National Academies Press: OpenBook

Issues in Risk Assessment (1993)

Chapter: Discussion

« Previous: CASE STUDY 3B: ECOLOGICAL RISK ASSESSMENT OF TCDD AND TCDF
Suggested Citation:"Discussion." National Research Council. 1993. Issues in Risk Assessment. Washington, DC: The National Academies Press. doi: 10.17226/2078.
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—specifically, the lowest reported LOAELs and NOAELs. The risk quotient (RQ) for each species considered was defined as the ratio of the estimate of exposure to the corresponding benchmark value. On the basis of transfer estimates for land disposal of paper sludges, RQs could exceed 60:1 for the most exposed species (robins, woodcocks, and shrews). To estimate soil concentrations of TCDD ''safe" for these species, two uncertainty factors of 10 could be applied: one to allow for interspecies variability in sensitivity and one for an extrapolation from laboratory to field and/or the use of a LOAEL as the benchmark value. The corresponding estimates of safe concentrations were estimates that would lead to RQs less than 0.01:1 for the most heavily exposed species considered. Under those assumptions, soil concentrations of TCDD safe for highly exposed species would be about 0.03 ppt.

Discussion

Led by L. A. Burns, U.S. Environmental Protection Agency, and D. J. Paustenbach, McLaren/Hart)

These case studies present only estimates of environmental concentrations—i.e., exposure assessment—and do not address other elements of risk assessment. Compared with traditional human health assessments, they show a greater concern for accuracy (as opposed "policy-driven conservatism"), a greater use of formal uncertainty analysis, and better opportunities for verifying accuracy of exposure and uptake models.

Criticism of the models focused on the omission of processes and on the assumed linear relationship between loading and environmental concentrations. Omitted processes include in-lake generation of solids (phytoplankton), transport in the benthic boundary layer, effects of water clarity on photolysis rates, and daily cycles in pH. A nonlinear relationship between loading and toxicant concentrations might occur if the toxicant reaches high enough concentrations to change the processes that control its own fate. For example, reduction in fish populations might allow for higher populations of zooplankton, which clarify the water column by decreasing populations of phytoplankton, thereby increasing photolysis rates and stabilizing pH.

Suggested Citation:"Discussion." National Research Council. 1993. Issues in Risk Assessment. Washington, DC: The National Academies Press. doi: 10.17226/2078.
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Next: CASE STUDY 4: RISK ASSESSMENT METHODS IN ANIMAL POPULATIONS: THE NORTHERN SPOTTED OWL AS AN EXAMPLE »
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The scientific basis, inference assumptions, regulatory uses, and research needs in risk assessment are considered in this two-part volume.

The first part, Use of Maximum Tolerated Dose in Animal Bioassays for Carcinogenicity, focuses on whether the maximum tolerated dose should continue to be used in carcinogenesis bioassays. The committee considers several options for modifying current bioassay procedures.

The second part, Two-Stage Models of Carcinogenesis, stems from efforts to identify improved means of cancer risk assessment that have resulted in the development of a mathematical dose-response model based on a paradigm for the biologic phenomena thought to be associated with carcinogenesis.

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