[gdavisson]

"They filter AT LEAST 95% of the particles above a certain size."

That's a common misunderstanding; they filter at least 95% of particles of all sizes. It's actually intermediate-sized particles (around 0.1 - 1 micron) that're hardest to capture. Smaller particles are more subject to Brownian motion, which makes them jiggle around more, and hence makes them more likely to bump into one of the respirator's strands... where they'll stick, thanks to Van der Waals forces.

N95 masks (if properly fitted) block at least 95% of particles in that 0.1 - 1 micron range (they're tested with particles around 0.3 microns), and even higher percentages of particles that're either larger or smaller than that.

Reference: https://blogs.cdc.gov/niosh-science-blog/2009/10/14/n95/ (especially figure 2)