A study was conducted to investigate the phase contrast method as a predictor of total fiber exposures and exposures to fibers longer than 5 micrometers in three major industries processing chrysotile (12001295) asbestos. Airborne fiber length and diameter distributions for these industries were compared. Historic samples collected during 1964 through 1971 were used for the study. The three industries examined included the asbestos textile industry, the friction products industry and the cement products industry. Phase contrast optical microscopy provided a good measure of occupational exposures to fibers longer than 5 micrometers in length but was not a good index of total fiber exposures. The inability of phase contrast microscopy to measure total airborne fiber concentrations was due to the fact that airborne fiber size distributions for asbestos, and particularly chrysotile, vary considerably between industrial operations. The authors suggest that phase contrast concentration measurements in practice be supplemented with size determinations by phase contrast and transmission electron microscopy.
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