Also, many filters actually can stop smaller particles than you might expect based on the rating. Most people think of a filter as working the way a net does. There is a gap size between the fibers, and anything smaller than that gets through easily, and anything larger does not.
It's more complicated because there are actually 4 mechanisms that might stop a particle from getting through.
1. For particles bigger than the gaps, it does work like a net as most people expect.
2. For particular smaller than that but massive enough that they cannot change direction fast enough to follow the air stream they can miss turns of the air flow and get embedded in the fibers.
3. For even smaller particles they can get jostled around significantly by hits from gas molecules (like with Brownian motion). This can knock them into fibers where they can become embedded.
4. Some filters materials have an electrostatic effect that can attract and hold passing particles.
One of the reasons filters are commonly rated by their efficiency at 0.3 micron is that this is in the region where the filter is least efficient. For bigger particles, the gains from #2 and later #1 more than make up for #3 being irrelevant at those sizes. For smaller particular, the gains from #3 increase efficiency.
So you have the filter roughly getting worse down to the 0.3 micron ballpark, and then getting better as particles become still smaller, and at some point as you continue getting smaller it is going to get worse again. (I don't know where that final turn around is).
It's more complicated because there are actually 4 mechanisms that might stop a particle from getting through.
1. For particles bigger than the gaps, it does work like a net as most people expect.
2. For particular smaller than that but massive enough that they cannot change direction fast enough to follow the air stream they can miss turns of the air flow and get embedded in the fibers.
3. For even smaller particles they can get jostled around significantly by hits from gas molecules (like with Brownian motion). This can knock them into fibers where they can become embedded.
4. Some filters materials have an electrostatic effect that can attract and hold passing particles.
One of the reasons filters are commonly rated by their efficiency at 0.3 micron is that this is in the region where the filter is least efficient. For bigger particles, the gains from #2 and later #1 more than make up for #3 being irrelevant at those sizes. For smaller particular, the gains from #3 increase efficiency.
So you have the filter roughly getting worse down to the 0.3 micron ballpark, and then getting better as particles become still smaller, and at some point as you continue getting smaller it is going to get worse again. (I don't know where that final turn around is).