Despite the significant increase in research on mask filtration testing since the COVID-19 pandemic, there remains considerable ambiguity regarding which parameters affect particle filtration efficiency (PFE) and how differences in standard testing protocols can lead to divergent PFE values. We evaluated the PFE (and differential pressure) of several common face masks and community face mask materials including woven cotton, spunbond polypropylene, and meltblown polypropylene, testing in accordance with ASTM F2100/2299 standards for medical masks, using polystyrene latex (PSL) aerosol, as well as NIOSH standards for respirators, using NaCl aerosol. In both cases, PFE was measured with and without aerosol charge neutralization, which is used to bring the particle population to a known, equilibrium bipolar charge distribution. Aerosols of either composition that were untreated (not neutralized) led to significant increases in measured PFE, especially in the case of PSL. In contrast, effective neutralization led to lower PFE measurements that also showed little to no dependence on aerosol composition across most materials. To investigate further, the bipolar charge distributions of PSL and NaCl aerosols, both neutralized and untreated, were characterized using an aerodynamic aerosol classifier operated in tandem with a scanning mobility particle sizer (AAC-SMPS). This technique illustrated the differences in the distribution of particle charge states between PSL and NaCl aerosols of the same size, and between PSL particles of different sizes, revealing the presence of highly charged particles in many cases. Most importantly, the equilibrium charge distribution after neutralization is shown to be independent of particle composition or initial charge distribution, highlighting the crucial role of aerosol charge neutralizers in preventing overestimates of mask performance (due to electrostatic effects) and promoting consistency in standard testing procedures.
