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About the Chernobyl Accident

The accident of April 26, 1986 at the Chernobyl nuclear power plant in Ukraine was the most severe ever to have occurred in the civilian nuclear power industry (UNSCEAR 2008). It happened during a routine test, intended to demonstrate how long the turbines would spin after a power loss. Prior to the test, the automatic shutdown mechanisms had been disabled. Improper, unstable operation of the reactor, which itself had design flaws, allowed an uncontrollable power surge to occur, resulting in steam explosions that completely destroyed the reactor.

Occupational Exposures and Health Effects

Two workers died in the immediate aftermath, and high doses of radiation to 134 plant staff and emergency personnel resulted in acute radiation syndrome that rapidly proved fatal for 28 of them. In addition to this group of emergency workers, several hundred thousand were involved in clean-up operations during the first four years following the accident. These workers were exposed to both external radiation and to radioiodines. The incidence of leukemia, thyroid cancer, and cataracts among groups of clean-up workers has been the subject of epidemiologic studies.

Environmental Exposures and Health Effects

The accident also caused the largest uncontrolled atmospheric releases of radioactive materials ever recorded for any civilian operation. Two radionuclides, short-lived iodine-131 (131I with a half-life of eight days) and long-lived cesium-137 (137Cs with a half-life of 30 years), were particularly significant with respect to the environmental contamination and to the radiation doses they delivered to vast numbers of people. The atmospheric releases lasted about 10 days, during which time the wind changed direction several times. This, coupled with rainfall, led to widespread, substantial contamination of an area within approximately 300 miles of the plant, principally to northern Ukraine, Belarus, and parts of Russia (see map of 137Cs fallout on the ground). The radioactive clouds further dispersed over the entire northern hemisphere, depositing lesser amounts of radioactive material over large areas of the former Soviet Union and some other countries in Europe.

For the short-lived 131I, the main pathway of human exposure was via the transfer of material deposited on pasture grass to cow’s milk. Young children residing in contaminated areas, who consume relatively large quantities of fresh cow’s milk, received high thyroid doses of 131I because iodine concentrates in the thyroid, and their thyroid mass is small. Since the Chernobyl accident, there has been a striking increase in the reported incidence of thyroid cancer among children who resided in parts of Belarus, Russia, and Ukraine.

Map from the 2008 United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) report showing 137-Cs deposition levels in Belarus, the Russian Federation and Ukraine as of December 1989



The increase in the reported incidence of thyroid cancer started some four to five years after the accident and has continued to manifest itself. As the underlying incidence rate of thyroid cancer in young children is normally very low, many of the cases diagnosed before age 15 have been attributed to the releases from the Chernobyl accident. As of 2005, four thousand cases of thyroid cancer have resulted, including 15 thyroid cancer deaths. Several thousand more are expected to develop among people who were children when the accident occurred. A number of epidemiologic studies of thyroid cancer and other thyroid diseases have been or are being conducted in Belarus and in Ukraine. To date, there has been no persuasive evidence of any health effect, other than thyroid cancer, that can be attributed to radioactive releases from the Chernobyl accident (UNSCEAR 2008). However, the accident is presumably the cause of a range of mental health issues, among both workers and members of the public.

Key Publication

UNSCEAR 2008 Report, Vol. II, Annex D: Health effects due to radiation from the Chernobyl accident (pdf, 5.5 MB)


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