Five PCM Samples and an Average That Hides a Failure
After removal of 60 sq ft of asbestos-containing ceiling texture in an occupied public housing unit, the contractor conducts clearance sampling. Five 25-mm PCM cassettes are run at 5 L/min for 240 minutes (1,200 L each) with no aggressive sampling. Results are 0.004, 0.006, 0.008, 0.012, and 0.005 f/cc; a field blank is 0.002. An outdoor background sample is 0.003 f/cc. The contractor's average is 0.007 f/cc, and he declares the unit clear. The visual inspection was performed by the abatement worker who did the removal; a small amount of dust remains on a window sill. A window was open during sampling. The housing agency's standard requires clearance at 0.01 f/cc or less by TEM after aggressive sampling.
1. Evaluate the clearance result and the contractor's decision.
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The unit is not clear. The contractor is applying the wrong statistic and the wrong method. Clearance is not decided by averaging the samples; each sample must meet the criterion, and one result of 0.012 f/cc exceeds the 0.01 f/cc criterion in the housing agency's standard. The average of 0.007 f/cc hides that failure. In addition, the samples were analyzed by PCM, not TEM. PCM counts all fibers longer than 5 µm with the right aspect ratio and cannot distinguish asbestos from background cellulose, glass, or other fibers. With an outdoor background of 0.003 f/cc and a field blank of 0.002 f/cc, the results are close enough to background that PCM cannot support a clearance decision. The open window and the dust on the sill further undermine the result.
2. What is wrong with the sampling design and the visual inspection?
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Several design elements are wrong. Clearance sampling should be aggressive: fans, blowers, or a brush should be used to re-suspend settled dust before and during sampling, then the air should be allowed to settle for a short period as the protocol requires. Simply running cassettes in still air under-samples dust that could be released by occupancy. The window should be closed and the space sealed so that outdoor background does not dilute or contaminate the samples. Five samples is a reasonable number for a small unit, but they must be analyzed by TEM with EDX for definitive clearance. The visual inspection must be performed by a competent person who is not the worker who performed the abatement-ideally the supervisor or an independent inspector-and it must occur before sampling. Dust on the sill means the visual failed.
3. What must be done before the unit can be reoccupied?
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Re-establish containment, HEPA-vacuum and wet-wipe all surfaces, clean the window sill and frame, and remove all visible debris. Seal the window and any other openings. Have the supervisor or an independent inspector repeat the visual inspection. Then conduct aggressive clearance sampling with five TEM cassettes at the agency's specified flow rate and volume, including a field blank, and analyze by TEM with EDX. The unit may be reoccupied only when every sample is at or below 0.01 f/cc, the visual is clean, and the results are documented. If any sample exceeds the limit, repeat cleaning and sampling. The contractor's 'average' clearance should be rejected in writing.