Chernobyl and childhood thyroid cancer

On 26 April 1986 an explosion and fires destroyed a reactor at the Chernobyl nuclear power plant in Ukraine, then part of the Soviet Union, causing the largest uncontrolled radioactive release in the history of civil nuclear power. Of 600 workers on the site that morning, 134 received doses of 0.8 to 16 gray and developed radiation sickness; 28 of them died within three months. About 115,000 people were evacuated in 1986 and about 220,000 more were relocated later.

The radionuclides that exposed people were mainly iodine-131, caesium-134 and caesium-137. Iodine-131 has a half-life of only eight days, but it passed quickly from the air into milk and leafy vegetables, and iodine collects in the thyroid. Because of the milk they drank, the small size of their thyroid glands and their metabolism, infants and children received higher thyroid doses than adults.

Before the accident, thyroid cancer in children under 10 occurred at about two to four cases per million a year. From 1990 and 1991 the rate rose steeply among people who had been infants or young children in 1986. Between 1991 and 2005 there were 6,848 cases among those under 18 at the time of the accident, 15 of them fatal. Children born after 1986 showed no increase. Case-control and cohort studies tied the excess to the estimated radiation dose to each child's thyroid, mostly from radioiodine.

By 2015 the number of cases among those under 18 in 1986 in Belarus, Ukraine and the four most contaminated Russian regions approached 20,000. Not all were caused by radiation: the population was ageing, awareness was higher and diagnosis had improved. The United Nations Scientific Committee on the Effects of Atomic Radiation estimated in 2018 that about a quarter of the cases among non-evacuated residents exposed as children could be attributed to radiation, with an uncertainty range of at least 7 to 50 percent. Apart from thyroid cancer, and some indication of more leukemia and cataracts among the workers, the committee found no clearly demonstrated rise in solid cancers or leukemia due to radiation in the exposed populations.