DEFINITION: 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 (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.
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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.
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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.
