When the air speed is high, some dirt particles are trapped immediately as they smash onto these filters. Those that are not trapped, are snagged by fibers even as they try to brush past. Then, at lower airflow speed, the particles wander randomly around the filter. As they do this, some get attached to its fibers. Using these mechanisms, it is highly efficient to capture particles of any size.
In order to qualify as HEPA, according to the US Department of Energy, the filter must remove 99.97% of the particles, larger than .3 microns, from the air which passes through it.
A HEPA filter under 90x magnification
Sizes of common household allergens
HEPA is an acronym that stands for High-Efficiency Particulate Air, sometimes seen as the S-class on vacuum cleaners models. A HEPA vacuum filter boast of the capacity to absorb at least 99.97% of all particles over .3 microns in size, that's about 1000 times smaller than the period at the end of this sentence.
These filters are so effective that they trap pollen, dust mite waste, mold spores, even many bacteria and tobacco smoke particles, all of which could make you sick, making it ideal for people with allergies and asthma.
The only downside to HEPA is the price. The HEPA filter costs more than the regular vacuum filter. Regardless, its benefits far outweigh this minor obstacle. It has many uses in medical facilities, automobiles, aircraft, homes, and the like.
HEPA is a technology that is included in vacuum cleaners to enable it to trap more dirt, most importantly, the tiny particles that cannot be seen. It was originally developed by the nuclear industry to clean radioactive particles. The public soon took advantage of the technology and came up with ways to use it in residential cleaning and air filtration for better health.
It is important to note that a good quality HEPA vacuum directs the dirty air through the filter before the motor and not after it. If the filter is placed after the motor, pressure from the airflow will push the filter away from the housing and let dirty air escape. If you've ever noticed a peculiar smell coming from a vacuum that you thought was HEPA, this is what's happening!
The HEPA filter consists of ultra fine strands in its randomly woven material. The thickness of the fibers varies from .5 to just over 2 microns. This creates an airflow pattern that is erratic inside the filter. It is this erratic airflow that makes the HEPA filter so effective at capturing microscopic particles that easily pass through other conventional filters. It is also interesting to note that HEPA filters get MORE efficient as they become dirty!
The arrangement of fibers trap particles three different ways: