher pressure drop. These products may be suitable to use as a disposable media in some homemade facial coverings, such as between cotton for an increase in filtration efficiency, though their performance in high humidity environments needs to be examined in future work. These products are both similar to thicker, randomly oriented cellulose fibers (Figure 1j,k). The difference between these two and printing paper is how compressed the sample is (Figure 1l). Printing paper is clearly a much more two-dimensional dense-packed structure, leading to its very high pressure drop with little or no pores in the material. All these materials have a low mechanical strength, which would require particular care if integrated into face coverings.

Among these common household materials tested, we see that PP-4 (polypropylene spunbond, 30 g/m2) is a relatively high performing common material with Q ∼ 17 kPa–1, 2–5 times higher than the other materials (arising from the lower pressure drop). While not as common as cotton, polypropylene spunbond is an inexpensive material that can be found in hobby fabric shops, some reusable bags, mattress covers, hygiene products, and disposable work wear. As Q does not change with multilayers, it suggests that the spunbond material may be suitable as a multilayer structure for facial coverings. In fact, using five layers of the spunbond (30 g/m2) experimentally yields filtration efficiency of ∼24% (8 Pa), Q of ∼15 kPa–1 (Table S1). Considering filtration properties alone, multilayer polypropylene material is superior or at least comparable to materials used in some medical face masks (polypropylene 2 and 3).

Both medical face masks and FFRs are typically composed of polypropylene nonwoven fabrics, and in both cases the primary filtration layer is produced via melt-blowing. (27,28) The meltblown layer’s polypropylene microfibers have diameters in the range of ∼1–10 μm and a fabric thickness of 100–1000 μm. The lofty nature, high porosity, and fine fiber diameter should not be adequate for fine particle filtration used in respirators by itself. (29) To improve the filtration efficiency while preserving high air permeability, these fibers are charged through “corona discharge” and/or triboelectric means into electrets with quasi-permanent dipoles. (30−32) Once charged, the filter can significantly increase its filtration efficiency without adding any mass or density to the structure. As the basis weights of the meltblown fabrics used in the medical face mask and FFR are similar, this suggests that the meltblown used in the FFR has been charged and the meltblown of both medical face masks may not have undergone any charging process. Qualitatively, the meltblown in the FFR and medical face mask (Figure 1a,b) also has some difference in the density, which contributes to the pressure difference. Thus, achieving a high-performing filter requires both a suitable filter morphology/geometry and a high degree of injected electrical charge.

It is worth exploring whether simple triboelectric charging can positively impact the filtration properties of the materials highlighted in blue in Table 1. While it is difficult to charge the samples in the same way as electrets are made in nonwoven meltblown media in a nonindustrial setting, the act of triboelectrically creating some surface charge to mimic an electret filter may be a way to increase the filtration efficiency for a time duration enough for the public’s temporary usage (Figure 2a).

Figure 2

Figure 2. Filtration properties of common materials before and after charging. Each uncertainty represents the standard deviation of three samples (excluding cotton which are from two samples). (a) Schematic of effect of charging fibers using triboelectricity. Particle filtration efficiency can be enhanced using charges generated from the triboelectric effect. (b) Filtration efficiency of materials before and after charging. Silk, nylon, polyester, and PP-4 all have increases in filtration efficiency after rubbing with latex. Cotton’s efficiency after charging was unchanged or decreased in efficiency, possibly due to abrasion and/or pore size expansion. (c) Quality factor, Q, of household materials before and after charging. Results are roughly in agreement with data presented in (b), except nylon which had lower performance due to pressure drop and PP-4 is much better performing due to low pressure drop. (d,e) Filtration efficiency (d) and Q (e) of various polypropylene spunbond fabrics with different basis weights.

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The triboelectric effect is a well-known method, commonly used to demonstrate static electricity. (33) However, the microscopic mechanism of triboelectricity is still not completely clear. Between solids, it has recently been found that the contact electrification is most likely due to an electron transfer between the two materials. (34) In general, when two different materials come into contact with one another, their electron clouds overlap (forming a transient chemical bond). As two different materials approach equilibrium chemical potential, a decrease in the interatomic distance may allow for electron transfer between the two atoms. The transferred electron is only slightly bound to the surface atoms after the two materials have separated from each other.

For the purposes of charging an arbitrary material, two materials with different charge affinities should be able to triboelectrically create surface charges on each other. This has led to well-known triboelectric series, (35) which has also recently been quantified. (36) We selected latex to initially charge the samples, being a commonly found rubber product. We rubbed the sample for 30 s using a pair of latex gloves and recorded the filtration performance before and immediately after treatment (Figure 2b,c and Supporting Movie M1).

All three cotton samples had a decreased or unchanged filtration efficiency, while all other samples had an increase in filtration efficiency. The decrease in the cotton fabrics’ filtration efficiency may be due to the pore size expansion produced by rubbing or even damage to the sample from the abrasion. It suggests that mechanical damage, friction, or stretching the cotton can all cause the filtration efficiency to decrease, and these effects should be considered for cotton face coverings. On the other hand, all other samples reported moderate to high increases in filtration efficiency when tested immediately after charging. Examining Q shows that PP-4 has the highest performance due to the low pressure drop; polyester is also within a comparable range after charging. The low Q value of nylon is due to the high pressure drop (because of the very tight weave of this synthetic) with slight increase after charging. After charging, silk has a high initial value (unsurprising as it is also a commonly used material to demonstrate static in the classroom setting), but the higher pressure drop of silk yields a lower Q, when compared to PP-4 or polyester.

In order to see if this effect can be generalized to other polypropylene fabrics, we tested additional polypropylene spunbond samples of different basis weights (Figure 2d,e, data in Table S2). We see that among three different basis weights of polypropylene spunbond (25, 30, 40 g/m2), the filtration efficiency and Q are very similar (∼5–10% initial efficiency charged to ∼20% efficiency with initial Q = ∼10–20 kPa–1 charged to ∼50 kPa–1). However, for the 60 g/m2 sample, the initial efficiency is much higher than the other samples. At the same time, the pressure is much higher as well (∼130 Pa, Table S2), which leads to a lower Q < 10 kPa–1. For the 70 g/m2 sample, it also has a higher filtration efficiency and pressure drop, but the effect of charging was not as significant as the lower basis weight samples. We note that the 70 g/m2 sample was colored pink (Figure S1), whereas the other spunbond samples were white. Addition of additives may affect charging, depending on the additives’ composition. While charging improves the efficiency (and Q) of all polypropylene samples, the effect was most prevalent in the lower basis weight samples tested here. Due to sample limitations as well as its behavior, PP-4 was used for all remaining experiments as a representative of polypropylene spunbond.

Static charge will inevitably dissipate due to adsorption of water molecules in the air, or discharge through contact with other surfaces. Therefore, we first evaluated natural, ambient decay (samples were placed on a tabletop without any covering, the temperature and humidity were approximately constant at 22 °C, 40% RH), as plotted in Figure 3a,b. Clearly, all the materials exhibited discharge from their initial charging value (denoted by time “0” here, where t0 denotes the values before charging). From Figure 3a, we see that both polyester and silk decay relatively quickly, reaching a plateau near the initial value at around 30 min. Nylon and PP-4 on the other hand have a much slower decay, with the PP-4 overnight value essentially remaining constant, within error.

Figure 3

Figure 3. Time evolution of filtration properties of common materials. Each uncertainty represents the standard deviation of three samples. (a) Filtration efficiency decay in ambient conditions (listed in the main text), up to overnight. (b) Evolution of Q as a function of time, up to overnight. (c) Evolution of filtration efficiency in a humid and warmer environment, up to an hour. (d) Evolution of the PP-4 Q as a function of the time, up to an hour.

