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 99.97o/o (9,997 out of 10,000) of particles 0.3
microns in size-about 300 times smaller than the diameter of a human hail 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.

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