Filtration efficiency, breathability, and reusability of improvised materials for face masks

Abstract

The goal of this study is to evaluate the filtration efficiency and breathability of improvised filtration and commonly available mask materials, as well as to assess their reusability. Materials readily available to the general public such as cotton, fragrance and additive-free dry baby cleaning wipes, and those abundantly available in the hospital setting, such as sterilization wraps, were chosen for testing, amongst others. In the COVID-important 2-5 µm particle range, two-layers of cotton provided filtration efficiency between 34%-66%. Amongst potential filter materials, 300weight sterilization wraps provided approximately 80% filtration efficiency and are readily available in the healthcare setting. The addition of sterilization wrap to cotton fabrics brought the filtration efficiency to above that of the sterilization wrap (80%-90%) at the expense of added pressure drop. Four-layers of dry baby wipes performed very well with a filtration efficiency of 85% and a reasonable pressure drop (1/3 of procedure mask). Since the material is advertised as pure spunlace polypropylene and designed to contact the skin during cleaning, it would appear generally safe as a filter insert. Of improvised filters, polypropylene electrostatic HVAC filters performed the best with filtration efficiencies of >99%, but are not recommended due to the risk of confusion with glass-based HVAC filters and uncertainty regarding trace materials used in the filter. The filtration efficiency of two-layers of cotton fabrics with one-layer of sterilization wrap slightly improved over 10 laundry cycles, while the performance of other non-wovens, like dry baby wipes, degraded more rapidly and should be considered disposable. In summary, we found that two-layers of cotton fabric can provide a comfortable, breathable and reusable option. The addition of a sterilization wrap or four-layers of pure spunlace fragrance free dry baby wipes can significantly improve filtration and block expiratory aerosols at the expense of an added pressure drop.

Strategies to minimize SARS-CoV-2 transmission in classroom settings: combined impacts of ventilation and mask effective filtration efficiency

“…The filtration performance of materials for both homemade cloth masks and commercially produced masks (cloth masks, disposable non-medical masks, medical masks, and KN95 and N95 masks) has been studied both before the COVID-19 pandemic (Davies et al, 2013;Jang Ji and Kim Seung, 2015;Jung et al, 2014;Rengasamy et al, 2010;van der Sande et al, 2008) and during (Bagheri et al, 2021;Crilley et al, 2021;Drewnick et al, 2021;Hill et al, 2020;Hao et al, 2020Hao et al, , 2021Joo et al, 2021;Kelly et al, 2020;Konda et al, 2020b;Lindsley et al, 2021b,a;Long et al, 2020;Mueller et al, 2020;Pan et al, 2021;Teesing et al, 2020;Whiley et al, 2020;Zhao et al, 2020) (also see the recent review by Clase et al (Clase et al, 2020)). However, the particle size range of the measurements, filtration conditions, and testing equipment and methods vary greatly between studies, making comparisons difficult.…”

“…Most studies have focused on measurements at small particle sizes (< 1 μm) similar to the range of sizes achieved with the NaCl aerosol used for certification of N95 respirators (count median diameter of 0.075 ± 0.02 μm and a standard geometric deviation not exceeding 1.86 (42 CFR Part 84, 2021)) (Crilley et al, 2021;Hao et al, 2020Hao et al, , 2021Joo et al, 2021;Lindsley et al, 2021a;Long et al, 2020;Rengasamy et al, 2010) or do not have size-resolved measurements (Davies et al, 2013;Long et al, 2020;Kelly et al, 2020;Lindsley et al, 2021a;Long et al, 2020;Mueller et al, 2020). A few studies have investigated a wider range of particle A c c e p t e d M a n u s c r i p t sizes with size-resolved measurements for sizes up to 10 μm (Bagheri et al, 2021;Drewnick et al, 2021;Konda et al, 2020b;Pan et al, 2021). Filtration face velocities studied vary from 0.78 cm/s (Hill et al, 2020) to 16.5 m/s (Kelly et al, 2020) with the majority of studies having face velocities in range of 2 to 25 cm/s (Bagheri et al, 2021;Crilley et al, 2021;Drewnick et al, 2021;Hao et al, 2020Hao et al, , 2021Joo et al, 2021;Lindsley et al, 2021a;Long et al, 2020;Pan et al, 2021;Rengasamy et al, 2010).…”

“…A few studies have investigated a wider range of particle A c c e p t e d M a n u s c r i p t sizes with size-resolved measurements for sizes up to 10 μm (Bagheri et al, 2021;Drewnick et al, 2021;Konda et al, 2020b;Pan et al, 2021). Filtration face velocities studied vary from 0.78 cm/s (Hill et al, 2020) to 16.5 m/s (Kelly et al, 2020) with the majority of studies having face velocities in range of 2 to 25 cm/s (Bagheri et al, 2021;Crilley et al, 2021;Drewnick et al, 2021;Hao et al, 2020Hao et al, , 2021Joo et al, 2021;Lindsley et al, 2021a;Long et al, 2020;Pan et al, 2021;Rengasamy et al, 2010). Studies generally measure both particle penetration (i.e., filtration efficiency) and pressure drop, although instrumentation and the method of generating the challenge aerosol can vary significantly.…”

“…The majority of studies have used NaCl aerosols, although some have utilized ambient aerosol particles (Bagheri et al, 2021;Drewnick et al, 2021;Teesing et al, 2020). The NaCl aerosols are typically dried and diluted.…”

“…where ηf is the MFE and P  is the pressure drop) ranging from qF = 0.0002 to 0.007 Pa -1 (Crilley et al, 2021;Bagheri et al, 2021;Drewnick et al, 2021;Hao et al, 2020;Joo et al, 2021;Pan et al, 2021;Rengasamy et al, 2010). An exception to this is the work of Konda et al (Konda et al, 2020b) which found very high filtration efficiency of 90-95% for woven cotton of different thread counts.…”