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 laboratories within a country or throughout the world is more important than making measurements that are more closely related to health effects. A number of factors which influence analysis results have been investigated, including the following. a. Microscope-related parameters Microscope magnification. The exact level of microscope magnification depends on microscope design, but most current methods use 450X (±10%) total magnification. Pang and coworkers investigated 1250X magnification to improve fiber detectability, but this has not been adopted in any established methods [16]. Pang also investigated the effect of using lower magnification (400X) and found that counts were lower for chrysotile asbestos by 25%, but that amosite fiber counts were unaffected [17]. Phase contrast optics. This contrast enhancement technique allows detection of asbestos fibers down to about 0.25 :m diameter for chrysotile and about 0.15-:m for amphiboles. Other techniques such as dark field microscopy may offer improved detectability, but also increase the background from non-fibrous particles. Test slide to check optics. A test slide was developed to allow a check of proper alignment and magnification in the microscope [18]. This ensures a reasonable level of uniformity in microscope setup and operation, including the operator’s visual perception. Improper setup can reduce detectability of fibers. There have also been cases where the optics were “too good,” and results were obtained that were higher than the reference count. Counting area in microscope field. Some early measurements with the phase contrast microscope were made using a rectangular graticule for defining the counting area, while others were made using the entire microscope viewing area. It was found that larger viewing areas resulted in lower counts, so the Walton-Beckett graticule [19] was developed that nominally gave a 100-:m diameter counting area (the area is calibrated more precisely for each microscope) and has been incorporated in all current methods. b. Sample preparation techniques Filter type. Virtually all measurements are made using 0.8-:m pore size mixed cellulose ester (MCE) filters. Some measurements are made using 1.2-:m pore size filters when sampling low concentrations to allow higher flow rate through the filter. Smaller pore size filters are used to ensure that fibers are deposited as near the surface of the filters as possible. This results in fibers ending up in the same plane so that they can be readily viewed with a minimum change of focus during fiber counting. Pore sizes smaller than 0.8 :m are only used with line-operated pumps because of limited suction power available with personal sampling pumps. Selection of the liquid for making filter transparent. A liquid is placed on the filter that closely matches the filter refractive index, yet has an index that is as far as possible from that of the fibers being detected. Rooker et al. showed that refractive index difference between cleared filter and fibers translated directly into detectability of small diameter fibers [20]. A viscous solution of dimethyl phthalate and diethyl oxalate mixed with cellulose filter material was commonly used in the 1970s and early 1980s. However, it did not result in a permanent

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