Low doses of dioxin may not be as carcinogenic as the Environmental Protection Agency (EPA) states in its draft risk assessment, according to a panel of the National Academy of Sciences (NAS). In an analysis released today, the panel called on EPA to clarify uncertainties and better justify some assumptions about the danger of dioxin.
Historically found in herbicides and industrial waste, today dioxins come mainly from incineration of municipal trash. Emissions have dropped by about 90% since 1987, yet the compounds have contaminated soils and water worldwide and have made their way through the food chain by accumulating in animal fat. In lab animals, dioxins cause tumors, birth defects, and many other problems. But the risks to the general public from long-term, low-dose exposure are difficult to establish.
In its first risk assessment of dioxin, in 1985, EPA called the compound a "probable human carcinogen." As more data came in, the agency reassessed the threat, and in 2000 it gauged that the cancer risks for the most exposed people were 10-fold higher than it previously thought (Science, 16 June 2000, p. 1941). This was controversial, however, as was the agency's view that cancer risk depends linearly on dose (which would mean there is no safe dose). In 2004, after an interagency working group couldn't agree on the report, the 1800-page document was sent to the NAS for review.
A key finding of the 18-member committee is that EPA should not simply assume a linear relationship between low doses and cancer risk. That "is not the most scientifically justified approach," says toxicologist David Eaton of the University of Washington, Seattle, who chaired the panel. Rather, EPA should consider a nonlinear dose response with a threshold below which dioxin has no impact on cancer. Further evidence for such a response comes from a series of animal tests on dioxin, published last year by the National Institute of Environmental Health Science's National Toxicology program, the panel found. A nonlinear dose response would imply a lower cancer risk, although the panel did not calculate the amount. The panel also said the agency should quantify the uncertainties surrounding the risks of cancer and other health problems.
The committee's recommendations are "pretty important," says Martin Van den Berg, a toxicologist at the University of Utrecht in the Netherlands, who reviewed the NAS report. "In practice, the risk of dioxins with respect to carcinogenesis might have been overrated." Eaton says that EPA should be able to follow its recommendations and finalize the report in a year or so. "We hope that they can complete this in a timely fashion."