In 1945, the cities of Hiroshima and Nagasaki met with the unprecedented human calamity of exposure to the atomic bombs and their radiation. In these two cities, the effects of the atomic bombs were primarily external and attributable to the blast, its heat, and accompanying irradiation. Though black rain containing radioactive material fell, the dose of internal radiation due to the absorption of radioactive fallout was relatively small. Early effects of that irradiation include, acute radiation syndrome, which occurred within a few months of the bombing. On average, radiation cataracts were observed two to three years following exposure. Leukemia emerged and peaked five to ten years after the bombing, and the risk of solid cancers started to increase ten years after the bombing. Recently, an increase of benign tumors and noncancer diseases has been reported for atomic-bomb survivors.

In 1986, a nuclear accident at the Chernobyl nuclear power plant in the former Soviet Union (the present Ukraine) resulted in the emission of a considerable amount of iodine and cesium into the air, contamination of a wide area, and both external and internal radiation exposure. Therefore, the radiation health effects from the Chernobyl nuclear accident differ from those of the atomic-bomb—a large part of which was due only to external exposure. One of the distinguishing features of the Chernobyl accident has been the increase of thyroid cancer among children since 1990, whereas increased leukemia, common in atomic bomb surbibors, has not been observed.