An international cancer research study has shown that low-dose radiation exposure over a long period slightly increases the risk of developing leukemia.

The study, funded in part by Japan’s health ministry, was reported in British medical journal The Lancet Haematology by a team of researchers, including from the Lyon, France-based International Agency for Research on Cancer.

The study analyzed data on the radiation exposure and health conditions of over 300,000 workers at nuclear facilities in Europe and the United States.

As little research on risks associated with protracted low-dose radiation exposure has been conducted, the latest findings may be useful for the health care of workers at nuclear power plants in Japan, and medical workers handling radiological equipment.

The team assembled data on 308,297 radiation-monitored workers employed for at least a year in France, Britain and the United States over about 60 years.

The study showed the relative risk of developing leukemia would increase by around three per thousand with a 1 millisievert radiation dose and would remain even with exposure to low doses of up to 100 millisieverts.

The workers’ average yearly radiation doses stood at 1.1 millisieverts and cumulative doses at 15.9 millisieverts. A total of 531 workers died of leukemia, according to the study.

According to the International Commission on Radiological Protection, the risk of cancer would increase with exposure to radiation doses above 100 millisieverts. Researchers are divided on the health impact of radiation exposure below 100 millisieverts.

As well as the healthy ministry, the research was also funded by the U.S. Department of Energy among other groups.