Filtration is an important aspect of clean rooms. Clean  rooms require  very high efficiency filters and for class 100 and below,   100% HEPA filter coverage is recommended. HEPA (high efficiency particulate arrestance)  filtration is 40% more efficient  then  the  highest  efficiency  rated ASHRAE filter.

Clean room air filtration technology centers around two types:

High efficiency particulate air (HEPA) : HEPA filters are replaceable extended media dry-type having a minimum particle collective efficiency of 99.97 to 99.997% for a 0.3 micron particle and a maximum clean filter pressure drop of 2.54 cm (1”) water gauge when tested at rated air flow capacity 0.3  micron  is 1/75,000  of an  inch or 1/300, the diameter of the human hair.

Ultra low penetration air (ULPA) : Most ULPA filters are replaceable extended media dry filters that have a minimum particle collection efficiency of 99.999% efficient for particles greater than or equal to 0.12-micron in size.