It is particles that are around 0.3 microns in size that filters have the most difficulty blocking. N95 particles block 95% of particles of 0.3 microns in size. N99 Masks block 99% of particles at 0.3 microns.

As I understand it smaller particles get ‘stuck’ in the maze of fibres and are statistically unlikely to find there way through the mask. The best analogy I can think of is a spec of dust finding its way into a centre of a huge labarynth that is vibrating randomly in all directions (and coated in honey). Some respirator masks also contain anti viral coatings and are electrostaticaly charged.