People argue that respirators and face masks are not effective against COVID-19 because the virus is much smaller than the 0.3 micron challenge particles against which respirators are tested. They compare the respirator to “trying to stop mosquitoes with a chain link fence.”
“Filtration Mechanisms of Particulate Respirators” by Erik Johnson (3M) explains why respirators are effective against particles smaller than 0.3 microns. The filtration efficiency actually goes UP for extremely small particles due to Brownian motion. The best way to imagine this is that, while the “mosquitoes” might be able to fly straight through the fence, they are buffeted from side to side by air currents that are likely to push them into the fence’s wire. The reference is worth reading and it has a self-assessment regarding knowledge of how respirators work.
Face masks (such as surgical face masks) are not the same thing as respirators and, most importantly, do not seal around the nose and mouth the way a respirator does. Surgical face masks that meet ASTM standards (ASTM F2100-19e1 Standard Specification for Performance of Materials Used in Medical Face Masks) are known quantities in terms of bacterial filtration efficiency (BFE) and particle filtration efficiency, and Cardinal Health has an excellent overview of the requirements.
Disclaimer; no information on this site constitutes formal engineering or OH&S advice.