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 It is likely that thoracic sampling will eventually be in routine use for measurement of asbestos and other fibers. This approach has several advantages. It places the fiber method in line with other dust sampling conventions. It removes some of the larger particles in the sample, resulting in a cleaner sample for the analyst. It removes the need for determining fiber diameter during counting and it is consistent with previous practice of using an upper diameter limit of 3 :m for fiber counting in some methods. Thoracic sampling has the disadvantage of requiring the flow rate for a specific sampler to be fixed. This reduces the flexibility to target the loading of the filter by adjusting the flow rate. However, several classifiers can be designed to operate at selected flow rates to allow some flexibility in sampling. 10.

CONCLUSIONS The capability for measurement of fiber size distributions is available through microscopy and, to a much lesser extent, through direct-reading instrumentation. Because of differences in counting rules, resolution capability, and ability to distinguish asbestos from interfering particles or other fibers, PCM, PLM, SEM, and TEM methods often do not produce results which are directly comparable. The traditional methods of microscopy are relatively inaccurate when compared to chemical analysis methods for most other analytes because of the many sources of error in the sampling and analysis procedure. To improve laboratoryto-laboratory agreement, counter training and quality control, including the exchange of samples among laboratories and proficiency testing, are important. Implementation of training through the use of Pang’s coverslips allows investigation of counting errors and potential improvement of PCM counting accuracy. Thoracic sampling could eliminate interfering particles and thereby improve measurement methods in the future.

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