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Respirator and Face Mask Particle Filtration Efficiency: Experimental Comparison of Test Methods
JOEL CORBIN, Gregory Smallwood, Ian Leroux, Jalal Norooz Oliaee, Fengshan Liu, Richard Green, Nathan F. Murnaghan, Timothy Sipkens, Triantafillos Koukoulas, Prem Lobo, National Research Council Canada
Abstract Number: 633
Working Group: Infectious Aerosols in the Age of COVID-19
Abstract
Respirators, medical masks, and face coverings all filter airborne particles using similar physical principles. Measurements of their filtration efficiency must also be made using comparable test methods. Current standardized test methods address this need by specifying environmental conditions, flow rate through the filter media (face velocity), key aerosol-particle properties (mean size, size variability, electric charge, density, and shape), and measurement techniques. However, these specifications vary between existing measurement test methods such as the NIOSH TEB-APR-STP-0059 under US 42 CFR 84 (for N95 respirators), ASTM F2299/F2100 (for medical face masks) and their international analogues. To quantify and understand the effects of each of the differences listed above, we have performed systematic experiments using a custom-built measurement system and combined the results with theoretical calculations and physical interpretations. The filtration efficiency is most sensitive to changes in face velocity and particle charge. For example, a material may filter charged and neutral particles with 99.5 % and 68 % filtration efficiency, respectively. Separately, the filtration efficiency may drop from 92% to 75% for face velocities representing adult breathing rates at rest and during exercise, respectively. Relative to the NIOSH method, users of the ASTM F2299 method have normally used non-neutralized (highly charged) aerosols at lower face velocities, although a strict reading of the latter method allows for higher face velocities and includes a recommendation for neutralization. On the other hand, the performance of some respirators was found to increase or decrease with environmental conditioning under the NIOSH method, allowing for potential false positives as well as false negatives in respirator testing. We discuss the application of our results to FFP2, KN95, P2, Korea 1st Class, and DS2 respirators, as well as barrier face coverings in the context of the COVID-19 pandemic.