All ionizing radiations, at sufficiently large exposures, can cause cancer. Many, in carefully controlled exposures, are also used for cancer therapy. Whether harmful or beneficial, exposures to ionizing radiation have been an inevitable part of the environment throughout the Earth's history. The nucleosynthesis processes that produced the elements created both stable and unstable nuclides. The unstable nuclides with very long half-lives, together with their radioactive progeny, constitute the natural radioactivity on Earth today. In addition, violent processes in the sun and elsewhere lead to the bombardment of the Earth by cosmic rays. Thus, radiation is an old and familiar, if unrecognized, pollutant.

However, human awareness of radioactivity and ionizing radiation has only a 100-year history starting with the discovery of x-rays and radioactivity. The first evidence that ionizing radiation could do harm came within months after the discovery of x-rays, when an early x-ray worker developed injuries to his skin. Serious efforts to understand and control radiation exposures started in the 1920s and greatly expanded during and after World War II.

Information on the effects of radiation comes from studies of exposed groups and individuals, from animal experiments, and from studies at the cellular and molecular level. It is now well established that ionizing radiation has both prompt and delayed effects. At very high radiation exposures, death will occur within several months or less. At moderate levels, radiation exposure increases the chance that an individual will develop cancer, with a time delay of ten or more years for most cancers. At low levels, the cancer risk decreases, but the relationship between cancer risk and the magnitude of the exposure is uncertain.

Other effects of radiation, in part inferred from animal experiments, include an increased risk of genetic defects and, for exposures of the fetus before birth, of mental retardation. In terms of frequency of occurrence and severity of effects, cancer is the most serious consequence and receives the greatest attention.

The importance of genetic effects has turned out to be much less than was originally expected. In the words of a 1993 NCRP report: "... the genetic risks have been found to be smaller and the cancer risks larger than were thought (in the 1950s)." Strikingly, no statistically significant genetic effects have been found among the extensively studied children of survivors of the Hiroshima-Nagasaki bombings. On the basis of animal experiments, however, one expects some genetic effects–even if so far not observable above the large background of "natural" defects. In addition, the Hiroshima-Nagasaki studies have shown an increased incidence of mental retardation among children who received large prenatal radiation doses, especially for exposures 8 to 15 weeks after conception.