The cleanliness of a cleanroom is determined by a U.S. Government specification known as Federal Standard 209D.
Six classes have been established to provide a qualified and standardized method for measuring how clean the air is in a cleanroom. The class number refers to the maximum number of particles bigger than half a micron that would be allowed in one cubic foot of cleanroom air. (The "Class" categories have now been superseded by ISO Classifications).
Primasil's cleanroom is certified ISO7 (Class 10,000), and so would not contain more than 10,000 particles bigger than half a micron in a cubic foot of air.
