“Invisible to all but the most powerful electron microscope, your typical virus (coronavirus, influenza, etc.), is around 0.125 microns in diameter, one six hundredth the size of the thinnest strand of human hair.  The microscopically tiny virus is one thousand times smaller than even the tiniest of gaps in the standard supermarket-issue cloth mask.  Even the very best quality surgical-type mask has a minimum 0.3 micron filter, two and a half times larger than the virus.  Lesser quality, but more common, surgical masks have gaping, to a virus, holes averaging 2-10 microns, sixteen to eighty times the size of a virus.”

https://philshannon.medium.com/the-hidden-curriculum-of-the-facemask-pt-1-79d4c09123d6