To guarantee a specific cleanroom classification requires a good cleanroom design (non-particulate materials, etc.) but also good cleanroom air flow design allowing clean filtered air sufficient to both dilute existing particulates and to drive “dirty” air out of the controlled space.

Fan-filter units (FFUs) are generally used in modular cleanrooms to provide HEPA- or ULPA-filtered air. FFUs provide air flow at velocities of about 90 to 110 feet per minute, fast enough to generate laminar flow but slow enough to prevent excessive turbulence.

Assuming a clean physical design, air flow determines the design cleanliness of a cleanroom. Cleanroom classes, ISO standards and general recommendations in terms of air-changes-per-hour are:

The cleanroom class is the number of particles >= 0.5 μm in diameter per cubic foot. For instance, a Class 1,000 cleanroom could contain an average of up to 1,000 particles per cubic foot of space.