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> So if your filter is 95% efficient at 0.3 μm, it's even better at bigger and smaller sizes. The SARS-CoV-2 virus is 0.05 to 0.2 μm in diameter, so an N95 mask will be substantially better than 95% for coronavirus.

small nit. the corona virus gloms onto other particles of differing sizes in an aerosol though it's not super clear what size they are.

> The size of infective airborne viral particles is not well known and the aerosol transmission of viral diseases is currently debated in the literature (Roy and Milton, 2004; Tellier, 2006).

https://academic.oup.com/annweh/article/52/3/177/312528

Same reason that we care about the concentration of PM2.5 particles in the atmosphere. Once we inhale them they manage to reach the very end of our lungs.

Is it? That's different from the counterintuitive N95 filtration efficacy. PM2.5 designates particles of diameter 2.5 microns and smaller, and I've read nothing that implies particles smaller than 2.5 microns are somehow easier for the respiratory tract to keep out of the lungs. The graph of filtration efficacy as a function of particle size is U-shaped for N95 masks, but not for the human respiratory tract.

You might find it interesting to know too that HEPA filters get more effective as they approach end-of-life.

So is the only reason to change filters to avoid blockage and reduced airflow but not filtering capacity?

I think they can also collect enough organic matter for mold to start growing on the downstream side. I base that on pictures of automotive cabin filters that are revolting.

Would some generous Lysol spray get rid of the mold?

Just flip it around

Yes, but it will ruin dust filtration, so the proper choice is hot water vapor, dry heat, or hydrogen peroxide. Perhaps silver ions too.

HEPA filters use exactly the same meltblown fabrics as respirators, except thicker and in accordion form.

For a more observable example, think about sorting American coins.

A dime is the easiest because it is the smallest. Quarters are much larger so are also easy. In the middle are pennies and nickels which are the hardest to separate from one another because they're so close in size.

Here's a video that demonstrates it: https://www.teachengineering.org/activities/view/cub_coinsor...

Since a filter sorts particles from air molecules, this would imply that the hardest particles to filter are just a couple atoms large.

To be fair, that’s probably true. It’s just that the particles at that scale are other molecules and they are indeed very hard to filter out mechanically.