The air change rate, or air changes per hour, is the amount of time a cleanroom can completely replace its air volume within an hour. The higher the air change rate, the more effective it will be at removing contamination, therefore achieving a higher class. Often abbreviated to ACPH, ACH, or alternatively called Air Change Rate.

ISO 14644-1 requirements describe contamination levels in terms of allowed air particulates in number and size. IEST Technical Guides serve as complementary documents to ISO Standards ISO 14644. They give guidance on recommended air change rates or airflow speed within the cleanroom, and outline the necessary percentage of ceiling coverage for filtration.