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It has been reported that polymers, especially hydrophilic polymers, are able to adsorb layers of water molecules from ambient humidity. Once adsorbed, water molecules in particular can essentially allow for a low concentration of ions (due to dissociation reactions of surface groups) that can discharge the generated triboelectric charge. (37) This is a reason why static charge is more apparent in dry environments, as the ambient moisture in the air is not enough to screen the charges generated through triboelectric means. In addition, cotton (and to a lesser extent silk) is hygroscopic, and this explains why cotton is very difficult to charge with latex, even when it is apparent that cotton clothes from a laundry dryer can have static on them.

When considering the surface chemistry of the fibers’ polymer groups, we note that polypropylene, predictably, is the most hydrophobic (it has only hydrocarbon linkages). The remaining nylon (polyamide), polyester, and silk (protein) have components which would make them more hydrophilic or less hydrophobic than polypropylene. This effect is further magnified when conditioning the materials in a humid environment of 38 °C, 85% (Figure 3c,d), which is used to mimic the exhalation temperature and moisture content. A humid environment was tested by charging the fabrics and placing them in an environmental chamber (SH-642) and measuring at the selected times (Methods in Supporting Information). We found that nylon (which was able to retain the charge well in ambient conditions) decayed to the initial value within 1 min in a humid environment and remained constant at this value for the remaining time. These conditions are also similar to the preconditioning used for FFRs in NIOSH STP0059 (85 ± 5% RH, 38 ± 2.5 °C for 25 ± 1 h) prior to measuring filter penetration. (38) On the other hand, the hydrophobic PP-4 was able to roughly stay consistent with the results that were conditioned in ambient conditions, and a considerable amount of static was retained on the sample after an hour (efficiency >10% after aging, with an uncharged value of ∼6%). The general observations here are consistent with previous studies which found that multilayers of water molecules can adsorb onto the surface polyamide (nylon), but hydrophobic polystyrene has little water adsorption. (37)

Hydrophobic polymeric materials may be considered for simple triboelectric methods for increasing filtration properties. Though polypropylene is the most common of these, some types of polyester or polyurethane fabrics can potentially be used in the same manner (or as an external/protective layer for polypropylenes like PP-4, if used as the filtration layer). More study is needed on this area to determine other common polymers which can be charged to retain their static, or multilayer polymers which can charge within a homemade face covering through interlayer friction.

In order to offer options for community use, we tested charging the PP-4 using various other common “charging” materials (Figure 4). We found that latex and nitrile rubbers were the most promising in increasing the filtration efficiency, and various other materials only had moderate (paper and wood) or negligible effect on charging the PP-4. Fortunately, latex and nitrile are commonly used glove materials, which would make the frequent charging of the material relatively easy from a user-application standpoint (i.e., rubbing the mask with gloved hands before putting on).

Figure 4

Figure 4. Efficiency after charging PP-4 with different materials. Each uncertainty represents the standard deviation of three samples.

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We note that recent work has also shown that face coverings of similar fabric materials to have reported filtration efficiencies of ∼10–90%. (26) The difference between these results and our findings may arise from differences in instrumentation, testing method, and source of material. Our study chose following a modified version of the test procedure that NIOSH uses for the approval of N95 filtering facepiece respirators.

A summary of the results described in this manuscript is presented in Table 2, ranked by filtration quality factor, Q. Cotton, polyester, and polypropylene multilayered structures can meet or even exceed the efficiency of materials used in some medical face masks. However, the exact number of layers, basis weight, and thread-count of material will need to be considered in addition to the fluid resistance and performance under breathing. In our work (Table S1), we found that a five-layer structure of PP-4 after charging can achieve filtration efficiency ∼50%. Although the medical face masks tested here (∼19–33% filtration efficiency) have a pressure drop of roughly 16–34 Pa, the five-layer structure can achieve a higher filtration efficiency and pressure drop <10 Pa. This is particularly relevant, as there is a concern in the sealing of both disposable medical masks and cloth face coverings. We reiterate that these filtration efficiencies are only applicable if there is no leakage in the seals of the masks, as loose-fitting devices such as these coverings and medical masks do not have any gasket or tight-fitting mechanism to ensure a proper seal. The leakage of air around the seal areas is significant and can contribute to real-world exposure to aerosols. (24,26) Previous reports suggest that 60% of users fail the fitting of surgical masks on first attempt. (24) This risk is also carried over in cloth face coverings that do not have any special form of sealing. When designing new facial coverings for community use, it is advisible to make users aware of this risk and to design cloth face coverings with pressure drop across the covering is as low as possible (with filtration efficiency as high as possible), otherwise air contaminants (particulates, viruses, infectious droplets, etc.) will preferentially flow through gaps and leaks at the skin and cloth contact points limiting any effectiveness of filtration in a de facto form of respiratory protection. The general public should be aware of the risks of self-contamination during removal and reuse of cloth face coverings. Finally, an important distinction to make is that surgical masks are designed and intended as a form of barrier protection and provide fluid resistance for use in hospitals. The materials evaluated in this study for cloth face coverings are not intended to be used by healthcare workers or any other workers as a form of respiratory protection. Further, we did not investigate the effects of cleaning or disinfecting of the materials studied. These effects could be evaluated in future work.

Table 2. Summary and Ranking of Materials Tested Here Based on Filtration Quality Factor, Q, with Relevant Comments for Each Material
∼Q (kPa–1) ∼filtration efficiency (%) material comments
>100 >95 polypropylene meltblown (charged) material found in FFRs (used for reference)
30 10–20 charged polypropylene (PP-4) charged value after overnight, polypropylene spunbonds can vary (different basis weight has different efficiency), charging increased the Q in all cases
15 5–10 uncharged polypropylene (PP-4) initial polypropylene spunbond fabrics can vary in efficiency, but most tested had low pressure drops
5–10 5–20 cotton cotton fabrics can vary in initial pressure drop, select cotton fabrics without any visible pores under light illumination or use multilayer configurations
5–10 20 polyester similar properties and comments as cotton
5 30 polypropylene meltblown (uncharged) material found in medical face masks (used for reference)
5 10–20 tissue paper, paper towel low mechanical strength, but may be possible to integrate into some masks with other cloths as a composite material
<5 5 silk silk can be considered for use if cotton and/or polyester are unavailable
<1 20 nylon (woven) the nylon tested in this study had very high pressure drop. If using nylon for masks the fabric needs to have a lower pressure drop to be effective

Personal protective equipment, such as surgical masks cleared for sale by the FDA and respiratory protection approved by NIOSH, comprise only one aspect of a hierarchy of infection prevention and control measures. The WHO and CDC recommend that other measures also be used with masks or respirators. (14,39) These additional measures as well as the efficacy of cloth coverings are covered in the Supporting Information.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.0c02211.

  • Discussion on COVID-19 symptoms and transmission, details of the testing procedures used for NIOSH and FDA approval for masks, public health measures suggested by the CDC and WHO, experimental methods, optical images, and additional tables (PDF)

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  • Corresponding Author
    • Yi Cui - Department of Materials Science and Engineering, Stanford University, Stanford California 94305, United States;  Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States;  Orcidhttp://orcid.org/0000-0002-6103-6352;  Email: [email protected]
  • Authors
    • Mervin Zhao - 4C Air, Inc., Sunnyvale, California 94089, United States;  Orcidhttp://orcid.org/0000-0002-7313-7150
    • Lei Liao - 4C Air, Inc., Sunnyvale, California 94089, United States
    • Wang Xiao - 4C Air, Inc., Sunnyvale, California 94089, United States
    • Xuanze Yu - 4C Air, Inc., Sunnyvale, California 94089, United States
    • Haotian Wang - 4C Air, Inc., Sunnyvale, California 94089, United States
    • Qiqi Wang - 4C Air, Inc., Sunnyvale, California 94089, United States
    • Ying Ling Lin - World Health Organization, Geneva, CH-1211, Switzerland
    • F. Selcen Kilinc-Balci - Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, National Personal Protective Technology Laboratory, Washington, DC 20201, United States
    • Amy Price - Stanford Anesthesia Informatics and Media (AIM) Lab, Department of Anesthesiology, Perioperative, and Pain Medicine, Stanford University School of Medicine, Stanford, California 94305, United States
    • Larry Chu - Stanford Anesthesia Informatics and Media (AIM) Lab, Department of Anesthesiology, Perioperative, and Pain Medicine, Stanford University School of Medicine, Stanford, California 94305, United States
    • May C. Chu - Colorado School of Public Health, University of Colorado, Aurora, Colorado 80045, United States
    • Steven Chu - Department of Physics, Stanford University, Stanford, California 94305, United States;  Department of Molecular and Cellular Physiology, Stanford University, Stanford, California 94305, United States
  • Notes
    The authors declare the following competing financial interest(s): Professors Steven Chu and Yi Cui are founders and shareholders of the company 4C Air, Inc. They are inventors on patent PCT /US2015/065608. Authors Mervin Zhao, Lei Liao, Wang Xiao, Xuanze Yu, Haotian Wang, and Qiqi Wang are employed by 4C Air, Inc.

