RE: The pore size was typically around 500nm. The virus is 100nm in diameter. So obviously they do not work.
So, why can’t someone manufacture a face mask with a pore size much smaller than the coronavirus’ diameter?
Because the pressure drop through the filter medium would be so high that air would no go through the medium, but around the leaks where it seals on your face. It’s a trade-off between air through the filter medium and the leakage.
In utility baghouses (used to capture coal flyash in the flue gas), the face velocity is measured in feet per MINUTE. The “face velocity” is the speed at which the gas passes through the filter fabric. The fabric itself does not do much of the filtration, either. It builds up a “filter cake” (filtered material deposited on the filter fabric) which itself does most of the filtration.
Human face masks have two serious strikes against them: 1) they do not have a filter cake and 2) they have a MUCH higher face velocity. Both of these lead to seriously degraded filtration efficiency.