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25th Anniversary of Chernobyl Accident

July 2011 - Linkage Newsletter

On April 26, 1986, an accident at the Chernobyl nuclear power plant in what is now Ukraine caused an explosion and the subsequent release of radioactive materials. More than 28 emergency workers at the plant died within the first four months as a result of exposure to extremely high levels of radiation. Across the region, more than 6 million people in Belarus, Russia, and Ukraine were exposed to varying levels of radiation from radioactive fallout, principally radioactive iodine (I-131) and cesium-137. Plant staff and emergency workers, clean-up workers (also known as liquidators), and children living in the contaminated areas experienced the largest share of adverse health effects from the accident.

Dr. Gilbert W. Beebe (1912–2003), a health statistician in the epidemiology program from which DCEG was created, played a leading role in launching multidisciplinary, collaborative projects to study the health consequences of the accident. Dr. Beebe and fellow NCI staff, along with many international radiation experts as well as investigators from Belarus and Ukraine, designed epidemiologic studies on the exposed populations. These studies, carried out with research partners at Columbia University in New York City, became part of DCEG's Radiation Epidemiology Branch (REB) portfolio in 1995 and included:

  • Two cohort studies of the long-term risk of thyroid cancer and other diseases among individuals in Ukraine and Belarus who were children or adolescents at the time of the accident and received relatively high doses of radioiodines. Later, individuals who were exposed in utero were added to the studies.
  • A case-control study of leukemia among a cohort of clean-up workers in Ukraine who were exposed mostly to external gamma radiation over a protracted period of time.

For these studies, REB dosimetrists worked closely with Belarusian and Ukrainian scientists, as well as epidemiologists, to reconstruct individual radiation dose estimates, which formed the basis for quantifying the radiation-related cancer risks. In addition, two external advisory groups—the Leukemia Advisory Group and the Thyroid Advisory Group—were formed to provide ongoing peer review and scientific oversight for the studies.

What Have We Learned from the Chernobyl Accident?

Dozens of papers have been published from these studies, describing their methodology and findings. "From the study of accidents such as Chernobyl, we can develop better understanding of the dose-dependent risk of adverse health effects, including both internal exposure to radioactive iodine and chronic external radiation exposure," said Kiyohiko Mabuchi, M.D., Dr.P.H., Deputy Chief of REB and head of the Chernobyl Research Unit. "Even 20 years after the accident, we see a persistently increased risk of thyroid cancer attributable to radioactive iodine," said Maureen C. Hatch, Ph.D. (REB), former head of the Chernobyl Research Unit.

Investigators used sophisticated models and measurements of thyroid radiation activity to develop reliable dose estimates and conducted standardized screening to identify cancer cases, which enabled them to estimate the risk from radiation exposure in the cohort of children and adolescents. The risk of thyroid cancer increased with radiation dose from I-131 throughout the study period of 1998 to 2007. The primary source of radioactive iodine among the children was from drinking milk produced by cows that had eaten contaminated grass.

Among clean-up workers in Ukraine, who were exposed to protracted low-dose radiation, the risk of leukemia also increased with radiation dose, and the magnitude of the risk was comparable to that of atomic bomb survivors, who experienced acute radiation exposure. Without a cancer registry in Ukraine at the time of the study's initiation, investigators identified leukemia cases by visiting a large number of medical institutions in various localities, and they confirmed the diagnoses through a review by a panel of international hematology experts. Doses were reconstructed based on data gathered through extensive interviews about work histories, because exposures differed by the type, duration, location, and timing of jobs performed after the accident.

Findings from these studies are contributing greatly to the ongoing development of standards for radiological protection and countermeasures needed in the event of a nuclear accident.

For detailed information on the Chernobyl studies, go to