The Chernobyl disaster of 26 April 1986 triggered the release of radioactive contamination into the atmosphere in the form of both particulate and gaseous radioisotopes. As of 2025^[update], it remains the world's largest known release of radioactivity into the natural environment.

The work of the Scientific Committee on Problems of the Environment (SCOPE) suggests that the Chernobyl disaster cannot be directly compared to atmospheric tests of nuclear weapons by simply saying that it is better or worse. This is partly because the isotopes released at the Chernobyl Nuclear Power Plant tended to be longer-lived than those released by the detonation of atomic bombs.

It is estimated that the Chernobyl disaster caused US$235 billion in economic damages.

In a 2009 United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) study, the Chernobyl accident had by 2005 caused 61,200 man-Sv of radiation exposure to recovery workers and evacuees, 125,000 man-Sv to the populace of Ukraine, Belarus, and Russia, and a dose to most other European countries amounting to 115,000 man-Sv. The report estimated a further 25% more exposure would be received from residual radioisotopes after 2005. The global collective dose from Chernobyl was earlier estimated by UNSCEAR in 1988 to be "600,000 man Sv, equivalent on average to 21 additional days of world exposure to natural background radiation."

The inhalation dose (internal dose) for the public during the time of the accident and their evacuation from the area in what is now the 30 km evacuation zone around the plant has been estimated, based on ground deposition of caesium-137, to be between 3 and 150 mSv.

Thyroid doses for adults around the Chernobyl area were estimated to be between 20 and 1000 mSv, while for one-year-old infants, these estimates were higher, at 20 to 6000 mSv. For those who left the area soon after the accident, the internal dose due to inhalation was 8 to 13 times higher than the external dose due to gamma/beta emitters. For those who remained until later (day 10 or later), the inhalation dose was 50-70% higher than the dose due to external exposure. The majority of the dose was due to iodine-131 (about 40%) and tellurium and rubidium isotopes (about 20 to 30% for Rb and Te).

The ingestion doses in this same group of people have also been estimated using the cesium activity per unit of area, isotope ratios, an average day of evacuation, intake rate of milk and green vegetables, and what is known about the transfer of radioactivity via plants and animals to humans. For adults, the estimated dose ranges from 3 to 180 mSv, while for one-year-old infants, the estimated dose ranges from 20 to 1300 mSv. Again, the majority of the dose was attributed to iodine-131.

Ukraine, Belarus and parts of Russia were exposed to radiation after the Chernobyl disaster in 1986, but prior to the disaster, the number of children affected by thyroid cancer was relatively low globally. Every year, about 0.1–2.2 individuals per million of all ages under 15 years old worldwide are affected by thyroid cancer. Research has shown after the Chernobyl disaster the level of thyroid cancer, particularly in children near the radiation exposure, increased. Although iodine-131 has a short half-life compared to other radioactive isotopes, iodine-131 made its way through the food chain through a milk-to-consumer pathway. 95% of iodine-131 was ingested through milk after the disaster. Communities were unaware of the contamination deposited in soil and the transforming capabilities of radiation into other food sources. Children also absorbed radiation after drinking milk.