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The International Agency for Research on Cancer also commissioned a study of more than 400,000 nuclear industry workers in 15 countries to estimate cancer risk following protracted low doses of radiation. The 2007 study found a dose-related higher risk of cancer death, but questions have been raised about its methods.
The results also were driven largely by higher rates in Canada; once that country's results were excluded, no increase is seen, Mettler said. There have been questions about the data from Canada, Mettler said. Also, the authors of the study say they need to do more work to assess how smoking and other factors affected their estimates.
So for now, the clearest information on cancer risk from a nuclear plant accident may come from Chernobyl. That disaster exposed 5 million people in Belarus, Russia and Ukraine to large amounts of radioactive material for 10 days, according to the 2008 report that Mettler helped write for the United Nations' Scientific Committee on the Effects of Atomic Radiation, which represents 22 nations on nuclear safety.
Exposure to cesium was a big concern because it affects the whole body, not just the thyroid gland. And exposure among cleanup workers and emergency responders ranged as high as a few hundred millisieverts over the following few years. Evidence suggests a higher rate of leukemia in these workers, "but it's not certain," Mettler said.
Research is continuing in that group, and longer follow-up should establish that more clearly, he said.
"Leukemia increases have not been seen in the children" who are now adults, he said. Nor have increases in breast, lung, stomach or other cancers been documented, though this population became very mobile after Chernobyl and the breakup of the Soviet Union, so the true rates are hard to establish, and rates before the accident in some cases are unknown, Mettler said.
As bad as Chernobyl was, the average radiation dose over 20 years to people who live in contaminated areas was "relatively low" -- 9 millisieverts, nearly the equivalent of a CT scan -- once the short-term doses to the thyroid were subtracted, the UN report said. That means there should not be "substantial health effects in the general population that could be attributed to radiation," the report concludes.
The NRC has said that typical annual background exposure to radiation shaves 18 days off the expected lifespan. Working in a nuclear plant under ordinary conditions -- not in a crisis like the one unfolding in Japan -- shortens life expectancy by 51 days. By comparison, being 15 percent overweight cuts two years; smoking a pack of cigarettes a day costs six years of life.
Thyroid cancer: http://tinyurl.com/5t5vpfu
Copyright 2011 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.
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