References

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    BACKGROUND: Respiratory protection devices are used to protect the wearers from inhaling particles suspended in the air. Filtering face piece respirators are usually tested utilizing nonbiologic particles, whereas their use often aims at reducing exposure to biologic aerosols, including infectious agents such as viruses and bacteria. METHODS: The performance of 2 types of N95 half-mask, filtering face piece respirators and 2 types of surgical masks were determined. The collection efficiency of these respiratory protection devices was investigated using MS2 virus (a nonharmful simulant of several pathogens). The virions were detected in the particle size range of 10 to 80 nm. RESULTS: The results indicate that the penetration of virions through the National Institute for Occupational Safety and Health (NIOSH)-certified N95 respirators can exceed an expected level of 5%. As anticipated, the tested surgical masks showed a much higher particle penetration because they are known to be less efficient than the N95 respirators. The 2 surgical masks, which originated from the same manufacturer, showed tremendously different penetration levels of the MS2 virions: 20.5% and 84.5%, respectively, at an inhalation flow rate of 85 L/min. CONCLUSION: The N95 filtering face piece respirators may not provide the expected protection level against small virions. Some surgical masks may let a significant fraction of airborne viruses penetrate through their filters, providing very low protection against aerosolized infectious agents in the size range of 10 to 80 nm. It should be noted that the surgical masks are primarily designed to protect the environment from the wearer, whereas the respirators are supposed to protect the wearer from the environment.
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    Journal of Occupational and Environmental Hygiene (2012), 9 (12), 681-690CODEN: JOEHA2; ISSN:1545-9624. (Taylor & Francis, Inc.)
    Few studies have quantified the dispersion of potentially infectious bioaerosols produced by patients in the health care environment and the exposure of health care workers to these particles. Controlled studies are needed to assess the spread of bioaerosols and the efficacy of different types of respiratory personal protective equipment (PPE) in preventing airborne disease transmission. An environmental chamber was equipped to simulate a patient coughing aerosol particles into a medical examn. room, and a health care worker breathing while exposed to these particles. The system has three main parts: (1) a coughing simulator that expels an aerosol-laden cough through a head form; (2) a breathing simulator with a second head form that can be fitted with respiratory PPE; and (3) aerosol particle counters to measure concns. inside and outside the PPE and at locations throughout the room. Dispersion of aerosol particles with optical diams. from 0.3 to 7.5 μm was evaluated along with the influence of breathing rate, room ventilation, and the locations of the coughing and breathing simulators. Penetration of cough aerosol particles through nine models of surgical masks and respirators placed on the breathing simulator was measured at 32 and 85 L/min flow rates and compared with the results from a std. filter tester. Results show that cough-generated aerosol particles spread rapidly throughout the room, and that within 5 min, a worker anywhere in the room would be exposed to potentially hazardous aerosols. Aerosol exposure is highest with no personal protective equipment, followed by surgical masks, and the least exposure is seen with N95 FFRs. These differences are seen regardless of breathing rate and relative position of the coughing and breathing simulators. These results provide a better understanding of the exposure of workers to cough aerosols from patients and of the relative efficacy of different types of respiratory PPE, and they will assist investigators in providing research-based recommendations for effective respiratory protection strategies in health care settings.
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    ACS Nano (2020), 14 (5), 6339-6347CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
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  • Abstract

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    Figure 1

    Figure 1. SEM images of the microscopic structure of various household materials. All images are given in pairs. The first of the pairs has a scale bar (left, black bar in white background) corresponding to 300 μm. The second of the pairs has a scale bar (right, white bar in black background) corresponding to 75 μm. (a–c) Polypropylene samples from PPE (a,b) and common spunbond (c). (d–f) Cotton samples, as given in Table 1. (g–i) Polyester, silk, and nylon samples, respectively. (j–l) Other cellulose-based products, paper towel, tissue paper, and printing paper, as per given in Table 1.

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    Figure 2

    Figure 2. Filtration properties of common materials before and after charging. Each uncertainty represents the standard deviation of three samples (excluding cotton which are from two samples). (a) Schematic of effect of charging fibers using triboelectricity. Particle filtration efficiency can be enhanced using charges generated from the triboelectric effect. (b) Filtration efficiency of materials before and after charging. Silk, nylon, polyester, and PP-4 all have increases in filtration efficiency after rubbing with latex. Cotton’s efficiency after charging was unchanged or decreased in efficiency, possibly due to abrasion and/or pore size expansion. (c) Quality factor, Q, of household materials before and after charging. Results are roughly in agreement with data presented in (b), except nylon which had lower performance due to pressure drop and PP-4 is much better performing due to low pressure drop. (d,e) Filtration efficiency (d) and Q (e) of various polypropylene spunbond fabrics with different basis weights.

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    Figure 3

    Figure 3. Time evolution of filtration properties of common materials. Each uncertainty represents the standard deviation of three samples. (a) Filtration efficiency decay in ambient conditions (listed in the main text), up to overnight. (b) Evolution of Q as a function of time, up to overnight. (c) Evolution of filtration efficiency in a humid and warmer environment, up to an hour. (d) Evolution of the PP-4 Q as a function of the time, up to an hour.

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    Figure 4

    Figure 4. Efficiency after charging PP-4 with different materials. Each uncertainty represents the standard deviation of three samples.

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    This article references 39 other publications.

