Exposure to ionizing radiation is known to increase the future incidence of cancer, particularly leukemia. The mechanism by which this occurs is well understood, but quantitative models predicting the level of risk remain controversial. The most widely accepted model posits that the incidence of cancers due to ionizing radiation increases linearly with effective radiation dose at a rate of 5.5% per sievert; if correct, natural background radiation is the most hazardous source of radiation to general public health, followed by medical imaging as a close second. Additionally, the vast majority of non-invasive cancers are non-melanoma skin cancers caused by ultraviolet radiation (which lies on the boundary between ionizing and non-ionizing radiation). Non-ionizing radio frequency radiation from mobile phones, electric power transmission, and other similar sources have been investigated as a possible carcinogen by the WHO's International Agency for Research on Cancer, but to date, no evidence of this has been observed.
Causes According to the prevalent model, any radiation exposure can increase the risk of cancer. Typical contributors to such risk include natural background radiation, medical procedures, occupational exposures, nuclear accidents, and many others. Some major contributors are discussed below.
Radon Radon is responsible for the worldwide majority of the mean public exposure to ionizing radiation. It is often the single largest contributor to an individual's background radiation dose, and is the most variable from location to location. Radon gas from natural sources can accumulate in buildings, especially in confined areas such as attics, and basements. It can also be found in some spring waters and hot springs.
Epidemiological evidence shows a clear link between lung cancer and high concentrations of radon, with 21,000 radon-induced U.S. lung cancer deaths per year—second only to cigarette smoking—according to the United States Environmental Protection Agency. Thus in geographic areas where radon is present in heightened concentrations, radon is considered a significant indoor air contaminant.
Residential exposure to radon gas has similar cancer risks as passive smoking. Radiation is a more potent source of cancer when it is combined with other cancer-causing agents, such as radon gas exposure plus smoking tobacco.
Medical In industrialized countries, medical imaging contributes almost as much radiation dose to the public as natural background radiation. Collective dose to Americans from medical imaging grew by a factor of six from 1990 to 2006, mostly due to growing use of 3D scans that impart much more dose per procedure than traditional radiographs. CT scans alone, which account for half the medical imaging dose to the public, are estimated to be responsible for 0.4% of current cancers in the United States, and this may increase to as high as 1.5–2% with 2007 rates of CT usage; however, this estimate is disputed. Other nuclear medicine techniques involve the injection of radioactive pharmaceuticals directly into the bloodstream, and radiotherapy treatments deliberately deliver lethal doses (on a cellular level) to tumors and surrounding tissues.
It has been estimated that CT scans performed in the US in 2007 alone will result in 29,000 new cancer cases in future years. This estimate is criticized by the American College of Radiology (ACR), which maintains that the life expectancy of CT scanned patients is not that of the general population and that the model of calculating cancer is based on total-body radiation exposure and thus faulty.
Occupational In accordance with ICRP recommendations, most regulators permit nuclear energy workers to receive up to 20 times more radiation dose than is permitted for the general public. Higher doses are usually permitted when responding to an emergency. The majority of workers are routinely kept well within regulatory limits, while a few essential technicians will routinely approach their maximum each year. Accidental overexposures beyond regulatory limits happen globally several times a year. Astronauts on long missions are at higher risk of cancer, see cancer and spaceflight.
Some occupations are exposed to radiation without being classed as nuclear energy workers. Airline crews receive occupational exposures from cosmic radiation because of reduced atmospheric shielding at altitude. Mine workers receive occupational exposures to radon, especially in uranium mines. Anyone working in a granite building, such as the US Capitol, is likely to receive a dose from natural uranium in the granite.
Accidental
Nuclear accidents can have dramatic consequences to their surroundings, but their overall global population impact on cancer is less than that of natural and medical exposures depending on dose rates.
The most severe nuclear accident at a commercial nuclear power plant is most notably the 1986 Chernobyl disaster. There is consensus that a total of approximately 30 people died from immediate blast trauma and acute radiation syndrome (ARS) in the seconds to months after the disaster respectively, with 60 in total in the decades since, inclusive of later radiation induced cancer.
However, there is considerable debate concerning the accurate number of projected deaths that have yet to occur due to the disaster's long-term health effects; long-term death estimates range from 4,000 up to 9,000 (per the 2005 and updated 2006 conclusions of a joint consortium of the United Nations) for the most exposed people of Ukraine, Belarus, and Russia, to 16,000 cases in total for all those exposed on the entire continent of Europe, with figures as high as 60,000 when including the relatively minor effects around the globe. Such numbers are based on the heavily contested linear no-threshold model.
