@wallabra@bark.lgbt @neckspike@indiepocalypse.social @eniko@mastodon.gamedev.place To an extent. 0.3 micrometers is the size that was taken as a general measurement target for testing because it apparently sits around where a particle is worst affected by both electrostatic effects (and other very-small particle effects) that better affect smaller particles, and physical obstruction that better affects larger particles. Something like that.
So if something has a rating meaning 99.95% filtration per pass would leave relatively little in its filtered exhaust, but with a sufficient saturation of some sufficiently virulent pathogen a single pass may not be enough. Air circulation in the room also becomes very important, because air has to actually go through the filtering process to be sanitized.
(This is incidentally why rock cutting workers (when properly equipped) use rebreathers and other air-cleansing equipment instead of just N95 respirators. Enough goes through such filters that there is still a health hazard.)
There is still stuff that isn't affected enough like scent particles that get very small or gasses, but those are also not biohazards (they can be toxic or allergenic though, among other issues, but that's an entirely different kind of problem and requires a different approach for handling).