A "cleanroom" is essentially a controlled environment in which the levels of particulate, microbes, and contamination of all kinds are reduced. Frequently this is accomplished by continuously "flushing" the cleanroom with highly filtered air that is forced in through High Efficiency Particulate air (HEPA) filters. HEPA filters can prevent over 99.97% of particles measuring greater than 0.3 microns in size from entering the cleanroom. This is remarkable considering that the outside air we breathe may contain up to 5 million suspended particles of dust, smog, and pollen in one cubic foot.

There are two basic types of cleanrooms: Horizontal Laminar Air Flow and Vertical Laminar Air Flow. In Horizontal Laminar Flow, HEPA filters in a wall force clean air from one side of the room to the other. In Vertical Laminar Flow, HEPA filters on the ceiling push clean air down to the floor. In both cases, air is forced through the filters at a rate of about 100 feet per minute, traveling in a uniform, piston-like fashion known as Laminar Air Flow.

As long as it is relatively unobstructed, the air will flow in a straight path where it will eventually be released or exhausted to the outside.

The contamination control industry currently uses a government specification known as Federal Standard 209D to provide a qualified and standardized method for measuring how clean the air is in a cleanroom. Six classes have been established to designate cleanroom cleanliness:

Class 100,000, 10,000, 1,000, 100, 10 and 1. The class number refers to the maximum number of particles bigger than one half of a micron that would be allowed in one cubic foot of cleanroom air. A Class 100 cleanroom, for example, would not contain more than 100 particles bigger than half a micron in a cubic foot of air.

The actual process of certification for a cleanroom is a subject of great debate. Certifications can be performed when cleanrooms are empty, or with people in the room simulating actual working conditions, at the HEPA filter face, or somewhere downstream. Ultimately, Federal Standard 209D allows the user to decide under what conditions to certify the cleanroom.