One Sample Above 0.01 and a Request to Average
A commercial office renovation in Philadelphia removes 1,200 square feet of asbestos-containing ceiling plaster. After final cleaning, the project monitor performs a visual inspection and finds no dust or debris. Five inside air samples are collected on 25-mm PCM cassettes at 2 L/min for 4 hours; one outdoor sample is collected. Results: 0.006, 0.008, 0.009, 0.011, and 0.004 f/cc inside; outdoor 0.002 f/cc. The contract says the area may be reopened when the visual inspection passes and "clearance air samples are below 0.01 f/cc." The abatement contractor calculates a mean of 0.0076 f/cc, notes that the outdoor sample is lower, and asks for permission to reopen. The building owner wants the space back for Monday. The monitor has not yet received the lab's field blank result or sample volumes.
1. Interpret the results and decide whether clearance has been achieved under the contract. Explain why averaging is not appropriate here.
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Clearance has not been achieved under the contract. The contract requires clearance air samples to be below 0.01 f/cc, and one inside sample is 0.011 f/cc. Averaging the five results to 0.0076 f/cc hides that exceedance. A single sample above the clearance level can indicate incomplete cleaning, a localized release, or contamination that was missed by the visual inspection. The outdoor result of 0.002 f/cc is useful background information, but it does not convert an indoor sample above 0.01 into a passing sample. PCM also counts all fibers, not only asbestos, so the outdoor comparison helps interpret background but does not override the contract criterion. The area should remain closed. The contractor should not receive permission to reopen until the exceedance is investigated, the area is re-cleaned, and re-sampling shows every required sample below 0.01 f/cc.
2. What quality assurance information is missing, and how could it change the interpretation?
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The monitor has not yet received the sample volumes, field blank result, pump calibration records, chain-of-custody documentation, or the laboratory's accreditation and analytical report. Sample volume matters because the fiber concentration is calculated from the number of fibers counted, the field of view, and the volume of air sampled; a low or uncertain volume can make a result less reliable. A high field blank can indicate contamination during sampling or handling and may bias the results upward. The laboratory should be accredited for PCM analysis, and the report should show the limit of detection and any analyst notes. If the 0.011 sample had a low volume or a questionable field blank, the result might need to be discounted or the sample re-analyzed, but that decision must be documented by the project monitor, not made by the contractor. If the material contains less than 1% asbestos or the fibers are ambiguous, the contract may require TEM confirmation.
3. If clearance fails, what should happen before re-sampling? Include cleaning, visual inspection, and sampling considerations.
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The area should remain under containment and restricted. The contractor should re-clean all surfaces with HEPA vacuuming and wet wiping, including ledges, equipment, and the floor, and should pay particular attention to the area near the sample that exceeded 0.01. The project monitor should repeat the visual inspection after cleaning and before air sampling. The containment should be allowed to settle, and the negative-air unit should be operated according to the project design so that air is filtered before sampling. Re-sampling should use the same or a greater number of samples, calibrated pumps, and documented volumes, with field blanks and chain of custody. The monitor should not sample immediately after cleaning while dust is still suspended. If the re-sample fails again, the contractor should investigate for a hidden source, damaged containment, or contaminated HVAC system before another cleaning cycle.