HEPA is an acronym that stands for High Efficiency Particulate Air. They are used in HEPA air purifiers, HEPA vacuums, industrial and laboratory exhaust hoods, and respirator masks. HEPA filters are defined as one which removes least 99.97% of airborne particles measuring at least 0.3 micrometers (µm) in diameter.
HEPA filters are made up of mats of randomly oriented fibres, usually fibreglass. The fibres are between 0.5 and 2.0 micron in diameter, while air space between the fibres in the filter mat is much greater than 0.3 μm.