Health Risks from Dioxin and Related Compounds Evaluation of the EPA Reassessment (2006) / Chapter Skim
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8 Conclusions and Recommendations
8 Conclusions and Recommendations
Pages 188-198
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From page 188... ...
8 Conclusions and Recommendations CLASSIFICATION OF TCDD AS CARCINOGENIC TO HUMANS In its charge, the committee was requested to comment specifically on the U.S Environmental Protection Agency (EPA) conclusion that 2,3,7,8tetrachlorodibenzo-p-dioxin (TCDD, also referred to as dioxin)
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From page 189... ...
CONCLUSIONS AND RECOMMENDATIONS 189 Risk Assessment (cancer guidelines) , the committee concludes that the classification of TCDD as "carcinogenic to humans" -- a designation suggesting the greatest degree of certainty about carcinogenicity -- versus "likely to be carcinogenic to humans" -- the next highest designation -- is somewhat subjective and depends largely on the definition and interpretation of the criteria used for classification.
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From page 190... ...
190 HEALTH RISKS FROM DIOXIN AND RELATED COMPOUNDS USE OF LOW-DOSE LINEAR VERSUS THRESHOLD (NONLINEAR) EXTRAPOLATION MODELS FOR QUANTITATIVE CANCER RISK ESTIMATIONS The committee unanimously agrees that the current weight of evidence for TCDD, other dioxins, and DLCs carcinogenicity favors the use of nonlinear methods for extrapolation below the point of departure (POD)
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From page 191... ...
CONCLUSIONS AND RECOMMENDATIONS 191 In its evaluation of the ED01 used for cancer risk assessment, the committee concluded that EPA had not adequately justified use of the 1% response level as the POD for the analysis of either the epidemiological or the animal bioassay data. Even though it is necessary to demonstrate that the POD is within the range of the observed data, that by itself is not sufficient to justify use of the ED01.
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From page 192... ...
192 HEALTH RISKS FROM DIOXIN AND RELATED COMPOUNDS CHARACTERIZATION OF UNCERTAINTY FOR RISK ESTIMATES Overall, the committee found that the Reassessment qualitatively addressed many sources of uncertainty and variability but that it failed quantitatively to sufficiently address uncertainty and variability that resulted from the numerous decisions EPA made in deriving point estimates of risk in the comprehensive risk assessment. In contrast, EPA used concerns about uncertainties and uncertainty factors as part of the justification for not setting an RfD for noncancer effects (see Chapter 7 for further discussion)
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From page 193... ...
CONCLUSIONS AND RECOMMENDATIONS 193 the Reassessment by providing a clear evaluation of the impacts of possible choices on the risk estimates. The committee recommends that EPA make greater use of mechanistic information to assess the biological plausibility of different mathematical models, use more rigorous criteria (e.g., goodness-of-fit test)
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From page 194... ...
194 HEALTH RISKS FROM DIOXIN AND RELATED COMPOUNDS EPA'S EXPOSURE ASSESSMENT OF TCDD, OTHER DIOXINS, AND DLCS IN THE UNITED STATES To assess total emissions of TCDD, other dioxins, and DLCs, EPA used a "bottom-up" approach, which attempted to identify all source categories and then estimated the magnitude of emissions for each category. However, a "top-down" approach that attempts to account for the levels measured in receptors (e.g., people, animals, and plants)
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From page 195... ...
CONCLUSIONS AND RECOMMENDATIONS 195 and DLCs are immunotoxic in laboratory animal studies -- together with limited human data -- EPA is prudent in concluding that these compounds are likely to be human immunotoxicants in the absence of more definitive human data. However, EPA's conclusion that TCDD and related compounds are immunotoxic at "some dose level" by itself is inadequate.
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From page 196... ...
196 HEALTH RISKS FROM DIOXIN AND RELATED COMPOUNDS disorders, and cardiovascular diseases. In humans, the relationship between dioxin exposure and risk of individual, clinically significant, noncancer end points remains uncertain, except for chloracne.
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From page 197... ...
CONCLUSIONS AND RECOMMENDATIONS 197 MOE values. (For cancer risk assessment, the threshold approach should be used in addition to the linear approach.)
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From page 198... ...
198 HEALTH RISKS FROM DIOXIN AND RELATED COMPOUNDS however, it does not advocate a substantial expansion in the length of the Reassessment. Rather, the committee encourages EPA to address the major concerns raised in this review and to finalize the current Reassessment as quickly, efficiently, and concisely as possible.
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