Cleanroom cleanliness is measured by how many micron sized particles pass through one cubic foot of air per minute (cfm). This is how a cleanroom’s classification is determined.

Cleanroom are classified by air cleanliness levels. In ISO 14644-1:1999(E), level of air cleanliness is expressed in terms of an ISO Class N, which represents maximum allowable concentration (in particles per cubic meter of air) for considered sizes of particles. The maximum permitted concentration of particles Cn for each considered particle size D is determined from Equation

Cn = (0.1/D)2.08 X10N

Where:

• Cn is the maximum permitted concentration (particles/ m3 ) of airborne particles that are equal to or larger than the considered particle size.
• N is the ISO classification number in the ranges from 1 to 9.
• D is the considered particles size (µm).