How much radiation exposure endangers health?

Kanagawa, near Tokyo, reports radiation levels 9 times normal after explosions at 2 reactors in Japan; how close is Japan to a serious crisis?

japan fukushima radiation (photo credit: Yuriko Nakao / Reuters)
japan fukushima radiation
(photo credit: Yuriko Nakao / Reuters)
Japan asked local governments to make more frequent radiation checks after explosions at two nuclear reactors, with reports of radiation levels nine times normal briefly detected in Kanagawa near Tokyo. Below are some facts about the health dangers posed by higher radiation levels.
* Chief cabinet minister Yukio Edano said radiation levels near the stricken plant on the northeast coast reached as high as 400 millisieverts (mSv) an hour, thousands of times higher than readings before the blast. That would be 20 times the current yearly level for some nuclear-industry employees and uranium miners.
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* Exposure to 350 mSv was the criterion for relocating people after the Chernobyl accident, according to the World Nuclear Association.
* People are exposed to natural radiation of about 2 mSv a year.
* Airline crew flying the New York-Tokyo polar route are exposed to 9 mSv a year.
* Exposure to 100 mSv a year is the lowest level at which any increase in cancer is clearly evident. A cumulative 1,000 mSv would probably cause a fatal cancer many years later in five out of every 100 persons exposed to it.
*A single 1,000 mSv dose causes radiation sickness such as nausea but not death. A single does of 5,000 mSv would kill about half of those exposed to it within a month.
*"Very acute radiation, like that which happened in Chernobyl and to the Japanese workers at the nuclear power station, is unlikely for the population," said Lam Ching-wan, a chemical pathologist at the university of Hong Kong.