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一篇关于日本核电站辐射问题的报道：Scientists lack complete answers on radiation riskShareretweetEmailPrint AP – Mother and daughter receive radiation exposure scanning in Fukushima, northern Japan Friday, March 18, … By The Associated Press The Associated Press – Fri Mar 18, 6:21 pm ETThyroid cancer for sure. Leukemia, probably. Too much radiation can raise the risk of developing cancer years down the road, scientists agree, and the young are most vulnerable. But just how much or how long an exposure is risky is not clear.Those are among the unknowns scientists are contemplating as the crisis unfolds at Japan's stricken nuclear power plant.In Japan, the Science Ministry said radiation levels about 19 miles northwest of the Fukushima Dai-ichi plant rose at one point Friday to 0.15 millisieverts per hour, about the amount absorbed in a chest X-ray. But levels have been fluctuating, and radiation at most sites that distance from the facility have been far below that.Long term, it is clear radiation can induce cancer. But researchers can't just count cancer cases after a disaster and declare radiation responsible. Rates before and after must be compared to know if more cases occurred than would be expected.That is why, 25 years after the Chernobyl accident, there is still controversy over its effects beyond the undisputed 6,000 cases of thyroid cancer. Of these cases, only 15 had proved fatal as of 2005, even though the Soviets were slow to treat victims of the catastrophe.The records necessary to spot trends in other types of cancer as a result of Chernobyl are poor, said Dr. Fred Mettler, a University of New Mexico scientist who led a United Nations-sponsored team investigating Chernobyl's health effects."At the end of the day, the scientific data isn't there. My instinct is, there probably is an increase there, but it's too small to see," he said.The U.S. Environmental Protection Agency says that no amount of radiation is absolutely safe above the 3 to 6 millisieverts a year that most of us get from normal living. In contrast, the Nuclear Regulatory Commission says that low doses — less than 100 millisieverts spread out over years — are not harmful. Researchers have not documented danger from such low levels, said Kelly Classic, a radiation physicist at the Mayo Clinic and a spokeswoman for the Health Physics Society, an organization of radiation safety specialists.High doses — over 500 millisieverts — can raise the risk of leukemia, breast, bladder, col