"To qualify as a "true" HEPA, the filter must allow no more than 3 particles out of 10,000 to penetrate the filtration media."

High-Efficiency Particulate Air filters, formerly called high-efficiency particulate arrestors, are another option of extended-surface media devices to consider.

This unique design was originally developed during World War II to prevent discharge of radioactive particles from nuclear reactor facility exhausts. They have since become a vital technology in industrial, medical, and military clean rooms and have grown in popularity for use in portable residential air cleaners.

Hepa filters have been traditionally defined as an extended-surface dry-type filtration system having a minimum particle removal efficiency of 99.97% for all particles of 0.3 micron diameter with higher efficiency for both larger and smaller particles. This rating is determined using a test challenge smoke that consists of particles of 0.3 micron average diameter.

Additionally, the 1990 review of indoor air pollutants and environmental controls published by the American Thoracic Society (1990) concludes that: "High-efficiency particulate filters are highly efficient in removing particles of a wide range of size. A room-size unit will control particles in that room, and a central unit will remove particles from the air of the building when the ventilation system is operating."

Overall, the American Lung Association recommends that proven source control strategies be employed as a primary means of reducing exposure to pollutants. However, physical studies which do not measure health effects do show that certain air cleaners are effective in removing certain indoor air pollutants. Thus, as an adjunct to effective source control and adequate ventilation, highly efficient air cleaners can be useful in further reducing levels of certain indoor air pollutants. More research on the health benefits of air cleaners is needed to provide complete evidence that would better address the circumstances of intended use. Manufacturers, clinicians, government agencies, and private industries can all assist with providing and interpreting this research in order to better inform the public.

The reality in most residences is that total elimination of a pollutant source is not always possible or practical. Individuals with severe allergy and asthma symptoms, whose symptoms are not alleviated by other source control and ventilation strategies, may want to try an effective air cleaner in an attempt to aid in further exposure reduction. Although there is no proven health benefit from such a measure, some individuals report that they perceive air cleaners as useful in improving their health status.