|What Are HEPA Filters? Do They Help?
"To qualify as a "true" HEPA, the filter must allow no more than 3
particles out of 10,000 to penetrate the filtration media."
Particulate Air filters (HEPA), formerly called
high-efficiency particulate arrestors, are another option of extended-surface media
filters 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 devices 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.
Nelson, et al. (1988)
state that: "The specific designation of these devices ensures a high degree of
efficiency. It should be sought if a mechanical filter is to be used."
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.
Based on the limited available data, we conclude that if allergen sources are present
in a residence, air cleaning alone has not been proven effective at reducing airborne
allergen-containing particles to levels at which no adverse effects are anticipated. Cats,
for example, generally shed allergen at a much greater rate than air cleaners can effect
removal. Dust mites excrete allergens in fecal particles in sequestered environments
(i.e., within the carpet or the bedding). For individuals sensitive to dust mite allergen,
the use of impermeable mattress coverings appears to be as effective as the use of a
laminar flow air cleaning unit above the bed. Source control should always be the first
choice for allergen control in residences.
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.