In a long-term study of more than 300,000 workers in France, the U.S. and the U.K., those with many years of exposure to low doses of radiation had an increased risk of dying from leukemia.

Medical workers and even patients are also exposed to much more radiation than was common decades ago, the study authors point out, but it’s unclear what amount of low-level exposure raises cancer risk, they say.

“A lot of epidemiological or radiobiological studies have brought evidence that exposure to ionizing radiation can cause cancer and leukemia,” said lead author Dr. Klervi Leraud of the Radiobiology and Epidemiology Department at Fontenay-aux-Roses in Cedex, France.

Workers exposed to ionizing radiation who are later diagnosed with leukemia can already ask for financial compensation in the U.S., the U.K. or France, Leraud told Reuters Health by email.

Leukemia is a cancer of the tissues that make blood cells, and it’s known to be caused by exposure to high doses of radiation, like that released by the atomic bombs dropped on Japan in 1945. In the years following those bombings, leukemia cases increased among the survivors, the authors note in The Lancet Haematology. But such high doses are rare today.

For the new study, researchers considered 308,297 nuclear energy workers whose radiation exposures were monitored. All had worked for at least a year for the French Atomic Energy Commission or similar employers or for the Departments of Energy and Defense in the U.S., or were members of the National Registry for Radiation Workers in the U.K.

The workers were followed for an average of 27 years, with data on exposure and health status through the early- to mid-2000s, depending on their country. Researchers looked for deaths from leukemia or lymphoma, a cancer of the immune system that also involves blood cells.

About 22 percent of the workers had died by the end of follow-up. There were 531 deaths due to leukemia and 814 due to lymphoma.

As cumulative dose of radiation exposure increased, so did the risk of dying from certain kinds of leukemia, the researchers found.

On average, the workers had been exposed to a cumulative dose of 16 milligray (mGy) of radiation over the years of the study, or about one mGy per year. For comparison, a medical computed tomography (CT) scan of the lumbar spine exposes the patient to between one and two mGy, according to the U.S. Food and Drug Administration.

In the U.S., the average person’s yearly exposure to ionizing radiation in 1982 was 0.5 mGy, but by 2006 it had risen to 3 mGy, largely due to medical exposures, Leraud and colleagues write.

In the new study, the researchers calculate that for each gray (1,000 mGy) of total radiation exposure, a worker’s risk of leukemia rose three-fold. The effect was greatest for chronic myeloid leukemia, with a 10.45-fold risk increase per gray.

“These findings are not really new as they just summarize findings of other studies,” said Dr. Maria Blettner of University Medical Center in Mainz, Germany. Blettner noted that occupational exposure policies have been changed over the last two years to reduce cumulative exposure.

Though the results appear to show an excess risk for leukemia with increased cumulative radiation exposure, the wide statistical margins in the paper do make ‘false positives’ possible, Blettner writes in a comment accompanying the paper.

For energy workers, “radiation safety standards in most – if not all – countries are very high,” Blettner told Reuters Health by email. “There is an occupational limit and if this limit is reached persons are not allowed to work in the areas with a potential exposure.”

People who take jobs in the nuclear industry are aware of the health hazards of radiation, Leraud said.

“These employees wear a badge so they are certainly aware of their potential of being exposed to radiation,” Leraud said.

We do know too little about what other factors play a role together with radiation to increase leukemia risk, and whether certain people are more sensitive to radiation exposure, Blettner said.

“There is so far no test with which we can find out whether people are more radio-sensitive than others,” she said.

But the exposure from ‘normal’ nuclear work is in her opinion not more dangerous than in other industrial jobs, she said.