Comment by aidenn0

This is how it works because the room is not sealed, nor is the filter being used to filter outside air into a positive pressure area.

It is (hopefully) easy to see that e.g. a filter that removes 99.5% of particles, but moves twice as much air per minute will remove almost twice as many particles per minute as a filter that removes 99.95% of particles.

Using the numbers from TFA (20% of the room for the 99.5 rather than 10%):

  .2 × (1-.995) + .8 = 0.801

vs

  .1 × (1-.9995) + .9 = 0.90005

Thus proving the point in TFA that the airflow matters more than E12 vs H13. The fact that the steady state (given that "dirty" air is being introduced somehow) is lower for the filter that moves more air follows from the fact that it removes particles at a faster rate.