SUMMARY 



Polychlorinated dibenzo- para - dioxins (PCDDs) are present as trace 

 impurities in some manufactured chemicals and industrial wastes. The chemical 

 and environmental stability of PCDDs and their tendency to accumulate in fat 

 have resulted in their detection within many ecosystems. In general, wherever 

 high levels of PCDDs have been detected, the source has been a hazardous waste 

 dump, an industrial discharge, or an application of PCDD-contaminated 

 herbicide. 



There are 75 PCDD isomers; some are extremely toxic, while others are 

 believed to be relatively innocuous. The most toxic and most extensively 

 studied PCDD isomer is 2,3,7,8-tetrachlorodibenzo- para - dioxin 

 (2,3,7,8-TCDD). In the United States and elsewhere, accidental contamination 

 of the environment by 2,3,7,8-TCDD has resulted in deaths in many species of 

 wildlife and domestic animals. High residues of 2,3,7,8-TCDD in fish, i.e., 

 more than 50 parts-per-tril 1 ion (ppt) wet weight, have resulted in closing 

 rivers to fishing. In the most seriously affected areas, hospitalization and 

 permanent evacuation of humans has been necessary. Laboratory studies with 

 birds, mammals, aquatic organisms, and other species have demonstrated that 

 exposure to 2,3,7,8-TCDD can result in acute and delayed mortality as well as 

 carcinogenic, teratogenic, mutagenic, histopathologic, immunotoxic, and 

 reproductive effects. These effects varied greatly among species. 



No regulations governing PCDD contamination exist at present to protect 

 sensitive species of wildlife and aquatic organisms. However, the limited 

 data available suggest that 2,3,7,8-TCDD concentrations in water should not 

 exceed 0.01 ppt to protect aquatic life, or 10 to 12 ppt in food items of 

 birds and other wildlife. Additional data are needed in several areas: 

 background levels of PCDDs in natural systems; identification of fish and 

 wildlife populations at risk; relative importance of PCDD sources; 

 toxicological effects of various PCDDs to aquatic biota and wildlife, 

 especially reproductive and immunosuppressive effects; and toxic and other 

 interaction effects of PCDDs with other groups of polychlorinated chemicals 

 having similar structure and properties, such as biphenyls, dibenzofurans, and 

 biphenylenes. 



m 



