a particle does not become trapped because it tries to go through a hole
that is too small. Rather, while flowing through the layers of filter media, a particle becomes attached to a fiber due to a number of different mechanisms. The most common of these are gravitational settling, inertial impaction, interception, diffusion, and electrostatic attraction
https://multimedia.3m.com/mws/media/409903O/respiratory-protection-against-biohazards.pdf