DNA damage
Exposure to high doses of radiation cause DNA damage, later creating serious and even lethal chromosomal aberrations if left unrepaired. Ionizing radiation can produce reactive oxygen species, and does directly damage cells by causing localized ionization events. The former is very damaging to DNA, while the latter events create clusters of DNA damage. This damage includes loss of nucleobases and breakage of the sugar-phosphate backbone that binds to the nucleobases. The DNA organization at the level of histones, nucleosomes, and chromatin also affects its susceptibility to radiation damage. Clustered damage, defined as at least two lesions within a helical turn, is especially harmful. While DNA damage happens frequently and naturally in the cell from endogenous sources, clustered damage is a unique effect of radiation exposure. Clustered damage takes longer to repair than isolated breakages, and is less likely to be repaired at all. Larger radiation doses are more prone to cause tighter clustering of damage, and closely localized damage is increasingly less likely to be repaired.Somatic mutations cannot be passed down from parent to offspring, but these mutations can propagate in cell lines within an organism. Radiation damage can also cause chromosome and chromatid aberrations, and their effects depend on in which stage of the mitotic cycle the cell is when the irradiation occurs.
Accidental
Accidental exposure may be the result of a criticality or radiotherapy accident. There have been numerous criticality accidents dating back to atomic testing during World War II, while computer-controlled radiation therapy machines such as Therac-25 played a major part in radiotherapy accidents. The latter of the two is caused by the failure of equipment software used to monitor the radiational dose given. Human error has played a large part in accidental exposure incidents, including some of the criticality accidents, and larger scale events such as the Chernobyl disaster. Other events have to do with orphan sources, in which radioactive material is unknowingly kept, sold, or stolen. The Goiânia accident is an example, where a forgotten radioactive source was taken from a hospital, resulting in the deaths of 4 people from ARS. Theft and attempted theft of radioactive material by clueless thieves has also led to lethal exposure in at least one incident.Exposure may also come from routine spaceflight and solar flares that result in radiation effects on earth in the form of solar storms. During spaceflight, astronauts are exposed to both galactic cosmic radiation (GCR) and solar particle event (SPE) radiation. The exposure particularly occurs during flights beyond low Earth orbit (LEO). Evidence indicates past SPE radiation levels that would have been lethal for unprotected astronauts. GCR levels that might lead to acute radiation poisoning are less well understood. The latter cause is rarer, with an event possibly occurring during the solar storm of 1859.