Addressing COVID-19 Spread: Development of Reliable Testing System for Mask Reuse

While the novel coronavirus pandemic (COVID-19) continues to wreak havoc globally, self-protection from possible infection by wearing a mask in daily life has become the norm in many places. The unprecedented demand for masks has now attracted attention on their filtration efficiency. Furthermore, the widespread use of disposable masks has led to shortage of filter materials and problems with their haphazard disposal. In this study, a testing system that is based on standardized methods has been established and enhanced to reliably measure the particle filtration efficiency (PFE) of masks. Quality control experiments that examine the filtration efficiency of polystyrene latex (PSL) particles that are 0.1 μm in size and sodium chloride (NaCl) particles that range from 0.01–1.0 μm are conducted to determine the reliability of the testing system. Moreover, various textile materials are tested to fabricate 3-layer face masks, and the PFE of these masks is tested by using the proposed testing system to find the most suitable materials and the likelihood of their reusability. Among the tested materials, polytetrafluoroethylene (PTFE) used as the membrane in the filter layer has the highest PFE of 88.33% ± 1.80%, which is mainly due to its dense and multilayer structure. The air permeability of the self-developed masks ranges from 1.41 ± 0.04 to 1.93 ± 0.08, less breathable than the commercial masks. The reusability of a mask that uses PTFE as the membrane in the filter layer is tested by gently washing the mask 30 times and then drying the mask in air before the PFE is measured. The PFE is only reduced by 10–20% after 30 washes, thus indicating the potential reusability of the mask. The findings in this study will contribute to reducing the pressure of mask shortages and are an environmentally friendly solution to the massive use of disposable masks.


Introduction 35
The new coronavirus (COVID-19) has become a global public health concern because it is 36 highly contagious with severe or even fatal health consequences. With the rapid spread of 37 COVID-19, self-protection has become crucial to avoid contracting this disease. As a highly 38 infectious disease, the two main routes of transmission include direct transmission (e.g. 39 spraying droplets emitted from aerosolized particles through sneezing, coughing or talking), 40 and indirect transmission (contact with surfaces that contain the virus or objects that have been 41 in contact with an infected individual) (Tomar and Gupta, 2020; Ningthoujam, 2020; Liu, 42 2020). However, the former has been documented to be a more prevalent form of transmission, 43 so an effective mask must be worn to obstruct the transmission of the virus and block its spread 44 when conducting daily life activities (Bałazy et al., 2006;MacIntyre and Chughtai, 2020). This 45 has led to the unprecedented demand and shortage of masks, along with the problem of 46 ineffective masks. On the one hand, there are a variety of different masks on the market. 47 Although most commercially available masks claim effectiveness against contact with airborne 48 particles, their actual performance has not been tested adequately. For example, Cheng (2020) 49 reported that there was no quality control in face mask production in Pakistan. In Italy, in order 50 to meet the huge demand of face masks, some industries have reset the production chain, 51 shifting from the usual target products to the production of masks. However, the quality of the 52 masks is questionable and needs to be tested (Amendola et al., 2020). In addition, Lam et al. 53 (2020) examined 160 mask brands on the market and found that 48.8% of them were 54 substandard and/or invalid. With COVID-19 as an urgent public health issue now, it has 55 become imperative to test and validate the performance of these masks with reliable testing 56 equipment. On the other hand, COVID-19 has led to the unprecedented demand for masks, 57 which has resulted in a shortage of materials to fabricate them. Moreover, most of the masks 58 currently found on the market are disposable (i.e. surgical, N95 and KF94 masks). The 59 widespread use of such disposable masks is having a detrimental effect on the environment, as 60 additional resources and human resources are needed to properly dispose of used masks which 61 might otherwise end up in the oceans or landfills. As such, it is timely to develop effective yet 62 reusable masks, not only to address the needs of the current epidemic but also to conserve 63 resources. 64 One of the key parameters of masks is their particle filtration efficiency (PFE) which reflects 65 the ability of masks and / or filters to block the inhalation of different types of particles.

