"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."
Michael Zimpfer, right, with Nicolai Korpan, said a higher dioxin dose could have been deadly.
(Ronald Zak -- AP)
Paul M. Wax, with the American College of Medical Toxicology, said two scientists he met in Volgograd, Russia, in 2002 told him that during the Soviet era they had investigated the potential of developing dioxin as a chemical weapon.
Wax expressed doubts that dioxin could be used in that way. "It was never on anyone's list," said Wax, a physician who now teaches courses on chemical terrorism. "We don't think about it as an acute poison that can kill you. . . . It's not going to cause someone to keel over on a battlefield," but could cause long-term illness.
A Russian government specialist challenged the Vienna doctors' findings. "It is impossible to get a dose of dioxin today and get poisoning tomorrow," said Yury Ostapenko, head of the Toxicology Information Center at the Russian Health Ministry, speaking on the Echo Moskvy radio station. "Dioxins do not belong to immediate-effect poisons: Poisoning develops for years and decades."
Doctors at the Austrian clinic had declined for weeks to issue a finding on the cause of the illness but said Saturday that tests conducted in an Amsterdam hospital had confirmed the presence of dioxin.
Yushchenko flew to Austria on Friday for further tests at the clinic. "What happened to me was an attempt to politically destroy a politician with opposing views," he said at a news conference in Kiev on Friday, shortly before his departure. "The aim was to kill me."
Speaking to 5,000 supporters in the city of Luhansk before the announcement in Vienna, Yanukovych said of Yushchenko: "He certainly is ill and I sympathize. Let him get well soon. As for the reasons, I know nothing. Let the specialists work on that."
After the statement was issued, there was no immediate response from the Yanukovych campaign, whose press secretary resigned Friday night, the latest in a series of defections. But Yanukovych's attorney before the Supreme Court, Stepan Havrysh, said: "I'm afraid, two weeks before the vote, it's all political technologies," the Associated Press reported.
Yushchenko supporters said the finding bore out what they had long believed.
"This official confirmation is another opportunity to speak the truth to the Ukrainian people and to show one more time the dirty methods that were used by the authorities," said Yuri Yekhanurov, a member of parliament, in a telephone interview.
In Vienna, Yushchenko struck an upbeat note Saturday. "I plan to live for a long time and I plan to live happily," he said. "I am getting better . . . every day."
Staff Writers Rob Stein, Rick Weiss, Juliet Eilperin, Joby Warrick and John Burgess in Washington contributed to this report.