Testing of Commercial Masks and Respirators and Cotton Mask Insert Materials using SARS-CoV-2 Virion-Sized Particulates: Comparison of Ideal Aerosol Filtration Efficiency versus Fitted Filtration Efficiency

Abstract

Shortages in the availability of personal protective face masks during the COVID-19 pandemic required many to fabricate masks and filter inserts from available materials. While the base filtration efficiency of a material is of primary importance when a perfect seal is possible, ideal fit is not likely to be achieved by the average person preparing to enter a public space or even a healthcare worker without fit-testing before each shift. Our findings suggest that parameters including permeability and pliability can play a strong role in the filtration efficiency of a mask fabricated with various filter media, and that the filtration efficiency of loosely fitting masks/respirators against ultrafine particulates can drop by more than 60% when worn compared to the ideal filtration efficiency of the base material. Further, a test method using SARS-CoV-2 virion-sized silica nanoaerosols is demonstrated to assess the filtration efficiency against nanoparticulates that follow air currents associated with mask leakage.

Keywords: COVID-19; aerosol; exposure testing; face mask; filtration; mask fit.

Figure 1.

A SiO₂ nanoaerosol was generated for filtration efficiency studies, with the peak concentration centered around 40 nm in diameter.

Figure 2.

Left: Base filtration efficiency of materials was tested in-line between a SiO₂ nanoaerosol-filled chamber and scanning mobility particle sizer (SMPS). Right: Filtration efficiency in mask form was tested using a soft headform modified with sampling ports in the nasal cavity. Components are not to scale.

Figure 3.

The base filtration efficiency of each tested fabric ranged from as low as 17.4% to greater than 99.98%.

Figure 4.

The measured pressure drop across filter media were largely similar; the FTR467 ULPA material exhibited a pressure drop that would likely preclude use in a passive filtration mask, as it is designed for use in pleated, high surface area filter cartridges for powered applications.

Figure 5.

When inserted into a 2-layer cotton mask, all filter materials exhibited a significant drop in filtration efficiency compared to measured base filtration efficiency. The 3M 8511 and KN95 respirators as well as the medical and dust masks were all tested as-received in mask form, whereas the remainder were tested through insertion between the two layers of the cotton mask. A KN95 respirator that was sealed to the headform before testing with thermoplastic adhesive (i.e. to demonstrate a leak-free fit) provided filtration efficiency very near its base filtration efficiency, proving that the cause of efficiency reductions was related primarily to fit quality.