Filtration Performance of Layering Masks and Face Coverings and the Reusability of Cotton Masks after Repeated Washing and Drying

Abstract

In this study, filtration efficiency of different respirators, face masks, and a 2-ply cotton handkerchief bandana was compared for particles in the size range of 60 nm-4 μm under a "perfect fit" condition. The filtration efficiency at the most penetrating particle size of 0.3 μm on average ranged from 83-99% for N95 and KN95 respirators, 42-88% for surgical masks, 16-23% for cloth masks, and 9% for bandana. We also investigated the effects of using double surgical masks or layering a cloth mask over various surgical masks in terms of their filtration characteristics. In most of these combinations, the filtration efficiency improved by ~25% for particles 0.3-1 μm in diameter without any substantial change in the filter quality factor when compared to the highest of the individual mask results. To investigate the reusability of cotton cloth masks, 2-layer cotton fabric sample coupons were machine washed and dried for 52 cycles leading to an increase in inhalation resistance (~20 Pa) without affecting size-resolved filtration efficiency. Scanning electron microscopy revealed that washing and drying led to the gradual deconstruction of cotton fibers at the scale of several micrometers to hundreds of nanometers in the form of delamination of the fiber wall and fibrillation of the nanofiber constituents. Results indicate that cloth masks may be layered over surgical masks for additional benefits, and that cloth masks made out of cotton fabric can be washed and reused numerous times without a significant loss in filtration efficiency.

Modeling the filtration efficiency of a woven fabric: The role of multiple lengthscales

“…Konda et al 13 found filtration efficiencies of 9% and 18% for (complete single layers of) woven fabrics of 80 and 160 TPI. Sankhyan et al 15 also found similar values.…”

“…Both groups find a large variability in filtration efficiency from one material to another, with filtration efficiencies in the range less than 10% to almost 100%, for particles with diameters of a few micrometers. Sankhyan et al 15 found comparable filtration efficiencies to Konda et al and Duncan et al They also found that the fabric masks were systematically less good at filtering than non-woven surgical masks.…”

“…The efficiency of filtration by fabrics can be improved by using multiple layers. 15 However, both multiple layers and higher TPI lead to higher impedance to air flow. Making a practical air filter always involves a trade-off between maximizing filtration and keeping the impedance (pressure drop) low enough to be acceptable to the user.…”

“…Konda et al 13 found an impedance of for a 180 TPI cotton/polyester blend. Sankhyan et al 15 reported pressure drops in the range 40–55 Pa for an air speed of , which gives impedances in the range 5– .…”

“…Konda et al , 12,13 Duncan et al , 14 and Sankhyan et al 15 have all measured filtration efficiencies for a number of fabrics. They studied the filtration of particles in the size range we consider, which is 1 μ m. Zangmeister et al 16 studied the mechanism of filtration for smaller particles.…”