In this review article, published after the Fukushima accident, medical scientists examine published information on the short- and long-term health risks of exposure to ionising radiation. The article describes two previous nuclear accidents — at Three Mile Island in the USA, and Chernobyl in Ukraine — and explains the types and doses of radiation that can damage biological systems. It discusses the mechanisms behind exposure, and radiation-induced illness and injury, including long-term cancer risks. The authors also review measures that can be taken to reduce the effects of radiation exposure, including potassium iodide tablets used in the aftermath of Chernobyl. The article stresses that clear communication on radiation exposure levels and health risks is a key component of the response to a nuclear incident.