People who helped clean up after the 1986 Chornobyl nuclear plant accident show a higher risk for developing leukemia, say researchers who hope the findings will help better define the cancer risk of low doses of radiation.

Researchers in the U.S, Ukraine and Russia followed more than 110,000 workers and overall, there were 137 cases of leukemia.

"We estimated that approximately 16 per cent of all leukemia cases in our Chornobyl cleanup worker population over a period of 20 years of follow-up were attributable to radiation exposure from the Chornobyl accident," lead researcher Dr. Lydia Zablotska, a professor of epidemiology and biostatistics at the University of California, San Francisco and her co-authors concluded in the journal Environmental Health Perspectives.

The findings were consistent with estimates for Japanese atomic bomb survivors, who received higher cumulative doses on average, they said.

Chronic lymphocytic leukemia was the most common type of leukemia among the workers and cases are expected to increase as they age, the researchers said.

But there is uncertainty in extrapolating radiation risks associated with acute exposures and protracted exposures to low doses, such as those faced by miners, nuclear workers or patients receiving "sizeable" radiation doses during medical diagnostic tests.

Scientists estimate cancer risk from radiation by extrapolating from the high doses that atomic bomb survivors were exposed to. But they were bathed in gamma or neutron rays while people getting medical tests are exposed to X-rays, a different type of radiation.

Chronic lymphocytic leukemia accounts for about three per cent of all cases of leukemia in Japan compared with one-third of all leukemia cases in the U.S. and 40 per cent in Ukraine, the researchers said.

Many of the Ukrainian workers were exposed to high levels of radiation from cleaning up contaminated debris.

A panel of hematologists and specialized pathologists confirmed each diagnosis.

Annual medical exams, including blood tests and a visit to a hematologist, were recommended for Chornobyl cleanup workers, which may have resulted in better detection than in the general population.

The research was funded by the U.S. National Cancer Institute.