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Issues in Risk Assessment —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.