TABLE 3.

N95 retesting strategies

Test	Methodology	Considerations	References
Aerosolized sodium chloride	Detection by photometry or particle count (e.g., PortaCount)	• NIOSH standard test (photometry) • Useful for N class masks (e.g., N95) • Capacity to test particles that are approximate the size of SARS-CoV-2	[42, 53, 54]
Aerosolized corn oil	Detection by photometry	• Tests particles larger than the N95 MPPS • Less electrostatic charge than sodium chloride testing • Useful for assessing lipophilic particle filtration (non-N class masks)	[55]
Beads (e.g., silica, latex, polystyrene)	Detection by spectrophotometry	• Can be fluorescently tagged • Can be neutral or charged • Capacity to match size to MPPS	[47, 56]
Bacteria (e.g., Staphylococcus aureus, Bacillus atrohaeus)	Detection by particle counting and viable growth	• Test biological particles with shapes, sizes, and charge similar to pathogen of interest	[55, 57]
Viruses (e.g., bacteriophage MS2 virus, T4, Bacillus subtilis phage, phiX, H1N1 influenza virus)	Detected by measuring viable virus	• Test biological particles with shapes, sizes, and charge similar to pathogen of interest • Similar sizes to SARS-CoV-2	[39, 47, 56, 57]

MPPS = most penetrating particle size; N = not resistant to oil; NIOSH = National Institute for Occupational Health and Safety; SARS-CoV-2 = severe acute respiratory syndrome-associated coronavirus 2