Another source from World Health Organization mentions that..."the average person is exposed to about
3.0 millisieverts (mSv) a year of radiation, from naturally-occurring, medical and other sources. But monitoring at the Fukushima Daiichi site has recorded radiation as high as
400 millisieverts an hour -- a level known to be a risk to human health.Exposure to 1,000 millisieverts (1 Sievert) of radiation can cause radiation sickness. " [
http://www.who.int/en/]