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Despite decades of work, the Environmental Protection Agency still has to redo its assessment of one of the most notorious chemicals in the world: dioxin. A committee convened by the National Academy of Sciences, says the EPA's methods are flawed and need to be reworked.

A report published yesterday, is the latest turn in a decade's long drama involving dioxin. NPR's Richard Harris reports.

RICHARD HARRIS reporting:

Dioxin first gained infamy as a contaminant in herbicides used in the Vietnam War. It was then found in the Love Canal neighborhood, near Buffalo, New York, and Times Beach, Missouri - both of which were evacuated as a result. It became a powerful symbol of toxic dangers in our environment.

David Eaton, from the University of Washington, chaired a committee to review the Environmental Protection Agency's decades-long effort to quantify the risk that dioxin poses to human health.

Dr. DAVID EATON (Environmental and Occupational Health Sciences, University of Washington): There is probably no other compound in the universe that has been more extensively studied for its toxicological properties.

HARRIS: Even so, scientists still can't agree on even the most basic questions. For example, the National Academy Committee was not prepared to say that dioxin falls into the EPA's categories of agents known to cause human cancer.

Dr. EATON: However, I'd emphasize that the committee was in complete agreement that dioxin was likely to be carcinogenic to humans and recognized that finding where that bright line is that distinguishes between these two categories, was a challenge.

HARRIS: It's even harder to figure out whether the chemical poses a risk at the extremely low levels now found in our environment. The Academy criticized the EPA's methodology in that regard.

The EPA assumes that, down to the lowest level, half the dose will do half the harm. But Eaton says that almost certainly overestimates the risk. So the committee suggests that the EPA use a different method to estimate the risk of very low doses.

Dr. EATON: The consequences of that, in general, will result in a lower risk estimate as you extrapolate it down. How much different, we don't know. We did not do the calculations to know how much of a difference that will make.

HARRIS: And committee member Josh Cohen, from the Tufts-New England Medical Center says there may be other instances in which the EPA underestimated the risk to human health.

Dr. JOSH COHEN (Internal Medicine, Tufts-New England Medical Center): More generally, what we are encouraging EPA to do is to look at the full range of plausible scientific assumptions. And in some cases that may imply higher risk, and in some cases it may imply lower risks. But it's important for risk managers to understand how much we know and how much we don't know. How imprecise are our estimates?

HARRIS: The EPA has been grappling with these questions for decades, and its latest draft, published in 2003, still seems far from finished.

The National Academy offered more than 100 pages of detailed criticism. But committee chairman, David Eaton, says he hopes the EPA can now move quickly.

Dr. EATON: Quickly, in my mind, would be a year or so. Not eleven years.

HARRIS: The good news is, while the EPA has been busy rewriting and rewriting its assessment, dioxin levels have been decreasing significantly in the environment. That's because paper mills and incinerators that used to produce it have been retooled or shut down.

But Peter deFur, at Virginia Commonwealth University, says it's still important to get a credible safety standard for dioxin.

Professor PETER DEFUR (Environmental Studies, Virginia Commonwealth University): Well, it hasn't quite gone away yet, you see. Emissions have gone down, exposures have gone down, but dioxin has not gone away. We all carry a body burden. We still have a large number of contaminated sites that have dioxin. So we're still dealing with this problem, and we will be for decades.

HARRIS: The next move is now up to the Environmental Protection Agency.

Richard Harris, NPR News.

INSKEEP: You can read the National Academy's report and the EPA's 2003 dioxin assessment by going to npr.org.

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