Dose-Response Based Distributional Analysis of Threshold Effects: A Case Study of Developmental and Reproductive Effects from Ethylene Oxide. S. J. S. Baird, The Baird Group, Inc., 36 Duffield Rd, Newton, MA 02466; L R. Rhomberg, Gradient Corp., 238 Main St., Cambridge, MA 02142; and J. S. Evans, P. L. Williams, and A. M. Wilson, Harvard School of Public Health, 667 Huntington Ave., Boston, MA 02115
Current non-cancer risk assessment methods do not permit quantitative characterization of the uncertainty in a reference dose or reference concentration (RfD/C). This limits the ability to effectively use RfD/Cs as risk estimates for risk management decisions because: 1) the level of protection for a human population afforded by a RfD is unknown and likely to differ across chemicals; 2) the risk from exposures greater than the RfD can not be characterized; and 3) risk management and risk assessment are intertwined. We present here a comprehensive framework for characterizing sources of uncertainty and for understanding their relative contributions to the uncertainty in human risk estimates. This framework extends the current methodology by using dose-response assessment of available animal data, data-based statistical distributions of extrapolation factors, and an explicit quantitative approach to account for the variation in heterogeneity among the human population. Applying this framework results in a probabilistic characterization of uncertainty in a risk estimate at a specified level of human population risk (e.g. 1/100, 1/1000, 1/10,000). Using a rich database on the reproductive and developmental effects of ethylene oxide, a probabilistic characterization of possible regulatory thresholds for ethylene oxide is presented as an example of implementation of the dose-response based distributional framework. This framework can be applied to comparative risk analysis, benefit-cost analysis, and provide harmonization with methods used for assessment of carcinogens.
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