The industry differentiates between the cleanliness of rooms by referring to class numbers. Federal Standard 209E, “Airborne Particulate Cleanliness Classes in Clean Rooms and Clean Zones”, September 11, 1992, categorize clean rooms in six general classes, depending on the particle count (particles per cubic foot) and size in microns ( m). The first three classes allow no particles exceeding 0.5 microns (m), and the last three allowing some particles up to 5.0 microns.
Interpreting the table above, a class 100,000 clean room limits the concentration of airborne particles equal to or greater than 0.5 microns to 1 00,000 particles in a cubic foot of air. ISO/TC209 clean room class ratings are slowly replacing the Federal Standard 209E ratings. ISO/TC209 is based on metric measurements whereas Federal Standard 209E that is based on imperial measurements. The classes, according to ISO/TC209 14644-1, are in terms of class levels 3, 4, 5…of airborne particulate cleanliness. A Class 5 means that less than 3,520 particles (0.5 microns in size) are present per cubic meter, which equals 100 particles per cubic foot. A Class 6 indicates less than 35,200 particles per cubic meter. The higher the class number, the more are the particles present.
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