Aggregate exposure as mandated by FQPA (Slide 13) requires the summation of exposures from all pathways and sources. If conservatism is to take place in estimating the exposure from each pathway and source, the aggregate exposure will become unrealistically high. In tackling this type of problems, U.S. EPA has moved to support the use of probabilistic analysis (also referred to as stochastic analysis or Monte Carlo simulation) for exposure assessment (1998). In response to this movement, Dong and Ross (in press) have also provided several case studies to illustrate the use of this simulation technique.

Another approach to coping with the challenge of aggregate exposure and risk assessment is the use of biological monitoring (herein also biomonitoring) to measure the total internal dose, which accounts for all intakes and uptakes of the chemical from all relevant exposure sources. Biomonitoring relies on the measurement of a biological parameter (biomarker) in the fluid collected from the exposed individuals. Well-designed and useful biomonitoring studies would require not only the compliance of human test subjects, but also an understanding of the disposition, metabolism, and elimination of the chemical in the human body. Moreover, an acceptable analytical method must be sensitive enough to detect the biomarker which is typically present at very low level. Routine collection of 24-hour samples in human volunteers is often impractical. Using PB-PK simulation, Dong et al. (1994, 1996) have provided examples demonstrating how spot urine sample results can be used to estimate the amount of a pesticide dermally absorbed in humans.