Glossary
PCM
Also: Phase Contrast Microscopy
An optical microscopy method (NIOSH 7400) used to count airborne fibers. The standard method for asbestos exposure monitoring under OSHA.
PCM stands for Phase Contrast Microscopy. The method, codified as NIOSH 7400, counts airborne fibers collected on a mixed cellulose ester filter using an optical microscope under phase contrast illumination.
PCM is the standard method for OSHA personal and area exposure monitoring under 29 CFR 1926.1101. The PEL of 0.1 f/cc and the action level of 0.1 f/cc are both expressed in PCM-countable fibers.
PCM has a key limitation: it counts any fiber meeting the dimensional criteria (length greater than 5 µm, length-to-width ratio at least 3:1, width less than 3 µm). It cannot distinguish asbestos from other fibers (mineral wool, gypsum, cellulose). In environments with significant non-asbestos fiber content, PCM can overstate asbestos exposure.
For clearance sampling in schools subject to AHERA, TEM is required because TEM can confirm fiber composition. For most non-school work, PCM is used for both exposure monitoring and clearance, with the project specifications stating which method applies.