The estimate of much higher levels of radioactive cesium-137 comes from a worldwide network of sensors. Study author Andreas Stohl of the Norwegian Institute for Air Research says the Japanese government estimate came only from data in Japan, and that would have missed emissions blown out to sea.
The study did not consider health implications of the radiation. Cesium-137 is dangerous because it can last for decades in the environment, releasing cancer-causing radiation.
The long-term effects of the nuclear accident are unclear because of the difficulty of measuring radiation amounts people received.
In a telephone interview, Stohl said emission estimates are so imprecise that finding twice the amount of cesium isn't considered a major difference. He said some previous estimates had been higher than his. Read More