Most often, cleanrooms are validated by third-party validation agencies. This entails a thorough inspection and several tests, whereafter the cleanroom is certified to a specific class indicating its level of control, usually to an ISO 14644-1 class.
Cleanroom validation has several phases, beginning with design qualification, and ending in final certification. Some of the tests performed in these phases include airflow volume and velocity tests, HEPA/ULPA filter leak testing, air movement visualization (smoke testing), room pressurization, room recovery, airborne particle count tests, relative humidity, temperature, and other testing conditions.
Once initial certification is complete, it is important to regularly recertify to ensure that your cleanroom continues to operate as it did when it was built. At a minimum, annual certification is recommended. Depending on industry and product, semi-annual or even quarterly certification may be required.