Comparison of Human Health and Ecological Risk-Based Screening Concentrations for Chemicals in Fish and Shellfish. B. K. Shephard and R. A. Thompson, URS Consultants, Inc., 1100 Olive Way, Suite 200, Seattle, WA 98101
Risk-based screening concentrations (RBSC's) are used by risk assessors to identify those chemicals which are at or above concentrations posing unacceptable risks to receptors. A common human health exposure scenario is determining the concentration of chemicals in aquatic biota which pose unacceptable risks to human consumers. While the computational procedures and several RBSC compendia under different exposure scenarios are readily available for human health risk assessors, no comparable RBSC tables are available which define the tissue residues posing ecological risks to aquatic biota themselves. Thus, the ecological risk of contaminants in aquatic biota cannot be readily described, unlike the human health risks posed by the same tissue residue. Two computational procedures are described for deriving ecological RBSC's for aquatic biota: a steady-state uptake model using ambient water quality criteria and bioconcentration factors to derive no effect concentrations in tissues; and a quantitative structure-activity relationship (QSAR) based on a one compartment first order kinetic model. Ecological RBSC's are currently available for nearly 100 chemicals, including metals, insecticides, volatile and semivolatile organic compounds. Comparison of the ecological RBSC's derived from the steady-state model to human health RBSC's based on the mean daily consumption rate of fish and shellfish for the U.S. population indicates that over 95% of the ecological RBSC's are lower than the corresponding human health RBSC's for non-carcinogenic modes of toxic action. This is believed due to a combination of factors, including smaller body size, higher metabolic rates and less highly developed detoxification pathways in ecological receptors compared to humans. For assessing risks of non-carcinogenic chemicals, definition of tissue concentrations of chemicals protective from ecological risks to aquatic biota will also be protective of human consumers of fish and shellfish.