HEPA is an abbreviation for High Efficiency Particulate Arrestor. These filters are used extensively in air cleaners and vacuum cleaners. HEPA was first developed and used by the USA Atomic Energy Commission in 1940, to prevent the spreading of airborne radioactive contaminants. It was used in the Manhattan Project during the development of the atomic bomb. It remained a classified gadget till the end of World War II, after which it was unclassified and released for commercial purposes.
HEPA air cleaners are manufactured by a number of different producers. One must be able to distinguish between a true HEPA cleaner and a HEPA like cleaner. While the latter ones are cheaper, the former are more expensive but come with a guarantee. Some filters can also require frequent change of parts and therefore are not really cheap in the long run.
HEPA air cleaners are theoretically reputed to stop 99.97% of pollutants with particles smaller than 0.3 micrometers. This has been determined by the EN 1822 [European norm] test. The efficiency of the cleaner also depends upon its location, that is, whether it is located after or before the motor. In case it is located before the motor, the carbon dust produced by it is also caught. Some of the cleaners are equipped with an UV light source to further destroy bacteria and viruses like the influenza virus too. The Clean Air Delivery Rate [CADR] varies from model to model.
These are different from other air cleaners. While the latter work with UV lights and ionizers, HEPA filters physically remove the particles. The filter doesn't act as a mere sieve for the particles. Instead, purification is affected through three methods, diffusion, interception and impaction.
Interception occurs when a particle comes within the distance of one particle radius to a filter fiber and gets stuck. It doesn't work if the distance is greater. Diffusion is best for smallest particles. Particles in the air constantly collide with each other. As a result they travel in a zigzag motion. The smaller the particle, the more it zigzags and the slower is its progress in the air current. Impaction works when a particle is so large that it's unable to adjust to the sudden changes on entering the filter. Due to its inertia, the particle continues on its original path, hits, and gets trapped by the filter fiber. Consequently, chances of hitting the filter fiber are greater. It must be kept in mind that these filters are effective only for airborne particles and cannot be depended on, solely for a healthy environment.
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