SMOOTHING OF EXPOSURE VARIABILITY AT THE RECEPTOR: IMPLICATIONS FOR HEALTH STANDARDS

Abstract

The random nature of occupational exposures to airborne toxicants suggests that monitoring programs focus upon the parameters (mean and variance) of the exposure distribution. This analysis attempts to identify situations where the mean exposure might be used as the sole assessment criterion. A single-compartment toxicokinetic model is used to investigate smoothing or dampening of exposure variability associated with accumulation of toxicant at the receptor. The input to the model is a series of random, lognormally distributed, 8-h exposure concentrations with a continuous cycle of five exposures per week; the output is the discrete series of post-shift receptor burdens. It is observed that the amount of smoothing, as indicated by the ratio of the coefficient of variation of the 8-h exposures to that of the post-shift burdens, is proportional to the elimination half-time between 10 and 1000 h. Estimates of smoothing were found to agree well with published data comparing individual workers' daily air exposures and blood levels for mercury and lead. It is concluded that assessment of exposures to toxicants with very long half-times, such as fibrogenic dusts and some heavy metals, focus upon the mean exposure. Evaluation of exposures to chronic toxicants with short or intermediate half-times is more problematic; it may be necessary to consider both the mean and the variance of the exposure distribution unless information concerning the relationships between burden and damage or disease suggests otherwise.