What is the difference between PMC & TEM sampling for asbestos?
Air sampling is used to detect the presence of asbestos fibers in an area. PCM and TEM are two methods used to analyze air samples associated with asbestos fibers. In addition to determining if the air in a particular area contains asbestos fibers, sampling can also be an indication of how well or how poorly an asbestos removal project is proceeding. During an asbestos removal project, it is important to keep fiber counts as low as possible to reduce potential exposure to workers, building occupants and to determine if and when the work area is suitable to be reoccupied.
Phase Contrast Microscopy analysis (PCM) is most often used in the asbestos abatement industry. PCM analysis is not as accurate as Transmission Electron Microscopy (TEM). The PCM method of analysis can count fibers but cannot positively identify the fibers as asbestos. Using PCM analysis, fibers of a certain width to length ratio are assumed to be asbestos fibers and counted accordingly. In comparison, TEM sampling methods use a higher magnification thereby allowing smaller fibers to be detected and allowing the technician to positively identify an asbestos fiber or a non-asbestos fiber (see photo below).
PCM air sampling analysis is the most common method of determining asbestos fiber counts. It is far less expensive as compared to TEM analysis and turn-around time is much faster with PCM methods. In fact, PCM air samples can be read on site immediately after the sample has been taken by the technician.
Some advantages and disadvantages of analysis methods are as follows.
METHOD ADVANTAGES DISADVANTAGES
PCM • Faster turn around • Not as accurate
• Less costly • Lower magnification
• Can’t positively identify asbestos
• Could result in a false-positive result
TEM • Greater accuracy • More costly
• Less chance of false-positive result • Slow turn-around, usually 2-3 days