What do the particulate exclusion ratings mean for protection against viruses?
Respirators also have a numeric designation (95, 99, 100) that indicate the percent of a standard particle size (0.3 micrometers or 300 nanometers (nm), which is about 1/25 the size of a red blood cell or small lymphocyte) that is excluded. Exclusion occurs from direct impact against the layers of fabric as air transverses the mask and also be electrostatic forces that “attract” particles.
The SARS-CoV-2 virus that causes COVID-19 illness is smaller than 300 nm (approximately 80-100 nm), but is released into the air in droplets or as an aerosol with sneezing, etc. These larger particles are on the order of micrometers in diameter (1 micrometer = 1000 nanometers) and are excluded by a properly fitting respirator.
NIOSH-Approved Particulate Respirators. The National Personal Protective Technology Laboratory (NPPTL). CDC/NIOSH. Accessed on May 6, 2020 at: https://www.cdc.gov/niosh/npptl/topics/respirators/disp_part/respsource1quest2.html