Federal Standard 209E
Federal Standard 209E (FED-STD-209E) was a standard that classifies cleanrooms in relation to the limits of airbourne particles in the air of the cleanroom per volume. The classification divides the cleanrooms into six classes.
The FED-STD-209E was superseded by the ISO standard 14644 under the recommendation of the IEST Working Group CC100. Despite the cancellation of the Federal Standard, it is still used throughout the cleanroom industry.