This practice describes the determination of the concentration of fibers, expressed as the number of such fibers per millilitre of air, using phase contrast microscopy and optionally transmission electron microscopy to evaluate particulate material collected on a membrane filter in the breathing zone of an individual or by area sampling in a specific location. This practice is based on the core procedures provided in the International Organization for Standardization (ISO) Standard ISO-8672, the National Institute for Occupational and Health (NIOSH) Manual of Analytical Methods, NIOSH 7400, and the Occupational Safety and Health Administration (OSHA) Method . This practice indicates the important points where these methods differ, and provides information regarding the differences. However, selecting portions of procedures from different published methods generally requires a user to report that they have used a modification to a method rather than claim they have used the method as written. This practice further gives guidance on how differential counting techniques may be used to indicate where a population of fibers may be asbestos.

The practice is used for routine determination of an index of occupational exposure to airborne fibers in mines, quarries, or other locations where ore may be processed or handled. The method gives an index of airborne fiber concentration. The method provides an estimate of the fraction of counted fibers that may be asbestos. This practice should be used in conjunction with electron microscopy.