A cleanroom has a controlled level of contamination that is specified by the number of particles/m3 and by maximum particle size. To give a perspective, external environment would be considered about a 5,000,000 class cleanroom.
Cleanrooms can be small or very large. Entire manufacturing facilities can be contained within a cleanroom with factory floors covering thousands of square meters. They are used extensively in Semiconductor manufacturing, Biotechnology, Life sciences and other fields that are very sensitive to environmental contamination.
The air entering a cleanroom from outside is filtered to exclude dust and the air inside is constantly recirculated through High Efficiency Particulate Air (HEPA) and Ultra Low Penetration Air (ULPA) filters to remove internally generated contaminants.