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      Dong, E.; Du, H.; Gardner, L. An Interactive Web-Based Dashboard to Track COVID-19 in Real Time. Lancet Infect. Dis. 2020, 20, 533– 534,  DOI: 10.1016/S1473-3099(20)30120-1
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      An interactive web-based dashboard to track COVID-19 in real time
      Dong, Ensheng; Du, Hongru; Gardner, Lauren
      Lancet Infectious Diseases (2020), 20 (5), 533-534CODEN: LIDABP; ISSN:1473-3099. (Elsevier Ltd.)
      The authors describe the development of an online interactive dashboard, hosted by the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University, Baltimore, MD, USA, to visualize and track reported cases of coronavirus disease 2019 (COVID-19) in real time. The dashboard, first shared publicly on Jan 22, illustrates the location and no. of confirmed COVID-19 cases, deaths, and recoveries for all affected countries. It was developed to provide researchers, public health authorities, and the general public with a user-friendly tool to track the outbreak as it unfolds. All data collected and displayed are made freely available, initially through Google Sheets and now through a GitHub repository, along with the feature layers of the dashboard, which are now included in the Esri Living Atlas.
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    2. 2
      Christian, M. D.; Loutfy, M.; McDonald, L. C.; Martinez, K. F.; Ofner, M.; Wong, T.; Wallington, T.; Gold, W. L.; Mederski, B.; Green, K.; Low, D. E. Possible SARS Coronavirus Transmission during Cardiopulmonary Resuscitation. Emerging Infect. Dis. 2004, 10, 287– 293,  DOI: 10.3201/eid1002.030700
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      Possible SARS coronavirus transmission during cardiopulmonary resuscitation
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      Emerging infectious diseases (2004), 10 (2), 287-93 ISSN:1080-6040.
      Infection of healthcare workers with the severe acute respiratory syndrome-associated coronavirus (SARS-CoV) is thought to occur primarily by either contact or large respiratory droplet transmission. However, infrequent healthcare worker infections occurred despite the use of contact and droplet precautions, particularly during certain aerosol-generating medical procedures. We investigated a possible cluster of SARS-CoV infections in healthcare workers who used contact and droplet precautions during attempted cardiopulmonary resuscitation of a SARS patient. Unlike previously reported instances of transmission during aerosol-generating procedures, the index case-patient was unresponsive, and the intubation procedure was performed quickly and without difficulty. However, before intubation, the patient was ventilated with a bag-valve-mask that may have contributed to aerosolization of SARS-CoV. On the basis of the results of this investigation and previous reports of SARS transmission during aerosol-generating procedures, a systematic approach to the problem is outlined, including the use of the following: 1) administrative controls, 2) environmental engineering controls, 3) personal protective equipment, and 4) quality control.
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      Yan, J.; Grantham, M.; Pantelic, J.; De Mesquita, P. J. B.; Albert, B.; Liu, F.; Ehrman, S.; Milton, D. K. Infectious Virus in Exhaled Breath of Symptomatic Seasonal Influenza Cases from a College Community. Proc. Natl. Acad. Sci. U. S. A. 2018, 115, 1081– 1086,  DOI: 10.1073/pnas.1716561115
      3
      Infectious virus in exhaled breath of symptomatic seasonal influenza cases from a college community
      Yan, Jing; Grantham, Michael; Pantelic, Jovan; Bueno de Mesquita, P. Jacob; Albert, Barbara; Liu, Fengjie; Ehrman, Sheryl; Milton, Donald K.
      Proceedings of the National Academy of Sciences of the United States of America (2018), 115 (5), 1081-1086CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Little is known about the amt. and infectiousness of influenza virus shed into exhaled breath. This contributes to uncertainty about the importance of airborne influenza transmission. We screened 355 symptomatic volunteers with acute respiratory illness and report 142 cases with confirmed influenza infection who provided 218 paired nasopharyngeal (NP) and 30-min breath samples (coarse >5-μm and fine ≤5-μm fractions) on days 1-3 after symptom onset. We assessed viral RNA copy no. for all samples and cultured NP swabs and fine aerosols. We recovered infectious virus from 52 (39%) of the fine aerosols and 150 (89%) of the NP swabs with valid cultures. The geometric mean RNA copy nos. were 3.8 × 104/30-min fine-, 1.2 × 104/30-min coarse-aerosol sample, and 8.2 × 108 per NP swab. Fine- and coarse-aerosol viral RNA were pos. assocd. with body mass index and no. of coughs and neg. assocd. with increasing days since symptom onset in adjusted models. Fine-aerosol viral RNA was also pos. assocd. with having influenza vaccination for both the current and prior season. NP swab viral RNA was pos. assocd. with upper respiratory symptoms and neg. assocd. with age but was not significantly assocd. with fine- or coarse-aerosol viral RNA or their predictors. Sneezing was rare, and sneezing and coughing were not necessary for infectious aerosol generation. Our observations suggest that influenza infection in the upper and lower airways are compartmentalized and independent.
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      Tellier, R. Review of Aerosol Transmission of Influenza A Virus. Emerging Infect. Dis. 2006, 12, 1657– 1662,  DOI: 10.3201/eid1211.060426
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      Review of aerosol transmission of influenza A virus
      Tellier Raymond
      Emerging infectious diseases (2006), 12 (11), 1657-62 ISSN:1080-6040.
      In theory, influenza viruses can be transmitted through aerosols, large droplets, or direct contact with secretions (or fomites). These 3 modes are not mutually exclusive. Published findings that support the occurrence of aerosol transmission were reviewed to assess the importance of this mode of transmission. Published evidence indicates that aerosol transmission of influenza can be an important mode of transmission, which has obvious implications for pandemic influenza planning and in particular for recommendations about the use of N95 respirators as part of personal protective equipment.
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      Huang, H.; Fan, C.; Li, M.; Nie, H. L.; Wang, F. B.; Wang, H.; Wang, R.; Xia, J.; Zheng, X.; Zuo, X.; Huang, J. COVID-19: A Call for Physical Scientists and Engineers. ACS Nano 2020, 14, 3747– 3754,  DOI: 10.1021/acsnano.0c02618
      5
      COVID-19: A Call for Physical Scientists and Engineers
      Huang, Haiyue; Fan, Chunhai; Li, Min; Nie, Hua-Li; Wang, Fu-Bing; Wang, Hui; Wang, Ruilan; Xia, Jianbo; Zheng, Xin; Zuo, Xiaolei; Huang, Jiaxing
      ACS Nano (2020), 14 (4), 3747-3754CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      A review. The COVID-19 pandemic is one of those global challenges that transcends territorial, political, ideol., religious, cultural, and certainly academic boundaries. Public health and healthcare workers are at the frontline, working to contain and to mitigate the spread of this disease. Although intervening biol. and immunol. responses against viral infection may seem far from the phys. sciences and engineering that typically work with inanimate objects, there actually is much that can-and should-be done to help in this global crisis. In this Perspective, we convert the basics of infectious respiratory diseases and viruses into phys. sciences and engineering terms, and through this exercise, we present examples of questions, hypotheses, and research needs identified based on clinicians' experiences. We hope researchers in the phys. sciences and engineering will proactively study these challenges, develop new hypotheses, define new research areas, and work with biol. researchers, healthcare, and public health professionals to create user-centered solns. and to inform the general public, so that we can better address the many challenges assocd. with the transmission and spread of infectious respiratory diseases.
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      Lindsley, W. G.; Blachere, F. M.; Thewlis, R. E.; Vishnu, A.; Davis, K. A.; Cao, G.; Palmer, J. E.; Clark, K. E.; Fisher, M. A.; Khakoo, R.; Beezhold, D. H. Measurements of Airborne Influenza Virus in Aerosol Particles from Human Coughs. PLoS One 2010, 5, e15100,  DOI: 10.1371/journal.pone.0015100
      6
      Measurements of airborne influenza virus in aerosol particles from human coughs
