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FAQs About HEPA Filters
(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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