PCM (Phase Contrast Microscopy) Fiber Counting

Phase Contrast Microscopy (PCM) is a widely used, cost-effective method of measuring airborne fiber concentrations in areas where asbestos contamination is suspected. For total fiber counting, Micro Analytical Laboratories, Inc. follows guidelines of the NIOSH 7400 method and the Code of Federal Regulations, 29 CFR 1910.1001. Fibers equal to or greater than approximately 0.25µm in diameter, greater than 5µm in length and with a length: width ratio > 3:1, are countable by this method.

Although PCM was developed for monitoring worker exposure during asbestos abatement projects, it has been applied for final clearance of areas that measure less than 1500 square feet and less than 500 linear feet, according to AHERA regulations (U.S. EPA, 1987). However, the method is limited in its use because positive identification of fibers as "asbestos" or "non-asbestos" is not possible, and because most fibers below the previously mentioned size limits are not detectable by PCM. For positive identification of asbestos fibers, Transmission Electron Microscopy (TEM) must be used.

Air filter samples tested by PCM are collapsed with acetone, mounted on slides with triacetin, and examined with positive phase contrast microscopes (PCM). Fibers meeting the required length and length/diameter criteria, which fall within the PCM's calibrated graticule field, are counted for several fields of view. Airborne fiber concentrations are calculated using the air volumes that were pumped through each filter.