Since we're talking about counterintuitive things related to the coronavirus crisis, I was surprised to learn that N95 masks, that remove 95% of particles of size 0.3 μm, are
even more efficient with smaller particle sizes. In turns out that 0.3 μm is the hardest particle size to filter--they call it the most penetrating particle size--hence filter testing is done at this worst-case size. The reason is that filters work due to several mechanisms which dominate at different particle sizes according to a curve:
"For very small particles, less than 0.1 μm in diameter, the primary filtration mechanism is diffusion and the filter is very efficient. For particles between approximately 0.1 and 0.4 μm the filter is less efficient as the particles are too large for a great diffusion effect and too small for a large interception effect. Particles above approximately 0.4 μm enter the region where interception along with inertial impaction are predominant and the filter is very efficient again."[1]
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.
[1] https://www.tsi.com/getmedia/4982cf03-ea99-4d0f-a660-42b24ae...
"For very small particles, less than 0.1 μm in diameter, the primary filtration mechanism is diffusion and the filter is very efficient. For particles between approximately 0.1 and 0.4 μm the filter is less efficient as the particles are too large for a great diffusion effect and too small for a large interception effect. Particles above approximately 0.4 μm enter the region where interception along with inertial impaction are predominant and the filter is very efficient again."[1]
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.
[1] https://www.tsi.com/getmedia/4982cf03-ea99-4d0f-a660-42b24ae...