KIEV, UKRAINE - With every cough and sore throat, every ache and pain, Valentyna Stanyuk feels Chernobyl stalking her.
"It's only a matter of time," she said as she waited for a thyroid test at a mobile Red Cross clinic in her village of Bystrichy, 150 miles west of Chernobyl.
The tests came back clean, but that's little reassurance to this 54-year-old or to millions of others who live in the parts of Ukraine, Belarus and Russia that were heavily irradiated when the nuclear reactor exploded 20 years ago, spewing radioactive clouds over Ukraine and much of Europe for 10 days.
The April 26, 1986, disaster forced the evacuation of large swaths of some of the former Soviet Union's best farmland and forests. The radiation spread far enough to be detected in reindeer meat in Norway and in rainfall in the U.S. Pacific Northwest. It shocked most European countries into a 20-year freeze on building nuclear plants. In so starkly exposing the failings of the communist system, the world's worst nuclear accident might even have hastened the collapse of the Soviet Union five years later.
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And the effect on the health of the people exposed to its invisible poisons? That is the most heatedly debated legacy of Chernobyl.
"There is so much that we still don't know," said Dr. Volodymyr Sert, head of a team of Red Cross doctors who canvass Ukraine's rural Zhytomyr region in search of thyroid abnormalities -- one of the few problems that all scientists agree are linked to Chernobyl's fallout.
"The most important thing we can do is reassure people that they aren't being forgotten," he said.
After the explosion, about 116,000 residents were evacuated from a 20-mile zone around the plant. About 5 million others in areas that got significant fallout were not evacuated.
Over the years, reports and rumors have spoken of thousands of these especially vulnerable people dying from radiation. But a September report by a group of U.N. agencies concluded that the accident wasn't nearly as deadly as feared.
Fewer than 50 deaths had been directly linked to radiation exposure as of mid-2005, the report said. A total of 4,000 of the 600,000 "liquidators" -- workers who were hastily mobilized to clean up the accident site -- are likely to die from radiation-related cancers and leukemia, it predicted. That's far below the tens of thousands many claimed were fatally stricken.
The researchers found that thyroid cancer rates have skyrocketed among people who were under 18 at the time of the accident, but they noted that more than 99 percent survive after treatment.
The report by the Chernobyl Forum -- a group comprising the International Atomic Energy Agency and several other U.N. groups -- said there was no convincing evidence of birth defects or reduced fertility. Most of the general population suffered such low radiation doses that the scientists decided not to make predictions about deaths, except to say that some increase -- less than 1 percent or about 5,000 -- might be expected, it said.
Venyamin Khudolei, director of the Center for Independent Ecological Expertise at the government-founded Russian Academy of Science, disagrees.
In the part of Russia most heavily hit by the fallout, mortality rates have risen nearly 4 percent since the explosion, indicating the Chernobyl toll in Russia alone could be calculated at 67,000 people, he said. His findings are cited by the environmental group Greenpeace, which said in a report Tuesday that more than 90,000 people were likely to die of cancers caused by radiation from the disaster.
Greenpeace suggested the U.N. report was deliberately misleading.
"It is appalling that the IAEA is whitewashing the impacts of the most serious nuclear accident in human history," said Ivan Blokov of the group's Russia office. "Denying the implications is not only insulting to the thousands of victims, but it also leads to dangerous recommendations and the relocation of people in contaminated areas."
Other experts point to studies that show increases in everything from schizophrenia among the liquidators to breast cancer.
The U.N. report suggested that people in heavily affected areas were gripped by "paralyzing fatalism" that induced them to see themselves as victims and blame Chernobyl for every ailment, even those caused by smoking or drinking.
That outraged Ukrainian officials.
"I am speechless that we can allow this blasphemy in front of the graves of those who died," said lawmaker Borys Oliynyk. The death toll immediately after the disaster was about 30.
Researchers trying to determine combined death tolls -- and predict deaths to come -- don't have an easy task. Soviet-era attempts to cover up the chaotic and often inhumane response made it difficult to track down victims. Lists were incomplete, and Soviet authorities later forbade doctors to cite "radiation" on death certificates.
The rural regions affected are impoverished and unemployment is high. Alcohol abuse is rampant, diets poor. It's hard to distinguish Chernobyl-related health problems from a more general post-Soviet malaise, scientists said.
"I'm sure we'll see claims of thousands, tens of thousands, hundreds of thousands, millions of deaths, but again we checked, we checked all the research, all the files," said Didier Louvat, a radiation waste expert with the International Atomic Energy Agency.
"The explosion was very concentrated around the facility, and the fallout was spread in great plumes that went high into the atmosphere and crossed Europe, diffusing the concentration. ... It could have been much worse."
About 1,000 people -- plant personnel, military conscripts, firefighters from the Kiev region, emergency workers -- bore the brunt of the inferno, and 134 were officially confirmed as suffering from acute radiation syndrome.
One person died during the explosion, and his body has never been recovered. The U.N. report says that 28 died from radiation sickness in 1986 and that 19 suffering from radiation syndrome died from 1987 to 2004, but not all the deaths were necessarily caused by radiation. The rest remain alive.
Wearing no masks or protective suits, dozens of firefighters were deployed. While the bosses sheltered underground, plant workers recall, people stood around awaiting instructions, breathing poisoned air.
The Chernobyl plant now is a cracked hulk in the eerie "dead zone." The last of its four reactors was taken out of service in 2000, and the main activity is to shore up the concrete-and-steel "sarcophagus" that covers the reactor.
But radiation infects a vast stretch of Ukraine, Belarus and Russia -- in the soil, in the berries and mushrooms, in the firewood needed to heat homes.
Oleksandr Nabok, 21, has never been near the nuclear station, 60 miles from his village, but he was diagnosed with thyroid cancer.
He is one of more than 5 million people who live in areas deemed contaminated but habitable, far removed from the villages circling the plant that were considered so irradiated that they were bulldozed under mounds of dirt. There, isotopes with half-lives of 24,390 years came to rest.
In Nabok's village, experts say, the biggest concern was radioactive iodine.
People there suffer from a lack of iodine, so when the radioactive iodine was released, their thyroids gobbled it up; children's thyroid glands work most actively, putting them at greatest risk. Many ingested the iodine in milk from cows that had grazed on radiated fields.
Accounts vary, but experts agree that between 4,000 and 5,000 people, children when the explosion happened, have been diagnosed with thyroid cancer in Ukraine and Belarus -- making it the single biggest Chernobyl-related medical problem. At least nine have died. Before the accident, the illness was so rare that in most years only about 10 children were diagnosed with it.
The numbers keep growing. The main spurt was expected to come around this time, but no one knows whether this is the beginning of the peak or its end.
The U.N. report found that the high anxiety levels persist and even appear to be growing among people who live in contaminated zones.
"It is scary. You try not to worry about it," said Valentyna Yanduk, whose face brightened into a smile after the Red Cross doctors gave her 12-year-old son Ihor's thyroid the all-clear. Technically he's not part of the risk group -- he wasn't even born at the time of the explosion -- but his mother worries.
"For 20 years, these people have been living as victims instead of survivors," said Louvat, the IAEA radiation expert. "We need to be telling them: 'Look, you survived this.' "
(AP correspondent Jim Heintz contributed to this report.)