Air Monitoring Results and Clearance Decisions
A containment for removal of 200 square feet of asbestos-containing ceiling tile (5% chrysotile) has been completed. The contractor conducted aggressive air monitoring using PCM (phase contrast microscopy) after final cleaning. Three samples were collected: 0.003 f/cc, 0.004 f/cc, and 0.005 f/cc. The background level outside the containment was 0.002 f/cc. The clearance criterion in the contract is 0.01 f/cc or less by PCM. The project monitor also collected a TEM (transmission electron microscopy) sample, which showed 0.008 f/cc of chrysotile fibers. The contractor wants to remove the containment and declare clearance. The building owner's representative is concerned because the TEM result is higher than the PCM results. The project is on a tight schedule, and the contractor argues that the PCM results meet the contract specification.
1. Should the containment be cleared based on the PCM results? Explain.
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The PCM results (0.003-0.005 f/cc) are below the contract clearance criterion of 0.01 f/cc, but the TEM result of 0.008 f/cc indicates that asbestos fibers are present. PCM cannot distinguish asbestos from other fibers, so the TEM result is more specific. The contract criterion may be based on PCM, but the presence of asbestos at 0.008 f/cc suggests incomplete cleaning or residual contamination. The project monitor should consider the TEM result and the background level. The difference between PCM and TEM is small, but TEM detects only asbestos, so 0.008 f/cc of chrysotile is significant. Best practice is to not clear if TEM shows asbestos above background or a threshold (e.g., 0.01 f/cc). The contractor should re-clean and re-sample. Clearing based solely on PCM could lead to occupant exposure and liability.
2. What additional steps should be taken before clearance is granted?
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The contractor should perform additional cleaning of all surfaces, including HEPA vacuuming and wet wiping, and then re-sample using both PCM and TEM. The number of samples should be adequate for the containment size (at least 3 for small containments, more for larger). The project monitor should ensure sampling is aggressive (e.g., using a leaf blower or air mover) to disturb settled fibers. The clearance criteria should be clearly defined: if TEM is used, the threshold might be 0.01 f/cc or background level. The building owner should be informed of the TEM result and the plan for re-cleaning. Documentation of the re-cleaning and new sampling results is required. The containment should not be removed until clearance is achieved by both PCM and TEM, or as specified in the contract.