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 ©, 1988 (D) and 1992 (E).
The actual class of a cleanroom is found by measuring the number of particles greater than or equall to 0.5 micrometers in one cubic foot of room air, and determining
which class limit is not exceeded; this is the cleanroom classification.
Class 100: Particle count not to exceed a total of 100 particles per cubic foot of a size 0.5µ (micron) and larger.
Here are the limits as per 209E federal Standard:
Particles counted per cubic foot volume of air
greater than or equall to 0.1 micrometer -Not applicable
Greater than or equall to 0.2 micrometer -750
Greater than or equall to 0.3 micrometer - 300
Greater than or equall to 0.5 micrometer -100
Greater than or equall to 5.0 micrometer - Not applicable