CLEANROOM ENCYCLOPEDIA
HEPA filter leak testing (DOP/PAO): why and how
The filter integrity test is the most important cleanroom acceptance test. We review the procedure, the equipment required and the acceptance criterion.
Purpose of the test
HEPA filters leave the factory with an efficiency of 99.97% or better, but transport damage, incorrect installation or a failed gasket can open a leak path. The integrity (leak) test injects a challenge aerosol (PAO or DOP) upstream of the filter and scans the downstream face with a photometer to reveal any local leak.
Procedure and acceptance criterion
The upstream concentration is usually set between 10 and 20 µg/L, and the photometer probe scans the filter face, frame and seal at about 5 cm/s with full overlap. Under ISO 14644-3, a local penetration above 0.01% of the upstream concentration counts as a leak and must be corrected by local repair or filter replacement.
The test must be repeated after installation, after any filter is moved, and at regular intervals (typically annually, or every six months in Grade A/B areas).
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