Document 99qjY7wwOxQzbqj5BbzymbQ8D

14256997033 2002-02-26 19:09:21 (GMT), page 27 26 readily conceded that counting only fibers longer than 5 um enumerates just a fraction of the total number of fibers present, there is incomplete awareness that the fraction counted is highly variable, depending upon the fiber type, the process or products used, and even die past history ofthe asbestos material (e.g., old versus new insulation material), among otter factors. For example, the fraction of chiysotile fibers longer than 5 um in an aerosol can vary by a factor of 10 (from as little as 0.5 percent ofthe total number to more than 5 percent). When amosite aerosols are counted, the fraction longer than 5 um may be 30 percent; extending the variability ofthe fraction counted to two orders of magnitude Even if consideration is restricted to fibers longer than 5 um, many are missed by optical microscopy. Using electron microscopy, Rendall and Skikne (1980) measured the percentage of fibers with a diameter less than 1-4 um (the approximate limit of resolution ofan optical microscope) in various asbestos dust samples.(Skikne et al. 1971;Rendall and Skikne 1980) In general, they found that more than 50 percent of the 5 um or longer fibers are less than 0.4 um in diameter and thus, are not visible using a standard phase contrast optical microscope." (Nicolson 1986a) Asbestos fiber counting is a "tip ofthe iceberg" phenomenon because fibers are counted by light microscopy. Since chiysotile fibers split longitudinally, some offile fibers are too narrow to be seen and are not counted. The first steps of the textile process are specifically designed to split fiber bundles; therefore textile exposures involve a higher percentage of tirin (invisible fibers) than mill or mine exposuresjU^H As a result of increased fiber splitting in file textile process each counted fiber represents many more uncounted fibers then is so in the mining and milling process. (Nicholson 1991) 26