      Lindsley, William G.; Blachere, Francoise M.; Thewlis, Robert E.; Vishnu, Abhishek; Davis, Kristina A.; Cao, Gang; Palmer, Jan E.; Clark, Karen E.; Fisher, Melanie A.; Khakoo, Rashida; Beezhold, Donald H.
      PLoS One (2010), 5 (11), e15100CODEN: POLNCL; ISSN:1932-6203. (Public Library of Science)
      Influenza is thought to be communicated from person to person by multiple pathways. However, the relative importance of different routes of influenza transmission is unclear. To better understand the potential for the airborne spread of influenza, we measured the amt. and size of aerosol particles contg. influenza virus that were produced by coughing. Subjects were recruited from patients presenting at a student health clinic with influenza-like symptoms. Nasopharyngeal swabs were collected from the volunteers and they were asked to cough three times into a spirometer. After each cough, the cough-generated aerosol was collected using a NIOSH two-stage bioaerosol cyclone sampler or an SKC BioSampler. The amt. of influenza viral RNA contained in the samplers was analyzed using quant. real-time reverse-transcription PCR (qPCR) targeting the matrix gene M1. For half of the subjects, viral plaque assays were performed on the nasopharyngeal swabs and cough aerosol samples to det. if viable virus was present. 58 Subjects were tested, of whom 47 were pos. for influenza virus by qPCR. Influenza viral RNA was detected in coughs from 38 of these subjects (81%). 35% Of the influenza RNA was contained in particles >4 μm in aerodynamic diam., while 23% was in particles 1 to 4 μm and 42% in particles <1 μm. Viable influenza virus was detected in the cough aerosols from 2 of 21 subjects with influenza. These results show that coughing by influenza patients emits aerosol particles contg. influenza virus and that much of the viral RNA is contained within particles in the respirable size range. The results support the idea that the airborne route may be a pathway for influenza transmission, esp. in the immediate vicinity of an influenza patient. Further research is needed on the viability of airborne influenza viruses and the risk of transmission.
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      NIOSH Interim Guidance on Infection Control Measures for 2009 H1N1 Influenza in Healthcare Settings, Including Protection of Healthcare Personnel. Miss. RN 2009, 71, 13– 18
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      CDC. Healthcare Infection Prevention and Control FAQs for COVID-19. https://www.cdc.gov/coronavirus/2019-ncov/hcp/infection-control-faq.html (accessed May 24, 2020).
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      Rosenstock, L. 42 CFR Part 84: Respiratory Protective Devices Implications for Tuberculosis Protection. Infect. Control Hosp. Epidemiol. 1995, 16, 529– 531,  DOI: 10.1086/647174
      10
      42 CFR Part 84: Respiratory protective devices implications for tuberculosis protection
      Rosenstock L
      Infection control and hospital epidemiology (1995), 16 (9), 529-31 ISSN:0899-823X.
      There is no expanded citation for this reference.
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    11. 11
      Matsuyama, S.; Nao, N.; Shirato, K.; Kawase, M.; Saito, S.; Takayama, I.; Nagata, N.; Sekizuka, T.; Katoh, H.; Kato, F.; Sakata, M.; Tahara, M.; Kutsuna, S.; Ohmagari, N.; Kuroda, M.; Suzuki, T.; Kageyama, T.; Takeda, M. Enhanced Isolation of SARS-CoV-2 by TMPRSS2- Expressing Cells. Proc. Natl. Acad. Sci. U. S. A. 2020, 117, 7001– 7003,  DOI: 10.1073/pnas.2002589117
      11
      Enhanced isolation of SARS-CoV-2 by TMPRSS2-expressing cells
      Matsuyama, Shutoku; Nao, Naganori; Shirato, Kazuya; Kawase, Miyuki; Saito, Shinji; Takayama, Ikuyo; Nagata, Noriyo; Sekizuka, Tsuyoshi; Katoh, Hiroshi; Kato, Fumihiro; Sakata, Masafumi; Tahara, Maino; Kutsuna, Satoshi; Ohmagari, Norio; Kuroda, Makoto; Suzuki, Tadaki; Kageyama, Tsutomu; Takeda, Makoto
      Proceedings of the National Academy of Sciences of the United States of America (2020), 117 (13), 7001-7003CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      A novel betacoronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which caused a large respiratory outbreak in Wuhan, China in Dec. 2019, is currently spreading across many countries globally. Here, we show that a TMPRSS2-expressing VeroE6 cell line is highly susceptible to SARS-CoV-2 infection, making it useful for isolating and propagating SARS-CoV-2. Our results reveal that, in common with SARS- and Middle East respiratory syndrome-CoV, SARS-CoV-2 infection is enhanced by TMPRSS2.
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    12. 12
      Loeb, M.; Dafoe, N.; Mahony, J.; John, M.; Sarabia, A.; Glavin, V.; Webby, R.; Smieja, M.; Earn, D. J. D.; Chong, S.; Webb, A.; Walter, S. D. Surgical Mask vs N95 Respirator for Preventing Influenza among Health Care Workers: A Randomized Trial. JAMA - J. Am. Med. Assoc. 2009, 302, 1865– 1871,  DOI: 10.1001/jama.2009.1466
      12
      Surgical mask vs N95 respirator for preventing influenza among health care workers: A randomized trial
      Loeb, Mark; Dafoe, Nancy; Mahony, James; John, Michael; Sarabia, Alicia; Glavin, Verne; Webby, Richard; Smieja, Marek; Earn, David J. D.; Chong, Sylvia; Webb, Ashley; Walter, Stephen D.
      JAMA, the Journal of the American Medical Association (2009), 302 (17), 1865-1871CODEN: JAMAAP; ISSN:0098-7484. (American Medical Association)
      Context Data about the effectiveness of the surgical mask compared with the N95 respirator for protecting health care workers against influenza are sparse. Given the likelihood that N95 respirators will be in short supply during a pandemic and not available in many countries, knowing the effectiveness of the surgical mask is of public health importance. Objective To compare the surgical mask with the N95 respirator in protecting health care workers against influenza. Design, Setting, and Participants Noninferiority randomized controlled trial of 446 nurses in emergency departments, medical units, and pediatric units in 8 tertiary care Ontario hospitals. Intervention Assignment to either a fit-tested N95 respirator or a surgical mask when providing care to patients with febrile respiratory illness during the 2008-2009 influenza season. Main Outcome Measures The primary outcome was lab.-confirmed influenza measured by polymerase chain reaction or a 4-fold rise in hemagglutinin titers. Effectiveness of the surgical mask was assessed as noninferiority of the surgical mask compared with the N95 respirator. The criterion for noninferiority was met if the lower limit of the 95% confidence interval (CI) for the redn. in incidence (N95 respirator minus surgical group) was greater than -9%. Results Between Sept. 23, 2008, and Dec. 8, 2008, 478 nurses were assessed for eligibility and 446 nurses were enrolled and randomly assigned the intervention; 225 were allocated to receive surgical masks and 221 to N95 respirators. Influenza infection occurred in 50 nurses (23.6%) in the surgical mask group and in 48 (22.9%) in the N95 respirator group (abs. risk difference, -0.73%; 95% CI, -8.8% to 7.3%; P =.86), the lower confidence limit being inside the noninferiority limit of -9%. Conclusion Among nurses in Ontario tertiary care hospitals, use of a surgical mask compared with an N95 respirator resulted in noninferior rates of lab.- confirmed influenza.
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    13. 13
      Bałazy, A.; Toivola, M.; Adhikari, A.; Sivasubramani, S. K.; Reponen, T.; Grinshpun, S. A. Do N95 Respirators Provide 95% Protection Level against Airborne Viruses, and How Adequate Are Surgical Masks?. Am. J. Infect. Control 2006, 34, 51– 57,  DOI: 10.1016/j.ajic.2005.08.018
      13
      Do N95 respirators provide 95% protection level against airborne viruses, and how adequate are surgical masks?
      Balazy Anna; Toivola Mika; Adhikari Atin; Sivasubramani Satheesh K; Reponen Tiina; Grinshpun Sergey A
      American journal of infection control (2006), 34 (2), 51-7 ISSN:0196-6553.
