There are four main advantages of PCM over other methods: All the fibers meeting defined criteria for asbestos are counted and considered a measure of the airborne asbestos concentration. A measured area (field) is viewed by PCM. A wedge shaped portion of the filter is removed, placed on a glass microscope slide and made transparent. Phase contrast microscopy has continued to be the method of choice for the measurement of occupational exposure to asbestos.ДЄir is drawn through a MCE filter to capture airborne asbestos fibers. The use of this technique was again required by OSHA in 1986. On May 29, 1971, OSHA specified filter membrane sampling with phase contrast counting for evaluation of asbestos exposures at work sites in the United States. This method was refined by NIOSH and published as P & CAM 239. In July 1969, the Bureau of Occupational Safety and Health published a filter membrane method for counting asbestos fibers in the United States. The British Asbestos Research Council recommended filter membrane counting in 1969. The disk is placed in one of the microscope eyepieces so that the design is superimposed on the field of view.Д®arly surveys to determine asbestos exposures were conducted using impinger counts of total dust with the counts expressed as million particles per cubic foot. There are marks around the periphery of the circle to demonstrate the proper sizes and shapes of fibers. It consists of a circle with a projected diameter of 100 + or - 2 um (area of about 0.00785 mm(2)) with a crosshair having tic-marks at 3-um intervals in one direction and 5-um in the orthogonal direction.

Walton-Beckett Graticule: An eyepiece graticule specifically designed for asbestos fiber counting. It includes any of these minerals that have been chemically treated and/or altered. Tremolite, Anthophyllite, and Actinolite: The non-asbestos form of these minerals which meet the definition of a fiber. Set: The samples which are taken, submitted to the laboratory, analyzed, and for which, interim or final result reports are generated. 3:1, 5:1 aspect ratios).Д¬leavage Fragments: Mineral particles formed by comminution of minerals, especially those characterized by parallel sides and a moderate aspect ratio (usually less than 20:1).Дetection Limit: The number of fibers necessary to be 95% certain that the result is greater than zero.Дifferential Counting: The term applied to the practice of excluding certain kinds of fibers from the fiber count because they do not appear to be asbestos.ДЇiber: A particle that is 5 um or longer, with a length-to-width ratio of 3 to 1 or longer.ДЇield: The area within the graticule circle that is superimposed on the microscope image. (Mg,Fe)(7)Si(8)O(22)(OH)(2)ДЄsbestos Fiber: A fiber of asbestos which meets the criteria specified below for a fiber.ДЄspect Ratio: The ratio of the length of a fiber to it's diameter (e.g. Nominal compositions are listed:Д¬hrysotile. The precise chemical formulation of each species will vary with the location from which it was mined. Asbestos includes chrysotile, crocidolite, amosite (cummingtonite-grunerite asbestos), tremolite asbestos, actinolite asbestos, anthophyllite asbestos, and any of these minerals that have been chemically treated and/or altered. Some terms used are unique to this method and are defined below:ДЄsbestos: A term for naturally occurring fibrous minerals. This method describes the collection of airborne asbestos fibers using calibrated sampling pumps with mixed-cellulose ester (MCE) filters and analysis by phase contrast microscopy (PCM). Similar products from other sources can be substituted. 2,400 LДЄnalytical Procedure: A portion of the sample filter is cleared and prepared for asbestos fiber counting by Phase Contrast Microscopy (PCM) at 400X.Д¬ommercial manufacturers and products mentioned in this method are for descriptive use only and do not constitute endorsements by USDOL-OSHA. The sampling time and rate are chosen to give a fiber density ofД«etween 100 to 1,300 fibers/mm(2) on the filter. The cassette must beД®quipped with an electrically conductive 50-mm extension cowl. 1.0 fiber/ccДЄ known volume of air is drawn through a 25-mm diameter cassetteД¬ontaining a mixed-cellulose ester filter. 0.1 fiber/ccД®xcursion Level (30 minutes). Appendix B to В§1910.1001 - Detailed Procedures for Asbestos

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