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CORVALLIS - After years of study and millions of dollars spent on research, scientists are not much closer today than they were 25 years ago to understanding the human health risks of exposure to dioxin, according to a researcher at Oregon State University.

"That is not to say that we don't know more about dioxin and its potential effects on humans and animals," said Nancy Kerkvliet, a professor of agricultural chemistry at OSU, in a presentation recently at the 1996 northwest regional meeting of the American Chemical Society.

"However, the vast amount of research that has been done on dioxin over the past two decades has created many more questions than it has answered," she added.

Dioxin is a by-product of many kinds of industrial manufacturing. It first became well-known in the early 1970s when it was identified as a cancer-causing component of the herbicide Agent Orange, used extensively in the Vietnam War.

One problem in unraveling the mystery of dioxin has been the difference in species' response to dioxin exposure in animal studies.

"For example, scientists found that a dioxin exposure level of one microgram per kilogram of body weight is lethal to guinea pigs, but in hamsters the lethal level is 5,000 micrograms per kilogram," said Kerkvliet, who is a toxicologist with the OSU Extension Service. "That's quite a difference for two animals that seem very similar in many ways.

"These kinds of extremely inconsistent findings in animal studies make it almost impossible to extrapolate animal study findings to humans."

There also is controversy surrounding how dioxin molecules affect the cells of animals and humans, Kerkvliet said.

"Researchers have found that molecules of dioxin bind very well with a component in the cells of animals and humans called the Ah receptor. However, exactly what happens after the binding takes place is not well understood.

"Scientists speculate that perhaps after binding, dioxin may mimic natural hormones in the body, or the binding may result in persistent activation of the receptor, leading to toxicity, or dioxin may activate the receptor at the wrong time during development of the cell," Kerkvliet said.

"To make matters even more confusing, scientists are unsure of the Ah receptor's function in the cell," she pointed out.

It all adds up to a lot of uncertainty about dioxin's effects on humans and animals, Kerkvliet said, but that hasn't stopped the Environmental Protection Agency from issuing strict guidelines for allowable human exposure to the chemical.

The first in-depth EPA study of dioxin, released in 1985, established acceptable exposure levels of dioxin for humans. These levels were considered excessively low by some scientists and at the urging of several large U.S. corporations, the EPA conducted a lengthy review of its first study of dioxin.

The second EPA report on dioxin was released early last year, and far from calming worries about the chemical, the report heightened concerns by announcing that the safety margin for exposure to dioxin was much smaller than scientists first thought.

The EPA review added that dioxin effects on the immune system, the reproductive system and disruption of hormones might even be greater than the risk of cancer from exposure to dioxin, Kerkvliet said.

"In practical terms, that finding meant that EPA scientists were no longer talking about levels of safe exposure to dioxin," said Kerkvliet. "They were saying that exposure to the chemical at any level is potentially harmful to human health and to the environment in general.

"Many mainstream scientists disagree with several of the assumptions the EPA made in arriving at this conclusion," Kerkvliet said. Nonetheless, she added, the EPA's current position dictates how the chemical will be regulated, and industries that generate dioxin as a by-product - such as paper and pulp mills - will have to learn how to live with it.

In addition, the EPA listed trash incineration as a significant source of dioxin in the environment, Kerkvliet pointed out. This includes general trash incineration as well as incineration of medical and industrial wastes.

"The storm of controversy unleashed by this report has been tremendous and appears to be gathering strength," said Kerkvliet. "Activists are extremely concerned about dioxin. In light of all the anxiety, it's ironic that this chemical was never intentionally manufactured and has no use."