Health Risks from Dioxin and Related Compounds Evaluation of the EPA Reassessment (2006) / Chapter Skim
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From page 1... ...
Much of the knowledge about the health effects of TCDD, other dioxins, and DLCs in humans comes from studies of relatively highly exposed workplace populations. Widespread use of certain herbicides containing TCDD, other dioxins, and DLCs, as well as some types of industrial emissions, resulted in local and global contamination of air, soil, and water with trace levels of these compounds.
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From page 2... ...
EPA should also improve how it handles and communicates the substantial uncertainty that surrounds its various estimates of health risks from low-level exposures to TCDD, other dioxins, and DLCs. This NRC report provides a critical review of EPA's Reassessment, but the report is not a risk assessment and does not recommend exposure levels for TCDD, other dioxins, or DLCs for regulatory consideration.
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EPA derived its estimates of TCDD, other dioxins, and DLCs in food from statistically based national surveys, nationwide-sampling networks, food fat concentrations, and environmental samples of air, water, soil, and food. According to recent estimates, background concentrations of TCDD, other dioxins, and DLCs continue to decline.
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The committee concluded that because the definition of "carcinogenic to humans" changed somewhat from previous EPA guidelines and after submission of the Reassessment, EPA should reevaluate its 2003 conclusion based on the criteria set out in its 2005 cancer guidelines. The committee agrees with EPA in classifying other dioxins and DLCs as "likely to be carcinogenic to humans." However, because mixtures of DLCs and other dioxins may include TCDD, EPA should reconsider its classification of such mixtures as "likely to be carcinogenic to humans" if it continues to classify TCDD as "carcinogenic to humans." Estimating Cancer Risks at Very Low Doses Nearly all relevant cancer-risk data from human epidemiological studies and experimental animal bioassays reflect doses much higher than those typically experienced by humans from exposure to TCDD, other dioxins, and DLCs in the general environment.
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The committee concludes that EPA's decision to rely solely on a default linear model lacked adequate scientific support. The report recommends that EPA provide risk estimates using both nonlinear and linear methods to extrapolate below PODs.
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The committee concludes that, although EPA discussed many of these factors qualitatively, the agency should strive to more comprehensively characterize the impact of these sources of uncertainty quantitatively. Estimating Noncancer Risk To characterize the risks of adverse health effects other than cancer, EPA typically identifies a dose, called the reference dose (RfD)
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The committee further recommends that, in areas with substantial amounts of human clinical data and epidemiological data, EPA establish formal, evidence-based approaches, including but not limited to those for assessing the quality of the study and study design for classifying and statistically reviewing all available data. Communicating Variability and Uncertainty in Risk Estimates Risk assessors must make many choices as they develop models to characterize risks, including selecting appropriate data sets for low-dose extrapolation, dose-response models, PODs, and so forth.
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Even with the inherent uncertainties, the committee concludes that the toxic equivalency factor methodology provides a reasonable, scientifically justifiable, and widely accepted method to estimate the relative potency of DLCs. However, the committee noted that the Reassessment should acknowledge the need for better uncertainty analysis of the toxicity values and should provide at least some initial uncertainty analysis of overall toxicity of environmental samples.
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· Transparency, thoroughness, and clarity in quantitative uncertainty analysis. The following points represent Summary recommendations to address the key concerns: · EPA should compare cancer risks by using both a linear model and a nonlinear model consistent with a receptor-mediated mechanism of action and by using epidemiological data and the new NTP animal bioassay data.
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· EPA should continue to use body burden as the preferred dose metric but should also consider physiologically based pharmacokinetic modeling as a means to adjust for differences in body fat composition and for other differences between rodents and humans. The committee encourages EPA to calculate RfDs as part of its effort to develop appropriate margins of exposure for different end points and risk scenarios.
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