To qualify as a true HEPA, a filtering system must allow no more than 3 particles out of 10,000 to penetrate the filter. You can find HEPA filters on standard central filtration systems and on HEPA portable units.
The filter of a HEPA air purifier is made of tiny glass fibers that produce a consistency similar to blotter paper. This means that a HEPA purifier is like a tremendous high-pressure system, with air is forced through the filtration system under great pressure.
As you can imagine, this makes HEPA filters an expensive proposition in a residence. With powerful fans and engine horsepower, HEPA in a home can be an enormous and consistent drain on electricity.
However, don't make the mistake of comparing horsepower or wattage of brands to determine performance ratings. When it comes to HEPA effectiveness, look into air displacement effectiveness.
Air displacement describes the amount air moved, measured in cubic feet per minute (CFM). Look for purifiers with high air-displacement HEPA filters for the best recovery of smaller particles such as bacteria, pharmaceutical dust, lead paint or asbestos fibers. A small room purifier may offer as little as 80 CFM displacement, while larger industry models can range on average from 500-80,000 CFM.
Finally, if you want ultimate protection and cost is not an issue, try ULPA (or ultra-HEPA) filters. These are designed to trap 99.999% of all particles that .3 microns or larger in diameter.
HEPA filters were actually designed by the Atomic Energy Commission during World War II to capture and remove radioactive dust particles from the air that posed a health hazard to researchers. Today, true HEPA filters are considered to be among the best forms of air filtration available and are used in hospitals, laboratories and even by NASA to create contamination-free clean rooms.