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 sample, with crystallization of the mount and movement of fibers often occurring several days after sample preparation. Permanent slides were needed for quality assurance purposes and the sample preparation technique was also slow and required some skill. A rapid acetonebased filter clearing technique was developed that could be used safely in field situations [21]. After clearing, filters were coated with triacetin to surround the fibers. This resulted in a longer lasting sample (typically months to years) and is currently specified in most methods. Another technique uses a resin called Euparal to surround the fibers and results in a permanent slide preparation [22]. Filter loading. The number of fibers on the filter is usually specified to be within a certain loading range to ensure consistent counting. Cherrie et al. demonstrated using a serial dilution technique that counting efficiency was a function of concentration of fibers on the filter [23]. At very low filter loadings (<100 fibers/mm2 ) there was a tendency to count high relative to an intermediate range of concentrations (100-1300 fibers/mm2 ), where the counts were a linear function of loading. This “overcounting” was apparently due to greater visibility of fibers in a clean visual field. This effect was noted for both human counters and an image analysis system. At high filter loadings (>1300 fibers/mm2 ), undercounting occurred due to overlap of fibers with other fibers and with nonfibrous particles. Most published methods indicate that optimum counting occurs within the 100-1300 fibers/mm2 range, while some restrict the range further to less than 650 fibers/mm2. Fiber counting rules. The basic fiber counting rules for most current methods indicate that a countable fiber should be longer than 5 :m, narrower than 3 :m, and have an aspect ratio greater than 3:1. These rules were selected because shorter fibers were difficult to detect by optical microscopy and the 3:1 aspect ratio was used to discriminate between fibrous and non-fibrous particles in occupational settings. There has been a great deal of controversy over these rules. The use of a longer fiber cutoff, e.g., 15 20 :m, has been suggested, based on two separate arguments: first, that most asbestos fibers are relatively long and thin (with high aspect ratio) and the longer fiber cutoff would discriminate better toward fibers that were truly asbestos fibers according to mineralogical definitions [24]; and second, that fibers that enter the lungs are removed readily by macrophages if they are shorter than about 15
 * m [3]. Longer fibers cannot readily be engulfed by macrophages, thus staying in the lungs

for a long period and causing continuing fibrosis. The aspect ratio criterion has also been questioned because many non-asbestiform particles have shape distributions that include particles with aspect ratios greater than 3:1. Since asbestos and other minerals often contain single crystal particles not in the asbestiform habit, it has been argued that these single crystals, or cleavage fragments, should not be counted. However, the Occupational Health and Safety Administration (OHSA) has supported the 3:1 minimum aspect ratio through legal precedent. The National Institute for Occupational Safety and Health (NIOSH) has noted that because of the great difficulty in differentiating whether individual high aspect ratio particles are cleavage fragments or asbestiform fibers, all such particles should be counted. These high aspect ratio particles may cause disease whether or not they are asbestiform. Other aspects of fiber counting have been investigated, including how to count non-standard fiber shapes, overlapping fibers, overlapping compact particles on fibers, and bundles of fibers. Each of these factors can have a noticeable effect on the final count. Cowie and

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