The nuclear explosions that occurred in Hiroshima and Nagasaki were weapons involving a fission reaction of enormous energy. Extreme heat, blast wave and ionizing radiation together can cause very severe harm, death of hundreds of thousand of people within weeks and contamination of large areas.
In addition there have been hundreds of atmospheric nuclear weapon tests between 1945 and 1980. They have contaminated the whole Globe and in particular the northern hemisphere by the end of 1950`s. While the world average annual dose due to these tests reached a peak of 0.15 mSv in 1963, it has since decreased by about 30-times to 0.005 mSv in 2000 (further info: http://www.unscear.org)
The pie chart of this Home Page shows the average radiation dose from all man-made sources was less than 1 % of the total exposure except the dose from diagnostic medical examinations (0.4 mSv/person/year). The dose from all man-made sources includes doses from former nuclear tests, the average annual dose due to the Chernobyl accident (0.002 mSv in 2000) and from nuclear power production (0.0002 mSv in 2000). For further information see the UNSCEAR web site above).