What is it that makes HEPA filters so efficient?
The ultra-fine, glass-fiber medium captures microscopic particles that can easily pass through other filters by a combination of diffusion, interception and inertial impaction. To qualify as a Type A HEPA filter, the filter must capture at least 99.97% (9,997 out of 10,000) of particles 0.3 microns in size–about 300 times smaller than the diameter of a human hair, and 25 to 50 times smaller than we can see. To a HEPA filter, catching a one-micron particle (1/1,000,000 of a meter) is like stopping a cotton ball with a door screen.
