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Issues in Risk Assessment Appendix E Case Studies and Commentaries CASE STUDY 1:Tributyltin Risk Management In the United States R. J. Huggett and M. A. Unger, Virginia Institute of Marine Sciences Tributyltin (TBT) is a chemical with a variety of biocidal applications, including use as an antifouling agent in boat paints (Blunden and Chapman, 1982). Biological effects of TBT on marine and estuarine organisms and the concentrations of TBT that induce them vary widely among species (Huggett et al., 1992). A water concentration of 1,000 ng/L (1 part per billion) is lethal to larvae of some species, and nonlethal effects have been observed at concentrations as low as 2 ng/L (2 parts per trillion, ppt). Both laboratory and field studies of toxicity were initially hampered by difficulties in measuring the low concentrations that were toxic to some organisms. Adverse effects on nontarget organisms, including commercially valuable species of shellfish, were observed in Europe in the early 1980s (Alzieu, 1986; Abel et al., 1986). Abnormal shell growth was documented in Crassostrea gigas (European oyster) and linked through laboratory experiments to TBT leached from antifouling paints. That connection led to restrictive regulations in France (in 1982) and Great Britain (in 1985 and 1987). In the United States, concentrations exceeding those determined experimentally to be effective have been found in many