PCM or TEM?
Phase contrast microscopy (PCM) is often used to monitor asbestos exposure, but most of the fibers we count using PCM are not asbestos, and, conversely, many of the fibers that are too small to count using PCM actually are asbestos. Often, asbestos fibers are so small, transmission electron microscopy (TEM) is required to analyze them. Despite this fact, the PCM method is still the primary method of compliance because the relationship between asbestos disease to exposure was established with PCM data from asbestos mills, factories and mines. To relate asbestos disease to TEM data would require finding or artificially producing areas that have high asbestos levels - much higher levels than those that happen in this day and age, even in asbestos mills, factories and mines, if such even exist any more.
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?
PCM analyses are useful in 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 purposes, and 2) when it is being used as a general indication of site cleanliness, or as a screening prior to TEM analysis. Furthermore, it is Fiberquant's position that PCM should not 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 an asbestos removal area, or 3) establish a background asbestos level for areas that not asbestos mills, factories or mines - i.e., almost everywhere.
What are the alternatives? The only instrumentation that can identify asbestos fibers fiber by fiber in an air sample is transmission electron microscopy (TEM).
---The 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-asbestos determined and applied to the other samples. This would be useful in those situations requiring OSHA compliance (e.g., 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 basic AHERA TEM methodology can be used on any number of filters, which we at Fiberquant 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, we recommend PCM only for strict OSHA compliance, and consider it misleading when used to represent asbestos background levels or after-clearance levels. Background and clearance determinations should be conducted using the AHERA or modified AHERA TEM methods.