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Chrysotile (TEM) (5kb)



PCM or TEM?


We labs are supposed to be counting asbestos fibers with phase contrast microscopy (PCM), but most of the fibers we count are not, and many of the fibers we don’t count are asbestos. Despite this fact of current analysis, OSHA decided in its recent revision, to keep the PCM method as the primary method of compliance. The reason was that all the data relating asbestos disease to exposure was PCM data, and they couldn’t change to a different method without repeating the studies using the new method, and the studies can not be repeated because they were done in asbestos mills and asbestos-containing factories that no longer exist.

So if PCM results no longer have anything to do with asbestos exposure, when, if ever, do they make sense to use, and what would be a better method?

My opinion is that PCM analyses are useful for two circumstances: 1) when OSHA compliance is the only consideration, i.e. inside of a containment during a removal, or any other situation where the worker is already wearing respiratory protection, but the exposure needs to be documented for OSHA., and 2) when it is being used as a general indication of site cleanliness, or as a screening prior to TEM analysis. My opinion is furthermore that PCM should never be used to 1) establish an exposure level for workers who are not going to wear respiratory protection (they might be breathing large numbers of small asbestos fibers, 2) clear a removal area, or 3) establish a background level.

What are the alternatives? The only instrumentation that can identify asbestos fibers fiber by fiber in an air sample is transmission electron microscopy (TEM).

NIOSH Method 7402 is a TEM method that is designed to augment PCM analysis. For a series of PCM samples taken from the same area or areas having a similar mix of fiber types, one such air sample can be analyzed via 7402 and a ratio of asbestos to non-asbestor determined and applied to the other samples. This would be useful in those situations requiring OSHA compliance (eg., inside a containment), but where interference from non-asbestos fibers has caused the PCM counts to fail. This TEM method ignores all asbestos fibers <5um long. The AHERA TEM method is the accepted state-of-the-art to determine background or clearance levels of asbestos.. Even small fibers are counted, so it gives a fair indication of whether it is healthy to breathe the air or not. The strict AHERA protocol calls for 5 filters inside an area to be tested, but the TEM methodology can be used on any number of filters, which we call modified AHERA. Since only a few fibers make the difference between clean and dirty, a single sample does not give as statistically representative a view of air levels as do multiple samples.

In conclusion, I recommend PCM only for strict OSHA compliance, and find that it is misleading when applied to background or clearance testing. Background and clearance determinations should be conducted by the AHERA or modified AHERA TEM methods.


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