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 commonly performed to improve consistency of counting. A description of several quality assurance techniques for asbestos fiber counting is described by Abell et al [28]. To fulfill Method 7400 requirements for an interlaboratory sample exchange, Tombes and Calpin have described a simple approach using appropriate statistical tests [29]. Quality check samples. In order to get agreement between laboratories within a country or internationally, several programs send out identical samples to participating laboratories to assess their relative performance [30-33]. These programs provide feedback, often tied to laboratory accreditation, which provides incentive for laboratories to ensure that their performance is similar to that of other laboratories. d. Qualitative fiber analysis In addition to simply counting the fibers, there are techniques available for providing at least tentative identification of fiber type; use of these techniques is commonly called differential counting. Fiber shape can be used to limit the type of fiber counted. For instance, glass fibers tend to be straighter, with smoother sides than chrysotile fibers. Polarizing light techniques can also be used to identify larger diameter (> 1 :m) fibers. These are based on the optical properties of the materials, including refractive index and crystallinity. These techniques can provide quite positive identification for the presence of certain types of fibers, but are limited in application to airborne fibers because they only work for the larger diameter fibers. These techniques are often used in analysis of bulk materials [34]. The use of identification techniques is not allowed in reporting fiber counts using Method 7400 so that the results are consistent between laboratories. Considerable confusion has been caused in the past by individual laboratories using some of these identification techniques to change the counting procedure and, hence, the final results. Several PCM fiber counting methods have been published by national [35-36] and international organizations [37-38]. Most countries have methods very similar to the ones referenced here. e. Sampling volume for asbestos abatement applications Sampling for asbestos after abatement requires the selection of a sampling volume so that one can have high confidence that the air meets acceptable concentration standards. The following is an example of how to calculate this sampling volume. The approach assumes that one wishes to select sampling parameters in order to have a high degree of confidence that a target exposure standard (e.g. NIOSH REL, OSHA PEL, EPA clearance standard) is met. Several factors need to be established in order to perform this calculation if the target exposure standard involves clearance monitoring. The U.S. Environmental Protection Agency (EPA) authorizes the use of PCM for some clearance monitoring applications and specifies that a level of 0.01 fibers/mL be met. On the method synopsis page, Method 7400 indicates that the limit of detection (LOD) for PCM analysis is 5.5 fibers/100 fields. This is based on intralaboratory variability. A major difference between Method 7400 and other analytical methods in the NIOSH Manual of Analytical Methods (NMAM) is that there is no

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