High-Efficiency Particulate Air – HEPA
To qualify as HEPA by US Government Standards, the air filter must remove 99.97% of all airborne particles larger than 0.3 microns
Contrary to what many people think, a HEPA filter dose not work like a screen or sieve trapping particles that are larger than the openings in the sieve. The space between filter fibers in a HEPA filters are sufficiently large enough to physically allow rather large particles to pass; however, the complex air flow design of a HEPA causes the particles to interact with the filter fibers in three distinct and rather complex way which can only truly be described in a complex discussion about physics.
HEPA filters cannot remove odors, or VOCs (volatile organic compounds). The best home HEPA filter systems will have additional pre-filters and after-filters for the removal of odors and VOCs.
Whole-House HEPA systems cannot filter 100% of the air moving thru your AC/Heating system; a certain percentage (depending upon the design of your system) of your return air is diverted into the HEPA fiter and the re-introduced back into the return air before the air reaches the cooling coil. This by-pass system can be a real advantage if your HEPA system is also fitted with a UV Light System. Air velocity is much lower in the by-pass filter area as opposed to your main system return duct. With lower velocities, the air is then exposed to the UV Light for longer periods, and as such, the the UV Light is even more effective at removing pathogens and other organics.