Of the 100 million curies (4 exabecquerels) of radioactive material, the short lived radioactive isotopes such as 131I Chernobyl released were initially the most dangerous. Due to their short half-lives of 5 and 8 days they have now decayed, leaving the more long-lived 137Cs (with a half-life of 30.07 years) and 90Sr (with a half-life of 28.78 years) as main dangers.
In March 2011, an earthquake and tsunami caused damage that led to explosions and partial meltdowns at the Fukushima I Nuclear Power Plant in Japan. Significant release of radioactive material took place following hydrogen explosions at three reactors, as technicians tried to pump in seawater to keep the uranium fuel rods cool, and bled radioactive gas from the reactors in order to make room for the seawater. Concerns about the large-scale release of radioactivity resulted in 20 km exclusion zone being set up around the power plant and people within the 20–30 km zone being advised to stay indoors. On March 24, 2011, Japanese officials announced that "radioactive iodine-131 exceeding safety limits for infants had been detected at 18 water-purification plants in Tokyo and five other prefectures".
Also in Japan was the Tokaimura nuclear accidents of 1997 and 1999. The 1997 accident was far less fatal than the 1999 accident. The 1999 nuclear accident was caused by two faulty technicians who, in their desire to speed up the process of converting uranium hexafluoride to enriched uranium dioxide, resulted in a critical mass that resulted in technicians Hisashi Ouchi being dosed with approximately 17 sieverts of radiation and Masato Shinohara to be dosed with 10 sieverts of radiation, which resulted in their deaths. The two's supervisor, Yutaka Yokokawa, who was sitting in a desk far from the tank where the uranium hexafluoride was being poured into, was dosed with 3 sieverts and survived, but was charged with negligence in October 2000.
In 2003, in autopsies performed on six dead children in the polluted area near Chernobyl where they also reported a higher incidence of pancreatic tumors, Bandazhevsky found a concentration of 137Cs of 40–45 times higher than in their liver, thus demonstrating that pancreatic tissue is a strong accumulator of radioactive cesium. In 2020, Zrielykh reported a high and statistically significant incidence of pancreatic cancer in Ukraine for a period of 10 year, there have been cases of morbidity also in children in 2013 compared with 2003.
Other serious radiation accidents include the Kyshtym disaster (estimated 49 to 55 cancer deaths), and the Windscale fire (an estimated 33 cancer deaths).Perhaps the Worst, Not the First TIME magazine, May 12, 1986.
The satellite Transit 5BN-3 accident. The satellite had a SNAP-3 radioisotope thermoelectric generator (RTG) with approximately 1 kilogram of plutonium-238 on board when on April 21, 1964 it burned up and re-entered the atmosphere. Dr. John Gofman claimed it increased the rate of lung cancer worldwide. He said "Although it is impossible to estimate the number of lung cancers induced by the accident, there is no question that the dispersal of so much plutonium-238 would add to the number of lung cancers diagnosed over many subsequent decades." Other satellite failures include Kosmos 954 and Kosmos 1402.
Mechanism Cancer is a stochastic effect of radiation, meaning it is an unpredictable event. The probability of occurrence increases with effective radiation dose, but the severity of the cancer is independent of dose. The speed at which cancer advances, the prognosis, the degree of pain, and every other feature of the disease are not functions of the radiation dose to which the person is exposed. This contrasts with the deterministic effects of acute radiation syndrome which increase in severity with dose above a threshold. Cancer starts with a single cell whose operation is disrupted. Normal cell operation is controlled by the chemical structure of DNA molecules, also called chromosomes.
When radiation deposits enough energy in organic tissue to cause ionization, this tends to break molecular bonds, and thus alter the molecular structure of the irradiated molecules. Less energetic radiation, such as visible light, only causes excitation, not ionization, which is usually dissipated as heat with relatively little chemical damage. Ultraviolet light is usually categorized as non-ionizing, but it is actually in an intermediate range that produces some ionization and chemical damage. Hence the carcinogenic mechanism of ultraviolet radiation is similar to that of ionizing radiation.
Unlike chemical or physical triggers for cancer, penetrating radiation hits molecules within cells randomly.In the case of internal contamination with alpha emitters, the distribution may not be so random. Transuranic elements are believed to have a chemical affinity for DNA, and any radioactive element could be part of a chemical compound that targets certain molecules. Molecules broken by radiation can become highly reactive free radicals that cause further chemical damage. Some of this direct and indirect damage will eventually impact chromosomes and epigenetic factors that control the expression of genes. Cellular mechanisms will repair some of this damage, but some repairs will be incorrect and some chromosome abnormalities will turn out to be irreversible.