      BACKGROUND: Respiratory protection devices are used to protect the wearers from inhaling particles suspended in the air. Filtering face piece respirators are usually tested utilizing nonbiologic particles, whereas their use often aims at reducing exposure to biologic aerosols, including infectious agents such as viruses and bacteria. METHODS: The performance of 2 types of N95 half-mask, filtering face piece respirators and 2 types of surgical masks were determined. The collection efficiency of these respiratory protection devices was investigated using MS2 virus (a nonharmful simulant of several pathogens). The virions were detected in the particle size range of 10 to 80 nm. RESULTS: The results indicate that the penetration of virions through the National Institute for Occupational Safety and Health (NIOSH)-certified N95 respirators can exceed an expected level of 5%. As anticipated, the tested surgical masks showed a much higher particle penetration because they are known to be less efficient than the N95 respirators. The 2 surgical masks, which originated from the same manufacturer, showed tremendously different penetration levels of the MS2 virions: 20.5% and 84.5%, respectively, at an inhalation flow rate of 85 L/min. CONCLUSION: The N95 filtering face piece respirators may not provide the expected protection level against small virions. Some surgical masks may let a significant fraction of airborne viruses penetrate through their filters, providing very low protection against aerosolized infectious agents in the size range of 10 to 80 nm. It should be noted that the surgical masks are primarily designed to protect the environment from the wearer, whereas the respirators are supposed to protect the wearer from the environment.
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      World Health Organization. Advice on the Use of Masks in the Context of COVID-19 (Interim Guidance) ; 2020.
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      Artenstein, A. W. In Pursuit of PPE. N. Engl. J. Med. 2020, 382, e46  DOI: 10.1056/NEJMc2010025
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      Ranney, M. L.; Griffeth, V.; Jha, A. K. Critical Supply Shortages — The Need for Ventilators and Personal Protective Equipment during the Covid-19 Pandemic. N. Engl. J. Med. 2020, 382, e41  DOI: 10.1056/NEJMp2006141
      16
      Critical supply shortages - the need for ventilators and personal protective equipment during the Covid-19 pandemic
      Ranney, Megan L.; Griffeth, Valerie; Jha, Ashish K.
      New England Journal of Medicine (2020), 382 (18), e41CODEN: NEJMAG; ISSN:1533-4406. (Massachusetts Medical Society)
      As the United States braces for a growing wave of patients with Covid-19 in our hospitals and ICUs, we must ensure that we have the key equipment needed to care for patients and to keep our health care workforce safe. Failure to act in a coordinated manner would keep many patients from getting the care they need and would lead to the situation we see in Italy, in which frontline clinicians are making difficult decisions about who will and who won't receive care. Furthermore, without adequate PPE, health care workers will get sick, endangering the functioning of the entire health care system. The human and economic costs of that scenario should not be underestimated.
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      World Health Organization. Rational Use of Personal Protective Equipment for Coronavirus Disease (COVID-19) and Considerations during Severe Shortages: Interim Guidance, 6 April 2020; World Health Organization, 2020.
      There is no corresponding record for this reference.
    18. 18
      Liao, L.; Xiao, W.; Zhao, M.; Yu, X.; Wang, H.; Wang, Q.; Chu, S.; Cui, Y. Can N95 Respirators Be Reused after Disinfection? How Many Times?. ACS Nano 2020, 14, 6348,  DOI: 10.1021/acsnano.0c03597
      18
      Can N95 Respirators Be Reused after Disinfection? How Many Times?
      Liao, Lei; Xiao, Wang; Zhao, Mervin; Yu, Xuanze; Wang, Haotian; Wang, Qiqi; Chu, Steven; Cui, Yi
      ACS Nano (2020), 14 (5), 6348-6356CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The coronavirus disease 2019 (COVID-19) pandemic has led to a major shortage of N95 respirators, which are essential for protecting healthcare professionals and the general public who may come into contact with the virus. Thus, it is essential to det. how we can reuse respirators and other personal protective equipment in these urgent times. We investigated multiple commonly used disinfection schemes on media with particle filtration efficiency of 95%. Heating was recently found to inactivate the virus in soln. within 5 min at 70°C and is among the most scalable, user-friendly methods for viral disinfection. We found that heat (≤85°C) under various humidities (≤100% relative humidity, RH) was the most promising, nondestructive method for the preservation of filtration properties in meltblown fabrics as well as N95-grade respirators. At 85°C, 30% RH, we were able to perform 50 cycles of heat treatment without significant changes in the filtration efficiency. At low humidity or dry conditions, temps. up to 100°C were not found to alter the filtration efficiency significantly within 20 cycles of treatment. UV irradn. was a secondary choice, which was able to withstand 10 cycles of treatment and showed small degrdn. by 20 cycles. However, UV can potentially impact the material strength and subsequent sealing of respirators. Finally, treatments involving liqs. and vapors require caution, as steam, alc., and household bleach all may lead to degrdn. of the filtration efficiency, leaving the user vulnerable to the viral aerosols.
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      CDC. Public Health Recommendations for Community-Related Exposure. https://www.cdc.gov/coronavirus/2019-ncov/php/public-health-recommendations.html (accessed May 24, 2020).
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      CDC. Recommendation Regarding the Use of Cloth Face Coverings, Especially in Areas of Significant Community-Based Transmission. https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/cloth-face-cover.html (accessed May 24, 2020).
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      Adams, J., Recommendation Regarding the Use of Cloth Face Coverings, CDC ; 2020.
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      Lindsley, W. G.; King, W. P.; Thewlis, R. E.; Reynolds, J. S.; Panday, K.; Cao, G.; Szalajda, J. V. Dispersion and Exposure to a Cough-Generated Aerosol in a Simulated Medical Examination Room. J. Occup. Environ. Hyg. 2012, 9, 681– 690,  DOI: 10.1080/15459624.2012.725986
      22
      Dispersion and exposure to a cough-generated aerosol in a simulated medical examination room
      Lindsley, William G.; King, William P.; Thewlis, Robert E.; Reynolds, Jeffrey S.; Panday, Kedar; Cao, Gang; Szalajda, Jonathan V.
      Journal of Occupational and Environmental Hygiene (2012), 9 (12), 681-690CODEN: JOEHA2; ISSN:1545-9624. (Taylor & Francis, Inc.)
      Few studies have quantified the dispersion of potentially infectious bioaerosols produced by patients in the health care environment and the exposure of health care workers to these particles. Controlled studies are needed to assess the spread of bioaerosols and the efficacy of different types of respiratory personal protective equipment (PPE) in preventing airborne disease transmission. An environmental chamber was equipped to simulate a patient coughing aerosol particles into a medical examn. room, and a health care worker breathing while exposed to these particles. The system has three main parts: (1) a coughing simulator that expels an aerosol-laden cough through a head form; (2) a breathing simulator with a second head form that can be fitted with respiratory PPE; and (3) aerosol particle counters to measure concns. inside and outside the PPE and at locations throughout the room. Dispersion of aerosol particles with optical diams. from 0.3 to 7.5 μm was evaluated along with the influence of breathing rate, room ventilation, and the locations of the coughing and breathing simulators. Penetration of cough aerosol particles through nine models of surgical masks and respirators placed on the breathing simulator was measured at 32 and 85 L/min flow rates and compared with the results from a std. filter tester. Results show that cough-generated aerosol particles spread rapidly throughout the room, and that within 5 min, a worker anywhere in the room would be exposed to potentially hazardous aerosols. Aerosol exposure is highest with no personal protective equipment, followed by surgical masks, and the least exposure is seen with N95 FFRs. These differences are seen regardless of breathing rate and relative position of the coughing and breathing simulators. These results provide a better understanding of the exposure of workers to cough aerosols from patients and of the relative efficacy of different types of respiratory PPE, and they will assist investigators in providing research-based recommendations for effective respiratory protection strategies in health care settings.
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      Podgórski, A.; Bałazy, A.; Gradoń, L. Application of Nanofibers to Improve the Filtration Efficiency of the Most Penetrating Aerosol Particles in Fibrous Filters. Chem. Eng. Sci. 2006, 61, 6804– 6815,  DOI: 10.1016/j.ces.2006.07.022
      23
      Application of nanofibers to improve the filtration efficiency of the most penetrating aerosol particles in fibrous filters
