air filter
Photo: Susan McWhinney
HEPA Filter
Getting Hep to HEPA

High-efficiency particulate air (HEPA) filters were invented during World War II as a way to prevent radioactive particles from escaping laboratories. The filters are made of various synthetic fibers; there is no construction standard because the term merely designates an efficiency rating: the ability to block 99.97 percent of all particles 0.3 micron or larger. (A micron is a millionth of a meter, or .000039 inch; the naked eye can't see anything smaller than about 25 microns.) That rating is for laboratory conditions, however. Experts say that at home you can realistically expect a HEPA filter to grab about 80 percent of such particles.

A new filter technology for the home, called ULPA (ultra low penetration air), has raised the stakes. ULPA filters block 99.99 percent of particles measuring 0.12 micron, quite a bit smaller than the HEPA threshold. But ULPA filters restrict so much air flow that in practice they are able to clean less air than HEPA filters. For best performance, look for those labeled "true HEPA."

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