1.1 This practice² 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(1)³, the National Institute for Occupational and Health (NIOSH) Manual of Analytical Methods, NIOSH 7400 (2), and the Occupational Safety and Health Administration (OSHA) Method ID 160 (3). 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.

1.2 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 (See Appendix X1) for assistance in identification of fibers.

1.3 This practice specifies the equipment and procedures for sampling the atmosphere in the breathing zone of an individual and for determining the number of fibers accumulated on a filter membrane during the course of an appropriately-selected sampling period. The method may also be used to sample the atmosphere in a specific location in a mine or in a room of a building (area sampling).

1.4 The ideal working range of this practice extends from 100 fibers/mm² to 1300 fibers/mm² of filter area. For a 1000-L air sample, this corresponds to a concentration range from approximately 0.04 to 0.5 fiber/mL (or fiber/cm³). Lower and higher ranges of fiber concentration can be measured by reducing or increasing the volume of air collected. However, when this practice is applied to personal sampling in mines and quarries, the level of total suspended particulate may impose an upper limit to the volume of air that can be sampled if the filters produced are to be of appropriate particulate loading for fiber counting.

1.5 Users should determine their own limit of detection using the procedure in Practice D6620. For reference, the NIOSH 7400 method gives the limit of detection as 7 fibers/mm² of filter area. For a 1000-L air sample, this corresponds to a limit of detection of 0.0027 fiber/mL (or fiber/cm³). For OSHA ID 160 the limit of detection is given as 5.5 fibers/mm² of filter area. For a 1000-L air sample, this corresponds to a limit of detection of 0.0022 fiber/mL (or fiber/cm³).

1.6 If this practice yields a fiber concentration that does not exceed one-half the permissible exposure limit or threshold limit value for the particular regulated fiber variety, no further action may be necessary. If the fiber concentration exceeds one-half of the regulated permissible exposure limit or threshold limit value for the particular regulated fiber variety, it is necessary to examine the data to determine if more than 50 % of the counted fibers are thinner than 1.0 μm, or thicker but with an appearance of asbestos (curvature, splayed ends, or the appearance of a bundle).

1.7 The mounting medium used in this practice has a refractive index of approximately 1.45. Fibers with refractive indices in the range of 1.4 to 1.5 will exhibit reduced contrast, and may be difficult to detect.

1.8 Fibers less than approximately 0.2 µm in diameter may not be detected by this practice. (4)

1.9 This standard may involve hazardous materials, operations, and equipment. This standard does not purport to address all of the safety problems associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific precautionary statements, see Section 7.