      Podgorski, Albert; Balazy, Anna; Gradon, Leon
      Chemical Engineering Science (2006), 61 (20), 6804-6815CODEN: CESCAC; ISSN:0009-2509. (Elsevier Ltd.)
      Conventional, mech. fibrous filters made of microfibers exhibit a local min. of fractional collection efficiency in the aerosol particle size-range between 100 and 500 nm, which is called the most penetrating particle size (MPPS). Simple theor. calcns. predict that this efficiency may be significantly increased using nanofibrous media. The main objective of this paper is an exptl. verification of these expectations and simultaneously checking whether this anticipated gain in the filtration efficiency is not overpaid with an excessive pressure drop. For this purpose we developed a modified melt-blown technol., which allowed us to produce filters composed of micrometer as well as nanometer sized fibers. One conventional microfibrous filter and five nanofibrous filters were examd. The complete structural characteristics, pressure drop and efficiency of removal of aerosol particles with diams. 10-500 nm were detd. for all media. The results of the expts. confirmed that using nanofibrous filters a significant growth of filtration efficiency for the MPPS range can be achieved and the pressure drop rises moderately. Simultaneously, we noticed a shift of the MPPS towards smaller particles. Consequently, the quality factor for bilayer systems composed of a microfibrous support and a nanofibrous facial layer was considerably higher than this one for a conventional microfibrous filter alone. Addnl., it was found that utilization of many-layer nanofibrous filters combined with a single microfibrous backing layer is even more profitable from the quality factor standpoint. Comparing exptl. results with theor. calcns. based on the single-fiber theory we concluded that for microfibrous filters a fairly good agreement can be obtained if the resistance-equiv. fiber diam. is used in calcns. instead of the mean count diam. detd. from the SEM images anal.; in the latter case, filtration efficiency computed theor. is slightly overestimated. This is even more evident for nanofibrous media, suggesting that in such case a structural filter inhomogeneity has a strong influence on the filter efficiency and its resistance and one should strive for minimization of this effect manufg. nanofibrous filters as homogeneous as possible. We can finally conclude that fibrous filters contg. nanofibers, which are produced using the melt-blown technique, are very promising and economic tools to enhance filtration of the most penetrating aerosol particles.
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    24. 24
      Oberg, T.; Brosseau, L. M. Surgical Mask Filter and Fit Performance. Am. J. Infect. Control 2008, 36, 276– 282,  DOI: 10.1016/j.ajic.2007.07.008
      24
      Surgical mask filter and fit performance
      Oberg Tara; Brosseau Lisa M
      American journal of infection control (2008), 36 (4), 276-82 ISSN:.
      BACKGROUND: Surgical masks have been used since the early 1900s to minimize infection of surgical wounds from wearer-generated bacteria. There is ongoing debate, however, whether surgical masks can meet the expectations of respiratory protection devices. The goal of this study was to evaluate the filter performance and facial fit of a sample of surgical masks. METHODS: Filter penetration was measured for at least 3 replicates of 9 surgical masks using monodisperse latex sphere aerosols (0.895, 2.0, and 3.1 microm) at 6 L/min and 0.075-microm sodium chloride particles at 84 L/min. Facial fit was measured on 20 subjects for the 5 masks with lowest particle penetration, using both qualitative and quantitative fit tests. RESULTS: Masks typically used in dental settings collected particles with significantly lower efficiency than those typically used in hospital settings. All subjects failed the unassisted qualitative fit test on the first exercise (normal breathing). Eighteen subjects failed the assisted qualitative fit tests; 60% failed on the first exercise. Quantitative fit factors ranged from 2.5 to 9.6. CONCLUSION: None of these surgical masks exhibited adequate filter performance and facial fit characteristics to be considered respiratory protection devices.
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    25. 25
      Rengasamy, S.; Eimer, B.; Shaffer, R. E. Simple Respiratory Protection - Evaluation of the Filtration Performance of Cloth Masks and Common Fabric Materials against 20–1000 Nm Size Particles. Ann. Occup. Hyg. 2010, 54, 789– 798
      25
      Simple Respiratory Protection-Evaluation of the Filtration Performance of Cloth Masks and Common Fabric Materials Against 20-1000 nm Size Particles
      Rengasamy, Samy; Eimer, Benjamin; Shaffer, Ronald E.
      Annals of Occupational Hygiene (2010), 54 (7), 789-798CODEN: AOHYA3; ISSN:0003-4878. (Oxford University Press)
      A shortage of disposable filtering facepiece respirators can be expected during a pandemic respiratory infection such as influenza A. Some individuals may want to use common fabric materials for respiratory protection because of shortage or affordability reasons. To address the filtration performance of common fabric materials against nano-size particles including viruses, five major categories of fabric materials including sweatshirts, T-shirts, towels, scarves, and cloth masks were tested for polydisperse and monodisperse aerosols (20-1000 nm) at two different face velocities (5.5 and 16.5 cm s-1) and compared with the penetration levels for N95 respirator filter media. The results showed that cloth masks and other fabric materials tested in the study had 40-90% instantaneous penetration levels against polydisperse NaCl aerosols employed in the National Institute for Occupational Safety and Health particulate respirator test protocol at 5.5 cm s-1. Similarly, varying levels of penetrations (9-98%) were obtained for different size monodisperse NaCl aerosol particles in the 20-1000 nm range. The penetration levels of these fabric materials against both polydisperse and monodisperse aerosols were much higher than the penetrations for the control N95 respirator filter media. At 16.5 cm s-1 face velocity, monodisperse aerosol penetrations slightly increased, while polydisperse aerosol penetrations showed no significant effect except one fabric mask with an increase. Results obtained in the study show that common fabric materials may provide marginal protection against nanoparticles including those in the size ranges of virus-contg. particles in exhaled breath.
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    26. 26
      Konda, A.; Prakash, A.; Moss, G. A.; Schmoldt, M.; Grant, G. D.; Guha, S. Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks. ACS Nano 2020, 14, 6339,  DOI: 10.1021/acsnano.0c03252
      26
      Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks
      Konda, Abhiteja; Prakash, Abhinav; Moss, Gregory A.; Schmoldt, Michael; Grant, Gregory D.; Guha, Supratik
      ACS Nano (2020), 14 (5), 6339-6347CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      The emergence of a pandemic affecting the respiratory system can result in a significant demand for face masks. This includes the use of cloth masks by large sections of the public, as can be seen during the current global spread of COVID-19. However, there is limited knowledge available on the performance of various commonly available fabrics used in cloth masks. Importantly, there is a need to evaluate filtration efficiencies as a function of aerosol particulate sizes in the 10 nm to 10μm range, which is particularly relevant for respiratory virus transmission. We have carried out these studies for several common fabrics including cotton, silk, chiffon, flannel, various synthetics, and their combinations. Although the filtration efficiencies for various fabrics when a single layer was used ranged from 5 to 80% and 5 to 95% for particle sizes of <300 nm and >300 nm, resp., the efficiencies improved when multiple layers were used and when using a specific combination of different fabrics. Filtration efficiencies of the hybrids (such as cotton-silk, cotton-chiffon, cotton-flannel) was >80% (for particles <300 nm) and >90% (for particles >300 nm). We speculate that the enhanced performance of the hybrids is likely due to the combined effect of mech. and electrostatic-based filtration. Cotton, the most widely used material for cloth masks performs better at higher weave densities (i.e., thread count) and can make a significant difference in filtration efficiencies. Our studies also imply that gaps (as caused by an improper fit of the mask) can result in over a 60% decrease in the filtration efficiency, implying the need for future cloth mask design studies to take into account issues of "fit" and leakage, while allowing the exhaled air to vent efficiently. Overall, we find that combinations of various commonly available fabrics used in cloth masks can potentially provide significant protection against the transmission of aerosol particles.
