Courtesy: http://www.mems.louisville.edu/index2.html
A cleanroom is an environment where airborn particulates are controlled
through an exchange of highly filtered air using a high effeciency
particulate air (HEPA) filtering system, and through minimization of
activities that generate particles.
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A class 100 cleanroom maintains less than one hundred
particles larger than 0.5 microns in each cubic foot of air space.
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A class 1000 cleanroom has less than 1000 such
particles per cubic foot.
In addition to particle control, the cleanroom is temperature
and humidity controlled to 70F, 45% RH.
Protocols help mantain the best possible environmental conditions when
something goes wrong with the air handling systems.
(S.M. SZE, Semiconductor Devices, Pg. 430)