HEPA (high efficiency particulate air) filters are air filters that remove particles from the air by forcing air through screens with microscopic pores. This makes them highly effective at reducing the number of allergens and asthma triggers (e.g., dust, pollen, mold, dander, tobacco smoke) in an enclosed space. Originally designed for industrial, medical and military use, HEPA filters are now being used to clean the air in individual homes and offices.

There are two types of filters that bear the HEPA name:

• True HEPA filters
• HEPA-type filters

A True HEPA filter must be capable of removing 99.97 percent of the particles that are 0.3 microns or larger from the air. It must allow, on average, only 3 particles out of every 10,000 to pass through the filter. This means a HEPA filter is very effective at filtering out allergens and asthma triggers. HEPA-type filters use the same kind of technology as True HEPA filters. However, they are generally cheaper and do not filter out particles as effectively. The efficiency rating on a filter is a very good indication of its performance.

HEPA filters are generally expensive, and most types are sold in standalone versions that clean the air in a single room. Though they are available, whole-house cleaning systems are often not in the price range of the average consumer. HEPA vacuum cleaners, which are capable of removing allergens from the surfaces on which they are used, are also available.