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    27. 27
      Angadjivand, S. A.; Brandner, J. M.; Springett, J. E. Molded Respirator Comprising Meltblown Fiber Web with Staple Fibers. US 7989372 B2, 2011.
      There is no corresponding record for this reference.
    28. 28
      Gaynor, M.; McManus, J. Spunbonded/Meltblown/Spunbonded Laminate Face Mask. US 20040000313 A1, 2002.
      There is no corresponding record for this reference.
    29. 29
      Ghosal, A.; Sinha-Ray, S.; Yarin, A. L.; Pourdeyhimi, B. Numerical Prediction of the Effect of Uptake Velocity on Three-Dimensional Structure, Porosity and Permeability of Meltblown Nonwoven Laydown. Polymer 2016, 85, 19– 27,  DOI: 10.1016/j.polymer.2016.01.013
      29
      Numerical prediction of the effect of uptake velocity on three-dimensional structure, porosity and permeability of meltblown nonwoven laydown
      Ghosal, Arkaprovo; Sinha-Ray, Suman; Yarin, Alexander L.; Pourdeyhimi, Behnam
      Polymer (2016), 85 (), 19-27CODEN: POLMAG; ISSN:0032-3861. (Elsevier Ltd.)
      This work describes the first detailed model of meltblowing process which allows prediction of such integral laydown properties as thickness, porosity and permeability. Also, such laydown properties as the detailed three-dimensional micro-structure, fiber-size distribution and polymer mass distribution are predicted. The effects of the governing meltblowing parameters on the variation of all these laydown properties are accounted for, with the influence of the collector screen velocity being in focus. For this aim numerical solns. of the system of quasi-one-dimensional equations of the dynamics of free liq. polymer jets moving, cooling and solidifying when driven by surrounding air jet are constructed. Multiple polymer jets are considered simultaneously when they are deposited on a moving screen and forming a nonwoven laydown. The results reveal the three-dimensional configuration of the laydown and, in particular, its porosity and permeability, as well as elucidate the dependence of the laydown structure on the forming conditions, in particular, on the velocity of the screen motion. It is shown and explained how an increase in the velocity of the collector screen increases porosity and permeability of the meltblown nonwoven laydown.
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    30. 30
      Angadjivand, S. A.; Jones, M. E.; Meyer, D. E. Method of Charging Electret Filter Media. US 6119691 A 1996.
      There is no corresponding record for this reference.
    31. 31
      Angadjivand, S. A.; Jones, M. E.; Meyer, D. E. Electret Filter Media. US 6119691 A, 1994.
      There is no corresponding record for this reference.
    32. 32
      Kubik, D. A.; Davis, C. I. Melt-Blown Fibrous Electrets. US 4215682 A, 1980.
      There is no corresponding record for this reference.
    33. 33
      Henniker, J. Triboelectricity in Polymers. Nature 1962, 196, 474– 474,  DOI: 10.1038/196474a0
      33
      Triboelectricity in polymers
      Henniker, J.
      Nature (London, United Kingdom) (1962), 196 (), 474CODEN: NATUAS; ISSN:0028-0836.
      A triboelec. series of 43 natural and synthetic polymers is presented, extending from SiO2-filled silicone elastomer at the pos. end to poly(tetrafluoroethylene). The Fermi level is the most reasonable series-detg. property, and it is suggested that mech. produced free radicals are the source of conducting electrons.
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    34. 34
      Xu, C.; Zi, Y.; Wang, A. C.; Zou, H.; Dai, Y.; He, X.; Wang, P.; Wang, Y.-C.; Feng, P.; Li, D.; Wang, Z. L. On the Electron-transfer Mechanism in the Contact-electrification Effect. Adv. Mater. 2018, 30, 1706790,  DOI: 10.1002/adma.201706790
      There is no corresponding record for this reference.
    35. 35
      Lee, B. W.; Orr, D. E. The Triboelectric Series. https://www.alphalabinc.com/triboelectric-series/ (accessed May 24, 2020).
      There is no corresponding record for this reference.
    36. 36
      Zou, H.; Zhang, Y.; Guo, L.; Wang, P.; He, X.; Dai, G.; Zheng, H.; Chen, C.; Wang, A. C.; Xu, C.; Wang, Z. L. Quantifying the Triboelectric Series. Nat. Commun. 2019, 10, 1– 9,  DOI: 10.1038/s41467-019-09461-x
      36
      All-small-molecule organic solar cells with over 14% efficiency by optimizing hierarchical morphologies
      Zhou, Ruimin; Jiang, Zhaoyan; Yang, Chen; Yu, Jianwei; Feng, Jirui; Adil, Muhammad Abdullah; Deng, Dan; Zou, Wenjun; Zhang, Jianqi; Lu, Kun; Ma, Wei; Gao, Feng; Wei, Zhixiang
      Nature Communications (2019), 10 (1), 1-9CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      The high efficiency all-small-mol. org. solar cells (OSCs) normally require optimized morphol. in their bulk heterojunction active layers. Herein, a small-mol. donor is designed and synthesized, and single-crystal structural analyses reveal its explicit mol. planarity and compact intermol. packing. A promising narrow bandgap small-mol. with absorption edge of more than 930 nm along with our home-designed small mol. is selected as electron acceptors. To the best of our knowledge, the binary all-small-mol. OSCs achieve the highest efficiency of 14.34% by optimizing their hierarchical morphologies, in which the donor or acceptor rich domains with size up to ca. 70 nm, and the donor crystals of tens of nanometers, together with the donor-acceptor blending, are proved coexisting in the hierarchical large domain. All-small-mol. photovoltaic system shows its promising for high performance OSCs, and our study is likely to lead to insights in relations between bulk heterojunction structure and photovoltaic performance.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXitlSqs7%252FJ&md5=b75c7ad1867b22dbd07808b557eda87e
    37. 37
      Németh, E.; Albrecht, V.; Schubert, G.; Simon, F. Polymer Tribo-Electric Charging: Dependence on Thermodynamic Surface Properties and Relative Humidity. J. Electrost. 2003, 58, 3– 16,  DOI: 10.1016/S0304-3886(02)00137-7
      37
      Polymer tribo-electric charging: dependence on thermodynamic surface properties and relative humidity
      Nemeth, Erno; Albrecht, Victoria; Schubert, Gert; Simon, Frank
      Journal of Electrostatics (2003), 58 (1-2), 3-16CODEN: JOELDH; ISSN:0304-3886. (Elsevier Science B.V.)
      Tribo-elec. charging of polymers can be considered as an interfacial phenomenon of interacting polymer surfaces. The electron pair acceptor/donor properties of the polymer surfaces det. the extent of charging, as evidenced by the excellent linear correlation between the electron pair acceptor/donor parameters α and β and the measured amt. of surface charges of various polymers. The polymers studied include polyethylene, polypropylene, poly(but-1-ene), poly(4-methylpent-1-ene), polyamide 12, polystyrene, poly(vinyl chloride), poly(ethylene terephthalate), poly(oxymethylene), and poly(Me methacrylate). The rate of charging corresponds also with the surface polarity parameters. Water adsorption influences the charging mechanism. Several polymers show an increased uptake of water which forms adsorption or swollen layers with increasing the atm. humidity. In these cases, the fundamental electron pair interaction mechanism is influenced by an addnl. ion cond.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3sXit12it7w%253D&md5=64ac19ce2e74ada94a78a5b4cfc8f55e
    38. 38
      CDC. Standard Respirator Testing Procedures. https://www.cdc.gov/niosh/npptl/stps/apresp.html (accessed May 29, 2020).
      There is no corresponding record for this reference.
    39. 39
      Sanche, S.; Lin, Y. T.; Xu, C.; Romero-Severson, E.; Hengartner, N.; Ke, R. High Contagiousness and Rapid Spread of Severe Acute Respiratory Syndrome Coronavirus 2. Emerging Infect. Dis. 2020, 26, 26,  DOI: 10.3201/eid2607.200282
      There is no corresponding record for this reference.

  • Supporting Information

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    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.0c02211.

    • Discussion on COVID-19 symptoms and transmission, details of the testing procedures used for NIOSH and FDA approval for masks, public health measures suggested by the CDC and WHO, experimental methods, optical images, and additional tables (PDF)

    • Video demonstrating the simple charging of fabric that clearly has static charge after charging is complete due to the attractive nature of the fabric (MP4)


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