TODAY'S PAPER

It would have taken as little as one milligram of dioxin -- an amount so small it is barely visible -- to cause the shocking chloracne that has disfigured Ukrainian presidential candidate Viktor Yushchenko, pharmacology experts say.

The chemical compound, which doctors have blamed for the sudden onset of Mr. Yushchenko's symptoms while he campaigned in the fall, likely would have little taste and would be easy to surreptitiously slip into food or drink. But accidental poisoning is virtually impossible, said Allan Okey, a professor of pharmacology at the University of Toronto who is an expert on dioxin.

"You need to be exposed to a real jolt before you develop chloracne, and dioxin exists in the environment only in trace amounts," he said in an interview.

Dr. Okey said the form of acne Mr. Yushchenko has developed -- notable for the fact that pimples appear on the ears and eyelids, not just oily surfaces of the skin -- is virtually the only symptom of dioxin poisoning. The acne will likely disappear, although it could take three or four years, he said.

"If you want to kill someone, you don't choose dioxin," Dr. Okey said. "But if you want to take a robust man and make him look really bad, this is your agent of choice."

Dioxin is not a single chemical, but rather a group of 210 chemicals that have similar properties. They are not intentionally produced but rather are byproducts of industrial and combustion activities.

The most toxic chemical in the group is 2,3,7,8-tetrachlorodibenzo-para-dioxin, the likely agent that poisoned Mr. Yushchenko. The substance was used extensively in a covert chemical and biological weapons-research program in the former Soviet Union.

Dr. Okey refused to speculate on the source of the chemical, but said that blood tests conducted on Mr. Yushchenko will likely be able to reveal the compound and its source.

"It requires some sophisticated chemistry to produce 2,3,7,8-TCDD and the process leaves a 'chemical fingerprint' that would probably tell us what lab it's from."

There have been media reports that Mr. Yushchenko's blood contained 1,000 times the normal rates of dioxin. But there is no single standard for an acceptable level.

The U.S. national Centers for Disease Control and Prevention considers 1,000 parts per trillion of dioxin to be of concern. Dr. Okey said that most people take in a couple of parts per trillion daily from food and environmental exposure -- but that amount varies depending on diet and where you live.

Sources of dioxins include the manufacture of chemicals and pesticides; the incineration of waste and household trash; and forest fires. But they also can be found at low levels in air, soil, water and sediment, and in foods such as meat, dairy, fish and shellfish.

In Canada, the highest levels of dioxin are found in people who live near pulp mills.

According to an article published in the medical journal Environmental Health Perspectives, the highest recorded dioxin exposure in a human was 144,000 parts per trillion. It occurred in 1998 in a 30-year-old woman who worked at a textile plant in Vienna. She and a second woman who was 27, whose dioxin levels topped 26,000 parts per trillion, were believed to have been poisoned deliberately. One of the women suffered severe chloracne, which cleared up after a couple of years, while the other had a very mild case of acne and some minor gastrointestinal problems.

The biggest exposure to dioxin occurred in 1976 in Seveso, Italy, when an explosion at a pesticide plant released it in the vicinity of thousands of people. About 200 residents developed chloracne, but there was no apparent increase in cancer rates.

"It's a suspected carcinogen, but there are no clear cases where dioxin has caused cancer in humans," said Jack Uetrecht, a professor of pharmacy and medicine at the University of Toronto, who is also an expert in dioxin.

He said the cancer concerns arise because dioxin accumulates in fat and has a long half-life -- it can take the body seven to 10 years to eliminate 50 per cent of its stores of the chemical.

Dr. Uetrecht, who is also the Canada research chair in immunotoxicology, said the impact of exposure to dioxin, and to 2,3,7,8-TCDD in particular, seems to vary considerably among individuals.

"A thousandfold increase in exposure . . . is not that high. It wouldn't necessarily cause symptoms in everyone," he said.

He said the suspected poisoning of the Ukrainian presidential candidate simply adds to the chemical's dark history. Dioxin was also a principal ingredient in Agent Orange, an infamous cocktail used to defoliate the jungle during the Vietnam War, and it was one of the substances found in the toxic soup that was Love Canal, the community in the state of New York that was built atop a chemical dump site.