Arnold Schecter, a dioxin expert at the University of Texas School of Public Health at Southwestern Medical Center in Dallas, said dioxins can be highly effective poison in people who are sensitive to their effects. If Yushchenko was deliberately given dioxin, it was done by someone who "was very clever and very knowledgeable," Schecter said.

"If someone put a drop of pure dioxin in his food, he wouldn't taste it, he wouldn't see it and a few days later he'd start to get sick," Schecter said.

"If you are trying to kill someone quickly, it's not the way to go," he said. "But if you want to disable someone and want to do it subtly and have it happen days or weeks or months after you have contact with someone, this can do it," Schecter said. "Plus there are very few labs in the world that can accurately detect dioxin in the blood."