Though exposure to radiation can cause acute, near-immediate effect by killing cells and directly damaging tissue, radiation can also have effects that happen on longer scale, such as cancer, by causing mutations in the DNA of living cells. Mutations can occur spontaneously, but a mutagen like radiation increases the likelihood of a mutation taking place. In theory, ionizing radiation can deposit molecular-bond-breaking energy, which can damage DNA, thus altering genes. In response, a cell will either repair the gene, die, or retain the mutation. In order for a mutation to cause cancer, it is believed that a series of mutations must accumulate in a given cell and its progeny. For this reason, it may be many years after exposure before an increase in the incident rate of cancer due to radiation becomes evident.
Among the long-term effects suffered by atomic bomb survivors, the most deadly was leukemia. An increase in leukemia appeared about two years after the attacks and peaked around four to six years later. Children represent the population that was affected most severely. Attributable risk—the percent difference in the incidence rate of a condition between an exposed population and a comparable unexposed one — reveals how great of an effect radiation had on leukemia incidence. The Radiation Effects Research Foundation estimates the attributable risk of leukemia to be 46% for bomb victims.
For all other cancers, incidence increase did not appear until around ten years after the attacks. The increase was first noted in 1956 and soon after tumor registries were started in both Hiroshima and Nagasaki to collect data on the excess cancer risks caused by the radiation exposure. The most thorough study regarding the incidence of solid cancer (meaning cancer that is not leukemia) was conducted by a team led by Dale L. Preston of Hirosoft International Corporation and published in 2003. The study estimated the attributable rate of radiation exposure to solid cancer to be significantly lower than that for leukemia—10.7%.
According to the RERF, the data corroborates the general rule that even if someone is exposed to a barely survivable whole-body radiation dose, the solid cancer risk will not be more than five times greater than the risk of an unexposed individual.
Regarding individuals who had been exposed to radiation before birth (
in utero), studies, such as
one led by E. Nakashima in 1994, have shown that exposure led to increases in small head size and mental disability, as well as impairment in physical growth.
Persons exposed in utero were also found to have a lower increase in cancer rate than survivors who were children at the time of the attack.