How Dangerous is Dioxin? One Part per 1,000,000,000,000,000? Some Scientists Say the Threat is Exaggerated

November 07, 1993|By MICHAEL K. BURNS

Just how dangerous is dioxin? Is it the world's most toxic chemical, harmful in a few parts per quadrillion (that's 1 with 15 zeros behind it)?

Or is it the ubiquitous byproduct of an industrialized society that each year releases enough into the ecosystem to exceed World Health Organization limits for every human being on Earth -- without killing off the planet?

The United States spent $33 million a decade ago to buy the town of Times Beach, Mo., and relocate its 2,200 residents because the roads had been sprayed with oil that was laced with dioxin. Cleanup of the contaminated soil there didn't start until 1991 -- just as the federal official who had recommended the evacuation said he had over-reacted and made the wrong decision. No significant illness has been reported in follow-up studies of the former residents.

Clouds of dioxin-tainted powder fell over Seveso, Italy, in 1976 after a chemical plant explosion. Children played in the "snowfall" for days before health authorities pulled them out. No one died, and no substantial abnormalities were found in hundreds of exposed people for a dozen years. But this summer, the first reports surfaced suggesting higher than normal cancer rates among the exposed.

Initial studies of Vietnam veterans who were exposed to dioxin in massive spraying of jungles with Agent Orange defoliant in the 1960s did not document the disorders claimed to be linked to dioxin. But this year, an extensive survey by the National Academy of Sciences found three cancers and two other ailments with a "statistical association" to dioxin exposure, qualifying vets for disability compensation from a fund administered by the Veterans Administration.

A government study of 5,200 workers in the chemical plants that made the Agent Orange weedkiller, where exposure levels were hundreds of times higher than for the Air Force personnel who handled it, discovered an excessive number of cancers, but not higher death rates. And National Institute for Occupational Safety and Health researchers could not finger dioxin as the direct cause. Smoking and other toxic chemicals in the workplace may have been decisive factors, they admitted.

What is one to make of this? Studies do show correlations between high dioxin exposure and human health effects. Repeated experiments show that dioxin can cause death, cancer and birth defects in laboratory animals at certain doses.

But there is still no smoking gun to link dioxin directly as the cause of cancer in humans. And the lower "threshold" level at which dioxin is associated with serious human harm remains a matter of belief and interpretation and judgment, rather than absolute scientific proof. The strongest defenders of the Environmental Protection Agency's tight dioxin standards lapse into the double-negative. EPA toxicology director Linda Birnbaum: "The human data are not inconsistent with the [laboratory] animal data."

According to some researchers, the accumulated background levels of dioxin in the environment may already be affecting our health. Dioxins have, after all, been on Earth since before ancient people discovered fire. Their widespread presence and the persistence of dioxins is a fact: They are in the air and soil and accumulate in the fatty tissues, including the fish, meat and dairy products we eat.

There are more than 200 chemicals in the larger dioxin family, of which less than two dozen are toxic. The more toxic varieties are created as the byproducts produced in chemical processes using chlorine, or in burning chlorine substances in incinerators. They appear in the manufacture of such things as herbicides and PVC plastic and paper.

The dioxin debate has also become a gender issue. Women's rights groups claim dioxin and related chlorine-based chemicals are to blame for much of the breast cancer in the United States, and they are calling for a phase-out of these chemicals. The statistical links between female exposure to these chemicals and breast cancer are convincing, even if there is no definitive clinical proof, insists the Women's Environment Development Organization.

Ellen Silbergeld, a University of Maryland toxicologist and dioxin researcher, pointed out recently that good dioxin studies on women are lacking, but added: "Dioxin is very toxic to the female reproductive system and the development of the fetus." The scientific evidence has not changed sufficiently to loosen EPA's dioxin standards, she said.

On the other hand, the NAS panel that examined 230 studies of dioxin related to human health found inadequate evidence to link the substance with female cancers or birth defects; another health study of female veterans is under way.

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