The Indoor Air Quality Association has a brief video that helps define HEPA filtration. It is useful to know what it will filter (particles) and what it won't filter (gases). HEPA filters remove a minimum of 99.97% of particles of 0.3 microns in size. The 0.3 micron size it picked because it the most difficult particle size to filter. It is easier to filter particles bigger or smaller than this size.
The last section of the video discusses the need to make sure the filters are properly set in the filter racks to prevent bypass. This is a significant problem. I have measured particle counts upstream and downstream of filter racks and found them to be the same because the filters were not placed properly in the filter rack.
HEPA filtration is often used in asbestos and fungal abatement. It is also part of the personal respiratory protection used by those abatement workers.
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