Air Quality and Cleanrooms
"Filtration removes microorganisms. In cleanrooms, this is achieved through HEPA filters, which are designed to remove up to 99.997% of particles from air. HEPA filters are protected from blockage by prefilters, which remove up to about 90% of particles from the air. Therefore, if air contains about 3 x 108 particles per m3, and there is one prefilter and one HEPA Filter, the prefilter removes a sufficient number of particles to leave about 3 x 107 per m3 as a challenge to the HEPA filter. The terminal HEPA filter will leave about 103 per m3 to enter the cleanroom. In EU GMP, this relatively low number is within the limits for Grade A / ISO Class 5 and Grade B / ISO Class 7 “at rest” (Annex 1.4)."
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