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FAQs About HEPA Filters by
Abatement Technologies
(Questions with links to answers
below)
-
What does the term HEPA mean?
- HEPA is an acronym
for "High Efficiency Particulate Air" or "High Efficiency
Particulate Arrestance." This acronym refers to a filter that is
manufactured, tested and certified to meet Institute of
Environmental Sciences and Technology (IEST) construction,
performance and certification standards as currently published in
IEST RP-CC001.3.
-
How long have HEPA
filters been in use?
- The first HEPA
filters were developed for the Atomic Energy Commission during World
War II for use in facilities manufacturing components for the
Manhattan (atomic bomb) project. These HEPA filters were originally
designed to capture microscopic radioactive particles too small for
effective removal by existing types of filters. HEPA filters used
today are much more efficient and economical than the products made
in the 1940's.
-
Where are HEPA filters used
today?
- HEPA filters are
generally specified for applications where microscopic airborne
particles or biopollutants could cause human health or product
quality problems. Typical users include military, nuclear,
pharmaceutical, electronics, biological and medical facilities.
-
What is it that makes
HEPA filters so efficient?
- The ultra-fine
glass-fiber medium captures microscopic particles that can easily
pass through other filters by a combination of diffusion,
interception and inertial impaction. To qualify as a Type A HEPA
filter, the filter must capture at least 99.97% (9,997 out of
10,000) of particles 0.3 microns in size–about 300 times smaller
than the diameter of a human hair, and 25 to 50 times smaller than
we can see. To a HEPA filter, catching a one-micron particle
(1/1,000,000 of a meter) is like stopping a cotton ball with a door
screen.
-
Are filters
this efficient really necessary for IAQ applications?
- Laser particle
counter measurements typically show that more than 99% of the
particles suspended in indoor air are one micron (1/1,000,000 of a
meter) or smaller in size. EPA calls these “lung-damaging”
particles, because they can lodge deep in the lungs when inhaled.
The ability of HEPA filters to capture particles this small is what
sets them apart from other types of filters. Regulations developed
by EPA, OSHA, CDC and other federal, state and local government
agencies responsible for human health and IAQ issues specify HEPA
filters for asbestos, lead and mold abatement, TB and SARS isolation
rooms and healthcare renovation projects.
-
Are all
filters made with HEPA filter media HEPA filters?
- Manufacturing a
filter with HEPA filter media does not mean that the filter itself
meets true HEPA efficiency requirements. Serious filter leakage can
go undetected if filters are not individually tested and certified
at the end of the manufacturing process. Even the tiniest pinhole
leaks in the media or breach of the seal between the media pack and
the filter frame can cause the filter to fail IEST requirements. The
testing requires very specific procedures using a thermally
generated mono-dispersed aerosol and a laser particle counter. Some
regulations also require field-testing by the user prior to going
into service and periodically thereafter.
-
Why is the testing done
with a 0.3-micron particle size test aerosol?
- Filter efficiency
studies have shown that 0.3-microns is the "Most Penetrating
Particle Size (MPPS)" for HEPA filter media. Efficiency is typically
greater than 99.97% against larger or smaller particle sizes.
Particles larger than 0.3 microns are typically more easily trapped,
or intercepted, by the media. Smaller particles often lack
sufficient mass to penetrate the media.
-
Is a "HEPA
Type" filter the same as a HEPA filter?
- No. In fact, the
differences are huge. According to the American Lung Association,
filters classified as "HEPA type" filters may capture as little as
55% of 0.3-micron particles (5,500 out of 10,000). By this
definition, the true HEPA filter could be more than 1,800 times as
efficient as the "HEPA type" filter.
-
Does HEPA
filter efficiency decrease as the filter gets dirty?
- No. Unlike
electronic air cleaners and other air purification technologies that
experience substantial loss of efficiency as they become dirty,
exactly the opposite typically happens with HEPA filters. In fact,
the dirtier a HEPA filter gets, the more efficient it can become.
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