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3 ASTM F2299 (the United States) and YY 0469-2011/ GB19083-2010 (Chinese), and the 68 standard provided by the National Institute for Occupational Safety and Health (NIOSH). In 69 ASTM F2299, polystyrene latex (PSL) particles of a specific size are used to evaluate the PFE 70 of a material. Although the applicable particle size range is 0.1 μm to 5.0 μm, PSL particles of 71 0.1 μm are generally used for testing as it is the most penetrating particle size. In YY 0469-72 2011, sodium chloride (NaCl) particles that range from 0.010 μm to 1.0 μm are used as the 73 index in PFE testing of masks, while the NIOSH standard is used to test masks to filter NaCl 74 particles that are 0.3 μm in size. Although these standard methods provide basic instructions 75 and parameters for PFE testing, they do not offer specific settings for testing systems and 76 details of the equipment involved. Moreover, some of the standard methods have limitations. 77 For instance, a device that is recommended for measuring particle concentration (an optical 78 particle counter (OPC)) is low in accuracy and size-resolution (Rengasamy et al., 2011). 79 Moreover, the standards do not strictly stipulate the size of the particles to be tested, thus 80 resulting in differences and uncertainties in the results with the different testing agents and 81 systems. Lastly, the impacts of different types of particles on the testing results have been 82 hardly considered and discussed. Apart from the PFE, air permeability is another important 83 parameter for a mask. The air permeability indicates how comfortable the mask is to breathe. 84 The more permeable a mask is, the lower air resistance it has and the more comfortable it is to 85 wear. In general, there is a balance between air permeability and filtration efficiency. A 86 satisfactory mask should own a good ability of filtration with an acceptable air permeability 87 (Konda et al., 2020). 88 Therefore, this study aims to establish a testing system that examines two types of particles to 89 accurately measure the PFE of various face masks. The proven testing system is subsequently 90 used to determine suitable fabrics for reusable masks. The materials for the most effective 91 reusable face mask are presented in this study. Finally, the air permeability of the masks is 92 measured to indicate the wearing comfort of the masks. The particle generation unit is an atomizer (Model 7.811, Grimm, Germany), which was used 102 to generate the PSL and NaCl particles and provide sufficient air flow in the testing system by 103 supplying particle-free dilute gas. By adding 2-3 drops of PSL solution (Thermo Scientific, 104 USA) into 8 mL of Milli-Q water, an aqueous solution of PSL particles was produced for 105 atomization, which generated the PSL particles. To generate the NaCl particles, 25 mL of 0.025 106 g/ml NaCl solution was added into the atomizer. The count median diameter (CMD) of the 107 generated PSL particles was 101.2 nm with the geometric standard deviation (GSD) of 4.6 nm, 108 while the CMD of the generated NaCl particles was 42.8 nm with the GSD of 0.9 nm. The 109 generated particles were introduced into an aerosol neutralizer (TSI Incorporated, Model 110 3077A) to eliminate the electrostatic charges and ensure stability of the particle surface charge. 111 Silica gel was used to remove the water droplets and water vapour produced from the atomizer 112 and maintain a constantly low humidity in the system. The particles were then mixed in a 113 mixing chamber (cuboid shaped with a volume of ~ 2.5 L) before they were passed through a 114 mask placed in the sample holder. The air flow, relative humidity and temperature were 115 measured in the upstream and downstream of the sample holder, respectively. The sample 116 holder is a customized accessory in which a mask with a cross-sectional area of ~14 cm 2 can 117 be properly placed, and the holder is sealed with a layer of rubber to prevent air leakage. The 118 detection unit is a scanning mobility particle sizer (SMPS) (Model 5.400, Grimm, Germany) 119 which can measure the number concentration of particles in the upstream and downstream of 120 the sample holder, respectively. The SMPS is a highly sensitive and accurate instrument which 121 electronically measures the size-resolved number concentration of particles with a maximum

