Clearance Sample Just Over 0.01 f/cc
A contractor has completed removal of 240 linear ft of friable pipe insulation inside a negative-pressure enclosure. The visual inspection is clean. Six PCM air samples were collected using aggressive sampling; five results are 0.003 f/cc and one is 0.013 f/cc. The laboratory reporting limit is 0.002 f/cc. A background sample outside the enclosure is 0.006 f/cc. The project supervisor wants to declare clearance because the average of the six samples is about 0.005 f/cc, well below 0.01 f/cc, and because the enclosure looked clean. The building owner is pressing to reopen the area the next morning. The samples were collected on mixed-cellulose ester filters and analyzed by phase contrast microscopy, so the report does not identify whether the fibers are asbestos. The supervisor asks whether clearance can be granted and what should happen next.
1. Should clearance be granted based on the six-sample average? Explain.
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No. Clearance is not decided by averaging the samples. Under the usual aggressive-sampling clearance approach, each sample must be below the clearance limit, commonly 0.01 f/cc by PCM. One sample at 0.013 f/cc exceeds that limit, so the enclosure has not passed.
A clean visual inspection is necessary but not sufficient; it does not prove that airborne fiber levels are acceptable. The supervisor should also check whether the high sample is valid: pump flow, sample time, filter loading, field blanks, and chain of custody. PCM counts all fibers of the relevant size, not just asbestos, so background at 0.006 f/cc matters. Even so, the correct interim decision is to keep the containment closed and investigate rather than average away the failure.
2. What actions should be taken before re-sampling?
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Before re-sampling, the contractor should find and correct the source rather than simply repeating the test. Actions include:
- Inspect the enclosure for tears, failed seams, or dust migration from the work area.
- HEPA-vacuum and wet-wipe all surfaces, including ledges and poly overlaps.
- Verify that HEPA air machines are operating and filters are seated.
- Allow the enclosure to settle, then repeat aggressive sampling with the same method and include a field blank.
- Confirm the sampling pumps are calibrated and the analytical laboratory is using the proper counting rules.
If background remains high, the contractor should consult the project specifications and the overseeing authority about background correction or TEM analysis. The containment stays up until a valid set of samples passes.
3. If re-sampling produces one result of 0.011 f/cc and background remains 0.006 f/cc, what is the proper next step?
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The area still has not passed if the applicable clearance criterion is 0.01 f/cc per sample. The result is close to the limit, but a single exceedance cannot be ignored. The contractor should not simply subtract 0.006 from 0.011 unless the written protocol specifically permits background correction.
Because PCM cannot distinguish asbestos from other fibers, TEM analysis can help determine whether the airborne structures are asbestos and whether they came from the work. The contractor should continue HEPA cleaning, check for hidden debris, and re-sample. If TEM shows asbestos above the project criterion, more cleaning and containment are needed. If TEM shows no asbestos but PCM remains high, the contractor should document the interference and obtain approval from the owner and regulator before releasing the area.