Radiation and the Body

How much radiation is 'unsafe' for humans? For those exposed to fallout from the disaster at Japan's Fukushima Daiichi nuclear plant, the question is all too real. But there is still no good answer: the accident has highlighted the enormous difficulties in estimating the long-term health risks of relatively low doses of radiation.

http://www.nature.com/news/2011/110930/full/news.2011.565.html