What are HEPA filters?
HEPA stands for High Efficiency Particulate Air or High Efficiency Particulate Arrestance. A HEPA filter is a highly efficient form of air filter which is designed to remove very small particles from the air. HEPA filters are often used in commercial and residential air purifiers, and in vacuum cleaners - as they excel at removing particles such as pollen, mold and dust from the air. HEPA filters are also often found in medical applications - a good indicator of their quality.
HEPA filters are commonly used in both stand-alone air purifiers and in HVAC (Heating, Ventilation and Air Conditioning) systems. The best HEPA filters are highly efficient at cleaning the air but require fans to draw the air through the filter - which can sometimes be noisy. Many HEPA filter units have specifications for their sound level - measured in decibels (dB). This factor is worth considering if for example you are thinking of putting a standalone HEPA filter unit in your bedroom.
To achieve true HEPA specifications the filter must be able to remove 99.97% of particles .0003 of a millimeter in size. This particle size is considered the hardest to filter from air - both smaller and larger particles are more efficiently removed. HEPA filters cannot remove toxic gases from the air - only solid particles.
HEPA filters are likely, however, be effective against viruses. Studies have shown that HEPA filters are 99.99% efficient against particles in the same size range as most viruses. ( source - http://www.envirco.com/faq.asp )
HEPA filters are also likely to be
effective against some radioactive particles. It is interesting to note that HEPA filters were originally developed in the
1940s as a way to filter radioactive particles from the air. Survivalists, take note! Some HEPA air cleaner manufacturers
such as Aller
appear to have designed units with survivalists in mind.
Often, filter systems with HEPA filters have more than one stage. The first filter stage is more of a "general air filter" or "pre-filter" which removes the larger particles and "dust bunnies" and protects the more expensive HEPA filter from early loss of performance. The HEPA filter then removes finer particles. HEPA filters do not typically require maintenance until it's time to replace - and may even increase in efficiency over their lifetime.