Cleanrooms are classified according to how clean the air inside them is. Air cleanliness is measured according to the number of particles that are equal to or greater than 0.5 millimeters per cubic foot of air. Cleanrooms are classified according to Federal Standard 209 in the United States, or the International Standards Organization's TC 209 standard, as of 2015. There are nine ISO classes for cleanrooms that are based on the number and size of the particles in the air.
Cleanrooms are often used in manufacturing and scientific research, and are very important to pharmaceutical manufacturers. Cleanrooms may be fitted with high-efficiency particulate air (HEPA) filters, ultra-low particulate air (UPLA) filters and dehumidifiers. They may be extremely large; microelectronics and semiconductor manufacturing plants may be thousands of square meters. Air is filtered as it enters the cleanroom, and is then constantly recirculated and filtered again.
Staff working in cleanrooms must wear protective clothing and enter and leave the room through an airlock. Cleanrooms are furnished with special furniture and cleaned with special equipment to minimize the number of particles in the space. Although cleanrooms have very few particles in the air, they are not necessarily sterile. Sterility refers to the number of uncontrolled microbes in an environment.