Modular Cleanrooms are classified by how clean the air is

In Federal Standard 209 (A to D) of the USA, the number of particles equal to and greater than 0.5mm is measured in one cubic foot of air, and this count is used to classify the cleanroom.

This metric nomenclature is also accepted in the most recent 209E version of the Standard. Federal Standard 209E is used domestically. The cleanroom classification standards FS 209E and ISO 14644 require specific particle count measurements and calculations to classify the cleanliness level of a cleanroom or clean area.

In the UK, British Standard 5295 is used to classify cleanrooms. This standard is about to be superseded by BS EN ISO 14644 .