Systematic experimental comparison of particle filtration efficiency test methods for commercial respirators and face masks

Abstract

Respirators, medical masks, and barrier face coverings all filter airborne particles using similar physical principles. However, they are tested for certification using a variety of standardized test methods, creating challenges for the comparison of differently certified products. We have performed systematic experiments to quantify and understand the differences between standardized test methods for N95 respirators (NIOSH TEB-APR-STP-0059 under US 42 CFR 84), medical face masks (ASTM F2299/F2100), and COVID-19-related barrier face coverings (ASTM F3502-21). Our experiments demonstrate the role of face velocity, particle properties (mean size, size variability, electric charge, density, and shape), measurement techniques, and environmental preconditioning. The measured filtration efficiency was most sensitive to changes in face velocity and particle charge. Relative to the NIOSH method, users of the ASTM F2299/F2100 method have commonly used non-neutralized (highly charged) aerosols as well as smaller face velocities, each of which may result in approximately 10% higher measured filtration efficiencies. In the NIOSH method, environmental conditioning at elevated humidity increased filtration efficiency in some commercial samples while decreasing it in others, indicating that measurement should be performed both with and without conditioning. More generally, our results provide an experimental basis for the comparison of respirators certified under various international methods, including FFP2, KN95, P2, Korea 1st Class, and DS2.

Particle filtration efficiency measured using sodium chloride and polystyrene latex sphere test methods

“…This test aerosol has also formed the basis for a number of recent research works, e.g. 2 – 8 ,. A roughly monodisperse polystyrene latex (PSL) aerosol having a mobility diameter of 100 nm.…”

“…Comparably few research studies have considered this challenge aerosol – e.g. 2 , 8 , 11 – 14 , – with results being more variable, given the lack of neutralization 8 .…”

“…(*) The PSL distribution was chosen to match measurements by Corbin et al . 8 (1 drop/mL), including the presence of doublets, triplets, etc. and residual particles.…”

Filtration and Breathability of Nonwoven Fabrics Used in Washable Masks

“…Comparing the neutralized to uncharged case, 180 we confirmed that the variation of net charge in the neutralized distribution was negligible (see 181 Supplemental Information A). It is important to note that the natural charge distribution produced 182 by a nebulizer will result in the measurement of a higher PFE than that measured for neutralized 183 particles (Corbin et al, 2021), which can be a reason for differences in PFE between studies. The 184 remaining tests were run without the DMA column.…”

“…Considering typical mask flow areas, this 195 corresponds to an inhalation flow of ~50 L min -1 , midway between a resting rate and that used for 196 N95 testing (Caretti et al, 2004). Higher face velocities would yield larger PFEs for particles above 197 several microns due to increased impaction, but lower PFE for the smallest particles captured by 198 electrostatic interactions (Corbin et al, 2021)…”