Ionizing radiations are x-rays, γ-rays, and small atomic particles that have sufficient energy to ionize atoms. The local deposition of energy involved can damage or kill cells. It may also produce subtle changes in a cell genetic make-up. Much has been learned about effects of radiation from experiments on cell cultures. These experiments have shown that large particle radiations are more damaging than x-rays for the amount of energy released. They have also shown that when similar doses of x-rays are delivered at lower dose rates, less damage is produced because the cells are able to effectively repair much of the damage. The effects of radiation on human populations have been assessed through epidemiological studies of groups exposed to very large doses, such as the survivors of the atomic bombs exploded over Japan. Exposure to large doses of radiation above a certain threshold produces tissue reactions in the short term. Longer-term effects may occur after exposure to low doses of radiation, which are referred to as stochastic effects, as their occurrence is governed by the laws of chance. Stochastic effects are the induction of cancer and of genetic effects. Cancer induced by radiation will not develop until many years after exposure and the risks are greater among those exposed as children. Risks have been estimated from epidemiological studies, by extrapolating dose-response data down to the low doses received by radiation workers, but debate exists about whether the current extrapolation model may be too conservative.