Respirators, medical masks, and barrier face coverings all filter airborne particles using similar physical principles yet are tested using a variety of variety of standardized test methods. To quantify and understand the effects of differences between the 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). We systematically compared the experimental performance of the NIOSH and ASTM F2299 test methods in terms of face velocity, particle properties (mean size, size variability, electric charge, density, and shape), measurement techniques, and environmental preconditioning. Filtration efficiency is most sensitive to changes in face velocity and particle charge. Relative to the NIOSH method, users of the ASTM F2299 method have normally used non-neutralized (highly charged) aerosols at smaller face velocities, each of which may enhance measured filtration efficiencies by approximately 10%. Environmental conditioning at elevated humidity increased filtration efficiency in some lots while decreasing it in others, which was observed to result in false positives in the NIOSH method. Our results provide an experimental basis for comparing respirators certified under various international methods including FFP2, KN95, P2, Korea 1st Class, and DS2.
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