The standards of reference for filter classification for air are today the EN779:2005 (filters for general ventilation) and the EN 1822:2002 (absolute filters HEPA & ULTRA)

filters application in the aeraulic installations in compliance with uni 10339

GENERAL VENTILATING FILTER

Filters group G are submissing to a gravimetric test that uses a synthetic dust; in this case the filtration efficiency depends of a ratio between the mass held by filter in relation with the mass of dust injected in frontside of filter.
Filters group F are submissing to a test of particle counting that uses polydispersible aerosol. In this case the efficiency of filtration depends of a ratio between the number of particles injected in backside of filter and the number of particles injected in the frontside.

ABSOLUTE FILTER HEPA & ULTRA

The method essentially consists in a preliminary test of filtrating medium and the test of the filter ready for use. The first test is necessary to establish the MMPS, the size of particles most difficult to hold; this test is executing with the same speed of the crossing that develops in the rated capacity filter ready for use.
The second test is based on the method of particle counting and uses the monodispersible aerosol around of MMPS, or in the conditions where the holding of particles is harder. The filtration efficiency depends of a number of particles injected in the frontside.

Standard ISO 14644-1 defines 9 classes of cleanliness of air for ambients
with controlled pollution according the following data schedule: