How serious is dioxin's cancer risk?

classify the carcinogenicity of dioxin from their in-house panel of experts, U.S. EPA policy makers were out of luck. In March, EPA’s Science Advisory Board (SAB) was unable to reach a consensus on whether 2,3,7,8–tetrachlorodibenzo-p-dioxin (TCDD) is a human carcinogen. This lack of consensus leaves EPA policy makers back where they started, as the U.S. government struggles to reach agreement on the seriousness of dioxin exposure to human health. The board was reviewing EPA’s reassessment of dioxin and related compounds, a revised document officially released by EPA last June. The report was the subject of major news coverage then, because it characterized TCDD as a human carcinogen, adding that other dioxins and some dioxin-like compounds are likely human carcinogens. The cancer risk for people exposed to these chemicals ranges from 1 in 100 to 1 in 1000, EPA wrote. EPA’s conclusion on TCDD is in agreement with both the World Health Organization’s International Agency for Research on Cancer (IARC), which labeled TCDD a human carcinogen in 1997, and the U.S. National Toxicology Program, said Richard Clapp of Boston University’s School of Public Health, also on the SAB panel. The U.S. Agency for Toxic Substances and Disease Registry also considers TCDD a known carcinogen. EPA began its review of dioxin and related compounds in 1991, following an industry-led outcry that the agency’s dioxin risk assessments were too conservative. The reassessment includes a summary of the science, exposure routes, and possible human and animal health risks related to exposures. In March, the SAB wrote that although there is “compelling evidence of carcinogenicity in laboratory animals for TCDD,” the same is not true for humans. The reason? “There is a lack of consensus” in the panel over whether TCDD meets current EPA cancer guidelines for a human cancer hazard. In addition, there is just not enough epidemiological data indicating that dioxin is carcinogenic in humans, nor are there adequate scientific data demonstrating similar modes of action in humans and laboratory animals, according to the SAB draft. But several panel members strongly support EPA’s conclusion that TCDD is carcinogenic, saying there was a “broad consensus” on