Dioxin-type carcinogens pose additive risks.

Pollutants known as dioxins and furans, and at least some polychlorinated biphenyls (PCBs), have additive cancer-causing effects when mixed together, a new toxicology study indicates. That finding lends validity to a previously unconfirmed assumption that researchers and regulatory. agencies in many countries use to calculate the risks posed by dioxins and dioxinlike chemicals, which include furans and some PCBs.

Toxicologists typically estimate the cancer risks posed by exposure to mixtures of these chemicals by measuring the threat posed to lab animals by each individual chemical and then adding up the results. However, certain compounds that coexist in the environment may interact in the body to multiply or counteract each other's effects.

To search for possible synergistic or antagonistic interactions among dioxins and related compounds, toxicologist Nigel Walker of the National Institute of Environmental Health Sciences in Research Triangle Park, N.C., and his colleagues investigated the carcinogenic effects of three of the most abundant dioxinlike chemicals. The rodents in this study developed similar numbers of tumors during up to 2 years of steady exposure to any of the three chemicals--2,3,7,8-tetrachlorodibenzo-p-dioxin, 2,3,4,7,8 pentachlorodibenzofuran, or PCB-126--or to a carefully proportioned mixture of lower concentrations of each. The results suggest that the chemicals' threats add simply without any chemical exacerbating or mitigating another's threat.

The researchers report their findings in an upcoming Environmental Health Perspectives.--B.H.