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5 particles was acquired at an interval of 10 seconds for a specific size of PSL particles. As for 124 the NaCl particles, particles that ranged from 0.01 µm to 1.0 µm were categorized into 44 size 125 bins and measured at a time interval of 7 mins. The differences in particle concentration of 126 each particle size between the upstream and downstream of the sample holder were used to 127 calculate the PFE values (PFE = 100% × (1 -average downstream concentration/average 128 upstream concentration)). Each test was about 10 -30 mins to obtain a stable PFE value. 129 Exhaust gas was discharged after all of the generated particles were removed by using a high-130 efficiency particulate air (HEPA) filter. Compared with the methods recommended in the 131 NIOSH and ASTM F2299 standards, different types of particles were generated for the 132 examination of the PFE of masks in the study. This study also adopted more advanced and 133 accurate instrument, i.e., SMPS. Besides, silica gel was used to remove the moisture generated 134 by the atomizer, thereby protecting the instrument and eliminating the influence of moisture on 135 the PFE results. Moreover, the duration of the PFE test was longer than previous methods to conditions of the testing system were controlled. PFE testing was conducted under a controlled 142 temperature (20-25℃) and relative humidity (40% -55%) with the testing system. Previous 143 study indicated that temperature and relative humidity in these ranges had no remarkable 144 effects on the PFE results (Yang et al., 2007). The air flow in the testing system was 7.3-7.5 145 L/min, which was consistent for all of the tests. The setting of air flow was referred to the 146 ASTM F2299 standard. The airtightness of the system was determined by the consistency of 147 the upstream and downstream air flows. The air velocity in the sample holder was 10 cm/s, 148 which meets the requirement and/or suggestion in the standard methods (i.e. 0.5 -25 cm/s). It 149 has been proved that air velocity ranging from 4 cm/s to 16 cm/s has no discernible effects on 150 the PFE results (Sachinidou et al., 2017). The initial range of number concentration of particles 151 produced by the atomizer was 10 2 -10 3 counts/cm 3 , following the suggestion in the standard.

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cc/cm 2 /sec). Both front and back sides of each sample were measured for three times, 157 respectively. Totally, six values were derived and averaged to represent the air permeability of 158 a mask. 159

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8 As mentioned earlier, the OPC has some drawbacks which may cause fluctuations in the PFE 209 results. That is, the OPC not only detects the PSL or NaCl particles but also water droplets that 210 are generated from the atomizer. Hence, the number concentration of the particles upstream 211 could be overestimated by unintentionally including the water droplets ( droplets. In fact, silica gel was used to remove the water droplets and water vapour produced 227 from the atomizer before particles entered the SMPS (Fig. 1)   respectively. Sample A has the lowest PFE of 22.9% ± 6.0 % (p < 0.1). The discrepancies in 242 the PFE are caused by several factors. First, the extremely low PFE of Sample A is due to the 243 lack of a filter layer between the outer and inner layers, which suggests the importance of a 244 filter layer (O'Kelly et al., 2020). This also implies that regular fabrics such as woven fabric 245 (W100) and knitted fabric (J92) cannot effectively block ultrafine particles. Secondly, the 246 better performance of Sample B as opposed to Sample C indicates that the weight of the filter 247 layer has an effect on the PFE. The filter used for Samples B and C is a non-woven interlining. 248 The fibres are randomly laid out on the surface and bonded by using adhesion or heat. It is not 249 surprising that a heavier filter with the same surface area has a denser and tighter structure, 250 which is more beneficial for particle filtration. Thirdly, the much higher PFE of Sample D as 251 opposed to Sample C is related to the material of the filter layer, which shows its key role in 252 manufacturing a mask with a high PFE (Satish et al., 2017). The PTFE membrane used in 253 Sample D has a denser structure than the PP membrane used in Sample C. 254 In order to better understand the role of the different materials used in masks to determine the 255

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10 the structure of W100 and J92 is neat and orderly, while that of the non-woven PP fabric and 258 PTFE membrane is criss-crossing and more complex. Specifically, while the pore size of the 259 non-woven PP fabric is comparable to that of the W100 and J92 fabrics, its random fibre or 260 criss-cross structure provides many tiny voids that trap particles, thereby promoting the 261 filtration of ultrafine particles. A multilayer structure can be observed for the PTFE membrane. 262 In addition to the same criss-cross structured layer as the bottom of the non-woven PP sample, 263 the surface of the PTFE membrane has a netlike structure. This netlike structure is composed 264 of millions of ultrafine fibres which impart a "shielding effect" on the top, and thus greatly 265 promotes particle filtration (Matulevicius et al., 2014). The multilayer structure effectively 266 prevents the penetration of particles, thus resulting in a high PFE. 267

