Cleanroom Products & Cleaning Supplies
About Your HEPA Filter
The HEPA in HEPA filter stands for High-Efficiency Particulate Air, although the A has also been said to mean absorber, arrestance, or arresting. To be a true HEPA filter, it must meet standards stipulated by the US Department of Energy (DOE). These standards demand that the filter must remove 99.97% of particles with a size of 0.3 microns.
Usage of HEPA Filters
HEPA is not a new term. It was commercialized in the 50s and was then a registered trademark. Now, however, it is a generic term for filters that are highly efficient.
A filter meeting the HEPA standard may be used in medical facilities, automobiles and aircraft, vacuum cleaners, and, of course, clean rooms.
Beware: if you are being sold a filter that claims to be HEPA style or 99% HEPA, it’s not the real deal. A true HEPA filter removes all but 0.03% of particles 0.3 microns in size.
For illustration, consider that one micron is one-millionth of a meter, which could be about 100th the width of a human hair (depending on the person, of course). A particle under 10 microns is invisible to the naked eye, so you cannot exactly see the HEPA filter working.
How does a HEPA filter work so efficiently?
Some filters work like a sieve. Particles that are too big to fit through the holes get stuck. Not so with a HEPA filter.
If we’re talking HEPA, we’re talking about particles that are so small you can’t see them. A HEPA filter removes these from the air by making them stick to a fiber. This happens through diffusion, interception, inertial impaction, and electrostatic attraction.
Impaction works best with higher air flow and with particles above 4 microns. Diffusion works best at lower air velocities and with particles below 0.1 microns. This is more than many people need to know, but what is particularly interesting is that between the two is an area of inefficiency.
For particles of 0.21 microns, described as the “most penetrating particle size,” impaction and diffusion are equally inefficient. The retention of particles near this size (0.3 microns), therefore, has been used to classify the performance of the filter you are purchasing.
HEPA filter with functional description ( image credit: LadyofHats created as part of the Philip Greenspun illustration project )