Two standards were used to classify cleanrooms:  Federal Standard 209E (FD-STD-209E) was used in the US.  The ISO standard was used in Europe.  In November 2001,  FD-STD-209E was superceded by ISO 14644.

Cleanroom Classifications

Federal Standard	ISO
	1
	2
1	3
10	4
100	5
1,000	6
10,000	7
100,000	8
	9