[johnohara]

A single virion of SARS-COV-2 is about 125nm. Approximately the same size as a single particle of smoke. N95 masks are supposed to be 95% effective for particle sizes 100-300nm in size. It also means that 5% get through.

Not sure about RSV (and its variants) or flu A/B.

My assumption about N95 masks is that they are used to effectively reduce the initial viral load and act as a first line of defense but are not a sole preventative measure.

It goes both ways too. If you have symptoms it makes sense to wear an N95 if only to reduce the amount of virion you are sharing.

FWIW, sanitation is every bit as important as wearing an N95 mask in movie theaters, on subways, or on airplanes. Rule of thumb is to be ever-vigilant.

[tambourine_man]

My understanding is that there’s a minimum threshold for viral load to evolve into symptoms.

You won’t get an infection from 100 viruses, but you may from 1 million.

[nikolay]

A higher viral load means a better start for the infection. If you get a limited viral load, you give your immune system enough time to deal with it without major damage. But even asymptomatic COVID-19 leads to permanent injury, so, maybe it's better to get a massive infection and build a better resistance than many small-grade infection and accumulate enough damage without building a good enough natural defense.