HEPA

The terminal HEPA filter is the last barrier between the air-handling system and the classified space, and its performance is a property of the installation rather than of the filter. A filter that passed its factory test can still leak through a damaged medium, a defective seal or a poorly seated frame, which is why the meaningful test is performed in situ after installation and repeated periodically — scanning the downstream face and the seal for penetration against an upstream challenge.

Filter integrity and airflow are separate questions and are commonly conflated. An integral filter passing insufficient air will not maintain the room’s air change rate or its recovery time; a filter passing ample air with a seal bypass delivers unfiltered air efficiently. Both tests belong to the classification evidence, and so does the recovery test that shows the room can return to its grade after a disturbance.

Filters are also the slow variable in a cleanroom. They load over time, which changes pressure drop and fan duty, and eventually the cascade. A differential-pressure trend across the filter bank is the leading indicator; the alarm is the lagging one.

• —Performance is a property of the installation — tested in situ after installation, not on the factory certificate.
• —Leaks occur at the medium, the seal and the frame seating.
• —Integrity and airflow are separate tests; passing one does not imply the other.
• —Recovery testing shows the room can return to grade after a disturbance.
• —Filter loading changes pressure drop and eventually the cascade — trend it rather than wait for the alarm.

ISO 14644-3 for installed filter system leakage testing; EU GMP Annex 1 (2022); ISO 29463 for filter classification.