The measure of the air quality of a clean room is described in Federal Standard 209. Clean rooms are rated as “Class 10,000,” where there exists no more than 10,000 particles larger than 0.5 microns in any given cubic foot of air; “Class 1000,” where there exists no more than 1000 particles; and “Class 100,” where there exists no more than 100 particles. Hard disk drive fabrication requires a Class 100 clean room.
People who work in clean rooms must wear special protective clothing called “bunny suits” that do not give off lint particles and prevent human skin and hair particles from entering the room’s atmosphere.