Nuclear radiation risk

Thyroid 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.