Robust Measurement of Particle Filtration Efficiencies: Evaluated Quantities and Experimental Sensitivities

TIMOTHY SIPKENS, Joel Corbin, Gregory Smallwood, Andrew Oldershaw, Ian Leroux, Jalal Norooz Oliaee, Fengshan Liu, Thierry Lavoie, Triantafillos Koukoulas, Richard Green, Prem Lobo, National Research Council Canada

     Abstract Number: 556
     Working Group: Aerosol Science of Infectious Diseases: What We Have Learned and Still Need to Know about Transmission, Prevention, and the One Health Concept

Abstract
The COVID-19 pandemic has led to a resurgence of academic studies and new standards targeting respiratory protection. Many academic studies and standards differ in the measured quantities – for instance, whether they employ size-resolved (mostly scientific studies), number-weighted (e.g., in the ASTM F2299 test method), or mass-weighted (e.g., in the NIOSH TEB-APR-STP-0059 test method) particle filtration efficiency (PFE) – and the testing parameters, such as face velocity. Variations in these quantities present challenges in comparing standards and studies and results in differences in terms of the quality of standards. Here, we examine a range of these parameters and systematically consider their effect on measured PFEs. We start by examining the aerosol physics underlying the associated measurements, providing a discussion of the different types of particle size (e.g., mobility and aerodynamic diameters) and the differences between number- and mass-weighted PFEs. We discuss the non-trivial relationship between mass- and number-weighted PFEs and show that the number-weighted PFE is typically more conservative (lower) for many of the conditions relevant to PFE standards. We also examine this relationship for a specific set of materials where the basis weight and charge state of the filter material were systematically varied. We then explore the role of face velocity, particle properties (size distribution, electric charge, density, and shape), measurement techniques, and environmental preconditioning. We combine systematic experiments with data mining of a database of over 900 samples. For the range of values relevant to filtration efficiency testing, most important parameters are found to be face velocity and particle charge. Our results provide guidance for future testing protocols and a basis for the comparison of respirators certified under various international methods, including FFP2, KN95, P2, Korea 1st Class, and DS2.