Identification and counting of asbestos fibers

A combined analytical electron microscopic/optical count method for the determination of airborne asbestos fibers was tested for precision and bias. A modified phase contrast microscopic count method (NIOSH Method 7400) was used to determine total fiber content. The analytical electron microscope (AEM) procedure was added to identify the fraction of amosite asbestos fibers in airborne, laboratory-generated samples containing amosite and wollastonite fibers. Then this fraction was applied to the routine optical counts of all the samples in the set to estimate the asbestos fiber concentration. The effects of sample to sample, wedge to wedge, within wedge and between and counter variability were examined. In addition, the variabilities of the elemental ratio within a fiber and between fibers was also determined to find their possible influence on the ability to identify the fiber as amosite in the presence of other silicate fibers. A precision of 20.1% relative standard deviation (RSD) and a bias of -9.1% for the AEM count method compared with the optical count procedure were found for these mixed fiber samples.