A HEPA filter in a ceiling grid is easy to forget once the room is qualified. It is also the last barrier between the air handling system and the product. Integrity testing checks two things: that the filter medium has no pinholes or damage, and that the seal between the filter and its housing is tight. The test is simple in principle. Getting a result you can trust takes some care.
The photometer method
The method most used in pharmaceutical cleanrooms is the aerosol photometer scan, described in ISO 14644-3. An aerosol, usually polyalphaolefin (PAO), is introduced upstream of the filter. The concentration upstream is measured. Then the downstream face of the filter, its edges and the seal are scanned slowly with a photometer probe, looking for any point where the aerosol comes through.
The upstream concentration needs to be high enough to make small leaks visible, normally in the range of 10 to 100 µg per litre. Too low, and a real leak may not register. Too high, and the aerosol can load the filter and generate false readings near the edges.
What counts as a leak
A leak is defined as a downstream reading above 0.01 % of the upstream concentration. Because the photometer is set to read upstream as 100 %, a leak shows as any reading above 0.01 % during the scan. When a reading goes above that level, the probe is held still at that point to confirm it.
Scanning technique
- Hold the probe about 2 to 3 cm from the filter face.
- Move at no more than about 5 cm per second, in overlapping strokes.
- Cover the whole face, the full perimeter of the frame and the seal.
- Do not touch the filter medium. It is easy to damage, and a scratch made during the test is still a leak.
Rushing the scan is the most common cause of a false pass. At twice the speed, a small leak passes under the probe too quickly to register.
Repairs
Small leaks in the medium can usually be repaired with a suitable sealant, within limits set by the filter manufacturer and your own procedure, often expressed as a maximum percentage of the filter face. A leak in the seal is normally fixed by re-seating or replacing the gasket. Whatever the repair, the filter is retested afterwards, and the repair is recorded with its position and size.
How often
Filters are tested at installation, after any work that could disturb them, and periodically after that. EU GMP Annex 1 expects filter integrity testing for Grade A and B areas, and many sites test every six months in those areas and annually in Grade C and D, with the interval justified by risk and history.
What the report should show
- Filter identification and location on a drawing.
- Aerosol type, upstream concentration and how it was measured.
- Photometer calibration status.
- Result for each filter, any leak positions and readings, and any repairs.
- Retest results after repair.
A filter that passes this test properly is one less thing to worry about when an environmental monitoring result goes the wrong way.
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