Reusable masks 275
Mask reusability was determined based on the PFE against PSL particles that are 0.1 µm in 276 size after the mask was washed. The washing process was gentle with no squeezing. The 277 samples were then air dried. Since Sample D (with a PTFE membrane as the filter layer) has 278 the best performance in filtering ultrafine particles among the four developed masks in this 279 study, it was selected to further examine its reusability after 30 washings. For comparison 280 purposes, the PFE of a disposable mask (i.e. the surgical mask) after one washing and Sample 281 B (with a non-woven PP material as the filter layer) after 30 washings was measured.  non-woven PP layer and PTFE membrane can almost retain the same structure as that before 296 washing (Fig. 4). As discussed in Section 3.2, the filter layer is critical to the PFE of a mask. 297 Therefore, even though the outer and inner layers of Samples B and D are slightly damaged, 298 the PFE values of Samples B and D do not decrease significantly, due to the intactness of the 299 filter layer after the washing process. Overall, it is impractical to reuse surgical masks because 300 after only one washing, the surgical mask is no longer effective in filtering particles (PFE < 301 20%). Instead, Sample D can be reused at least 30 times as the material retains a high PFE (> 302 70%). The intactness of the filter layer is paramount to the reusability of a mask. Therefore, 303 using a PTFE membrane as the filter layer of masks would provide optimal reusable masks. 304 305

3.4
Air permeability 313 Table 4 shows the air permeability of N95, KF94, surgical and self-developed masks. For the 314 commercial masks, surgical mask is the most breathable with an air permeability of 0.64 ± 0.02 315 kPaꞏs/m, followed by the N95 mask (1.06 ± 0.04 kPaꞏs/m) and the KF94 mask (1.39 ± 0.05 316 kPaꞏs/m). The results are analogous to a recent research report, indicating the accuracy of the 317 results in this study (Suen et al., 2020). As for the self-developed masks, the air permeability 318 is 1.41 ± 0.04 kPaꞏs/m, 1.61 ± 0.05 kPaꞏs/m, 1.55 ± 0.04 kPaꞏs/m and 1.93 ± 0.08 kPaꞏs/m for 319 Samples A -D, respectively. The air permeability is anti-correlated with the PFE for the self-320 developed masks. Although a multi-layer structure and/or dense filter can lead to higher PFE, 321 the breathability of the mask may be poor. It is therefore essential for masks to achieve high 322 PFE with acceptable air permeability (Li et al., 2006

Conclusions 331
In the study, a testing system of the PFE of face masks has been developed based on 332 standardized methods. The system is validated by measuring the PFE of three commercial 333 masks (i.e. KF94, N95 and surgical masks) against standard PSL particles that are 0.1 µm in 334 size and NaCl particles that range from 0.01 -1.0 µm. The comparable PFE of the N95, KF94 335 and surgical masks found here against the values reported in previous studies validate the 336 reliability of our testing system. Afterwards, different fabrics are tested to produce reusable 337 masks, and their PFE is also measured with the proposed testing system. The results indicate 338 that a filter layer is vital for a mask to have a high PFE. Heavier filters with the same surface 339 area as that of the lighter filters contribute to increasing the PFE. Among the four developed 340 masks in this study, the sample with a PTFE membrane (Sample D) is the most effective, with 341 a PFE of 88.3% ± 1.8%, which is mainly due to the dense and multilayer structure of the PTFE 342 membrane. The permeability of the self-developed masks is between 1.41 and 1.93 kPaꞏs/m, 343 which is not as breathable as the commercial masks. After 30 washes, the PFE of Sample D is 344 slightly reduced to 72.0% ± 2.5%, thus indicating its potential reusability. The reusability is 345 mainly attributed to the intactness of the filter layer during the washing process. 346 and Control of COVID-19 in Hong Kong (SST/029/20GP). We would like to express our 351 heartfelt thanks to Professor Ping-kong Wai and Professor Hau-chung Man for their 352 coordination efforts. 353

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18 442   443   444 Fig. S1 Size distribution of particles generated from atomizer with PSL solution 445