The existence of long term occupational exposure limits (LTA OEL) and the issues that need to be considered when designing an exposure monitoring program for a LTA OEL were examined. The current time weighted average (TWA) threshold limit values (TLV) set by the American Conference of Governmental Industrial Hygienists and TWA permissible exposure limits (PEL) set by the Occupational Safety and Health Administration were intended to be used as upper control limits for each single shift TWA exposure, but instead were being used as upper control limits for each worker's long term mean exposure. The use of mean testing to determine compliance with exposure limits was discussed. Mean testing requires numerous assumptions, several of which may be difficult to evaluate. Assuming that a valid LTA OEL existed, issues to be addressed then included determination of the averaging time for the measurements, selection of sample size, strategy for collecting measurements, stability of the work environment, and selection of the mean testing procedure. The problems associated with mean testing were discussed. The author believes the primary problem is that, given realistic sample sizes and geometric standard deviations, the true long term mean would have to exceed an LTA OEL by a considerable degree before the inspector's mean test will reliably detect an unacceptable exposure condition. The author concludes that serious consideration should be given to these issues before an exposure monitoring program based on mean testing can be successfully implemented.
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