How fibre filters catch particles
Masks and air filters are not sieves: their fibres catch particles by impaction, interception and diffusion as the air swerves past.
The gaps between the fibres of a mask or an air filter are usually much bigger than the particles it stops, so it does not work like a sieve. As air flows through, it swerves around each fibre. A big droplet is too heavy to swerve and slams into the fibre: this is inertial impaction. A smaller particle that follows the air can still brush against a fibre and stick: interception. The very smallest particles are jostled by air molecules, wander off the airflow and touch a fibre: diffusion.
Because big particles are caught by impaction and tiny ones by diffusion, the hardest particles to catch are in between, roughly 0.1 to 0.3 micrometres across, which is why respirators are tested near that size. Every extra layer gives a particle another chance to be caught, at the cost of harder breathing.
A common misconception is that a better filter material is all that matters. Air leaking around a loose edge is not filtered at all, so the fit of a mask can matter as much as the fabric.
