National Academies Press: OpenBook

Issues in Risk Assessment (1993)

Chapter: Discussion

Suggested Citation:"Discussion." National Research Council. 1993. Issues in Risk Assessment. Washington, DC: The National Academies Press. doi: 10.17226/2078.

—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.


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.
Page 300
Issues in Risk Assessment Get This Book
Buy Paperback | $65.00
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

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.

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook,'s online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!