Science Advisers Say Some Analyses Lacking In EPA Reassessment of Dioxin Health Risks

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The Environmental Protection Agency has failed to provide some of the basic analyses and explanations that would justify its latest effort to assess the health risks of dioxins and related compounds, agency science advisers said at an Oct. 29 meeting.

The Dioxin Review Panel of the EPA Science Advisory Board has not yet formalized its review of EPA's Reanalysis of Key Issues Related to Dioxin Toxicity and Response to NAS Comments, but remarks made during the meeting indicated the scientists found some important elements of the EPA document unconvincing and unresponsive to earlier criticisms from the National Academy of Sciences.

The reassessment, still at the draft stage, could result in an estimate of a greater risk of cancer from dioxins than previously thought and is expected to establish a reference dose for dioxin, or more formally 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). A reference dose is not a regulation but can used to guide regulatory decisions. Those changes could have an impact on many industries and government agencies. Dioxin and related compounds are byproducts of waste incineration and various industrial activities, such as coal combustion and paper bleaching (34 CRR 525, 5/31/10).

“These proposed actions will have significant consequences for all companies and government agencies who have dioxin/furan or polychlorinated biphenyl (PCB) liabilities in any environmental media,” consulting company Arcadis said earlier this year in comments submitted to EPA on the reanalysis.

Panel members found much to praise in the EPA draft, including the agency's choice of two studies as crucial for identifying health effects for the calculation of an oral reference dose. Panel members generally agreed that the two studies were well-designed and well focused on appropriate measurements--of decreased sperm count in men in Mocarelli et al 2008, and of increased thyroid-stimulating hormones in newborn infants in Baccarelli et al 2008.

Threshold Question for Dioxin Risk.

The National Academy of Sciences in 2006 urged EPA to explore the option of a health risk assessment that includes the possibility of a threshold below which dioxins stop being carcinogenic. But EPA has dismissed that option, sticking with a “linear” dose-response model that assumes risk diminishes with exposure but does not vanish above zero exposure.

Members of the Dioxin Review Panel, like the National Academy of Sciences four years earlier, said the agency should provide both linear and nonlinear analyses or do an adequate job of justifying its refusal to do so. Much of the research literature suggests there is a threshold for cancer risk from dioxins, the science advisers indicated.

The advisory panel also made clear that its review would call on EPA to do a better job of justifying its exclusion of many health risk studies from its risk estimates--notably those studies that produced negative results in terms of attempting to identify health risks.

The agency should “consider a more balanced assessment of the negative studies in the context of hazard identification,” said the panel's chairman, environmental health scientist Timothy Buckley of Ohio State University, who was trying to summarize what he had heard from panel members.

In many cases it was impossible to understand why a study had been excluded, said panel member Paige Lawrence, a University of Rochester School of Medicine professor. She added that she was not suggesting the panel members were saying the criteria for inclusion or exclusion had to change, only that they had to be explained adequately.

Complications, Uncertainties.

Members of the panel also said EPA needs to justify its exclusion of dioxin-like chemicals from some of its calculations. They may complicate the calculations, but they are too important to dismiss, panel members suggested.

Arnold Schecter, a professor of environmental and occupational health sciences at the University of Texas School of Public Health, said he reviewed about a dozen dioxin papers a year for publication and found that related compounds often were accorded more significance than dioxins.

The National Academy of Sciences had encouraged EPA to include a quantitative uncertainty analysis, but EPA's draft reanalysis of dioxin risk dismissed such an analysis as infeasible. The science advisers sharply disagreed. They suggested various quantitative analyses would be useful and sufficient.

By Alan Kovski

EPA's Reanalysis of Key Issues Related to Dioxin Toxicity and Response to NAS Comments and related documents, including public comments, are available at

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