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The world has never seen a nuclear accident as severe as the one that unfolded when a reactor exploded in Chernobyl on April 26 1986, sending vast amounts of radiation into the skies around Ukraine, Belarus and Russia.

The planet had experienced massive releases like this before, in the bombings of Hiroshima and Nagasaki in 1945. But Chernobyl-related radiation exposure had a more protracted character.

It was the first time in history that such a large population, particularly at a very young age, was exposed to radioactive isotopes, namely iodine-131 and cesium-137, not just through direct exposure, but through eating contaminated food as well.

In 2006, the International Agency for Research on Cancer (IARC) published estimates of how many excess cancers would occur as a result of this contamination.

While noting that these estimates are subject to substantial uncertainty, the authors found that 1,000 cases of thyroid cancer and 4,000 cases of other cancers had already been caused by the accident. They further estimated that by 2065, 16,000 cases of thyroid cancer and 25,000 cases of other cancers could be attributed to the effects of Chernobyl radiation.

Research on the health impact of the Chernobyl disaster has mainly focused on thyroid cancer, in particular in those exposed to radioactive iodine isotopes in childhood and adolescence. Large amounts of iodine-131 were released into the atmosphere after the explosion, and children were exposed by consuming locally produced milk and vegetables.

Efforts were made to better understand the mechanisms of radiation-induced thyroid cancer and which factors could modify the radiation risk. This allowed us to identify a molecular “radiation fingerprint,” which can point to changes that are specific to radiation exposure, as opposed to any other factors.

Studies were also conducted to evaluate the risk of hematological malignancies — tumors that affect the blood, bone marrow, lymph, and lymphatic system — in children and Chernobyl clean-up workers in the three most affected countries. Studies of cancer incidence and mortality, cardiovascular diseases and all-cause mortality were also conducted on clean-up workers. Although of variable quality, the list of studies done on people affected by the blast is long.

What we found

Today, there is an overall agreement among scientist that thyroid cancers increased following exposure to radiation in childhood and adolescence. Several studies have also indicated an increase in haematological malignancies and thyroid cancer in Chernobyl clean-up workers.

Findings on radiation-associated risk both for chronic lymphocytic leukaemia and other types of leukaemia in clean-up workers were reported in 2013. Before then, chronic lymphocytic leukaemia was not considered to be sensitive to radiation. Further research will be required to confirm these findings.

Some studies focused on non-cancer health consequences of exposure to radiation. Convincing results on eye lens cataracts among Chernobyl clean-up workers led to the revision and considerable reduction in the recommended radiation dose limit for the lens of the eye.

Chernobyl also led to a greater knowledge on optimising treatment and follow-up of survivors of acute radiation sickness. A better understanding of thyroid cancer radiation risks allowed us to respond better to other disasters, such as Fukushima, to minimize potential adverse health consequences.

What we still don’t know

Despite these important findings, many grey areas still remain. For example, we still have no convincing evidence for childhood leukaemia associated with Chernobyl. It is unclear if this is due to methodological limitations or for other reasons.

Nor do we know how radiation risk changes over time after a someone is exposed as a child, as a longer follow-up study is required. We also don’t yet understand the potential transgenerational affects on children born to exposed parents.

The need for more research is immense, yet funding is declining. We need a sustainable approach to Chernobyl health research – similar to that taken after the Hiroshima and Nagasaki bombings in Japan. Without this, it is unlikely that the true impact of Chernobyl will ever be fully understood.

Ausrele Kesminiene, Deputy Section Head Section of Environment and Radiation at IARC, International Agency for Research on Cancer (IARC)