A Strategy for Assessing Exposures with Reference to Multiple Limits

Abstract

This article addresses the problem of assessing exposures to chemical agents which are regulated with multiple limits such as a permissible exposure limit (PEL) and an action level (AL). If exposures are described adequately by a lognormal distribution, the mean exposure is correlated with the frequency of large excursions above any particular value. This implies that by controlling the mean value below the lower exposure limit, one can constrain the frequency of exposures above the upper limit and vice versa. For example, if the mean exposure is maintained below the AL = 0.5 (PEL), then a maximum of 12 percent of the lognormally-distributed exposures are expected to exceed the PEL regardless of the degree of temporal variability. Furthermore, this property is independent of the averaging time of the measurements, thus samples collected over eight hours or longer can be used to limit the frequency of exposures in excess of a short-term exposure limit (STEL). Several applications regarding pairs of exposure limits (e.g., AL-PEL and PEL-STEL) are illustrated with actual monitoring data for benzene and toluene diisocyanate.