Cleanrooms are classified by the cleanliness of their air. This is done according to ISO 14644-1. This is an international standard, and was adopted by the European Union in 1999, and the USA in 2001. However, the most easily understood classification of cleanrooms is the obsolete Federal Standard 209 of the USA; it is still widely used.

The first Federal Standard 209 was published in 1963 in the USA, and titled "Cleanroom and Work Station Requirements, Controlled Environments". It was revised in 1966 (209A), 1973 (B), 1987 (C), 1988 (D) and 1992 (E), and withdrawn in 2001. The cleanroom class limits, given in the earlier 209 A to D versions, are shown in table 1. The class of a cleanroom is found by measuring the number of particles 0.5 m in one cubic foot of room air, and determining which class limit is not exceeded; this is the cleanroom classification.

Table 1 Federal Standard 209 class limits

In the last edition of Federal Standard 209 (E) the airborne concentrations in the room was also given in metric units, i.e. per m³ . This nomenclature never became established, and was only used in the occasional published article. The earlier version's nomenclature shown in table 1 continues to be used and is likely to be used for many years.