Workers in the nuclear industry are more likely to die from heart disease if they are exposed to radiation, according to the first study to establish a link between disorders of the circulatory system and radiation in the workplace.
However, the scientists who conducted the study emphasised that they have not yet proved that radiation causes heart disease, only that there appears to be a statistically significant association that may be the result of cause and effect.
They also said that they the study of nearly 65,000 nuclear workers shows that they are healthier overall than the general population, even after taking into account the extra health risks resulting from exposure to radiation in the workplace.
The study covered the past 60 years and investigated the health records of 64, 937 employees at the nuclear facilities at Sellafield, Springfields, Capenhurst and Chapelcross - 42,426 of the employees were classified as radiation workers and their exposures were assessed from film-badge records.
Professor Steve Jones of Westlakes Scientific Consulting, who was commissioned to carry out the study by BNFL and the Nuclear Decommissioning Authority, said that the workers who received the highest radiation doses were employed prior to the 1980s when safety standards were not as high as they are now.
For these workers there was an increased risk of heart disease and other fatal disorders of the circulatory system that would have accounted for about two extra deaths per year in the group. The increased health risk to younger people starting in the nuclear industry now would result in a fractionally lower life expectancy, Professor Jones said.
"What we have shown is an association between relatively high levels of occupational exposure to radiation and mortality from circulatory system disease. However, we have not been able to take account of all the other possible causes of circulatory system disease," he said.
The scientists found that social class - whether someone was a blue or white collar worker - had a bigger effect on the risk of heart disease than radiation, but they could offer no biological explanation for why radiation exposure might lead to circulatory diseases in the first place.
"The possible biological mechanisms that might explain a link with radiation are tentative at best, and so the results of our analysis are not consistent with any simple causal interpretation," Professor Jones said.
"We also found an overall 'healthy worker' effect. That is, workers had lower mortality rates than the local general population, and the overall mortality of occupationally exposed workers was no different from workers who were not exposed at all," he said.
Studies of A-bomb survivors, who in one dose received between five and 10 times the maximum lifetime exposure of the nuclear workers, had already established a link between radiation and heart disease. This has also been found in studies of cancer patients exposed to high levels of radiotherapy.
Professor Dudley Goodhead, an expert on the health effects of radiation, said that there is emerging evidence of the links between long-term exposure to high levels of radiation and certain fatal diseases other than cancer.
"The findings of the present study clearly suggest that even chronic exposure to radiation, spread over long periods of time such as received by some radiation workers in the past, may also be able to cause increased heart disease," Professor Goodhead said.
"On its own this study cannot prove such a relationship, especially since no firm mechanism has yet been identified, but it is valuable addition to further research that is needed," he said.
The study is published on-line in the International Journal of Epidemiology.
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