Sunlight may be associated with the development of certain autoimmune diseases, particularly in women, U.S. researchers said.

Researchers at the National Institute of Environmental Health Sciences, part of the National Institutes of Health, said women are more likely than men to develop many autoimmune diseases, but the reasons for this have not been clear.

"We only found the association between ultraviolet radiation exposure and dermatomyositis -- a connective-tissue disease characterized by inflammation of the muscles and the skin -- in women and not in men, and it could be that inherent differences in how women and men respond to UV radiation may play a role in the development of certain autoimmune diseases," Dr. Frederick W. Miller said in a statement.

"Although we have not shown a direct cause and effect link between UV exposure and this particular autoimmune disease, this study confirms the association between UV levels and the frequency of dermatomyositis that we found in a previous investigation."

Linda Birnbaum, director of the NIEHS, said while the causes of autoimmune diseases are not known, emerging research suggests they develop after one or more environmental exposures in genetically susceptible people.

The study is scheduled to be published in the August issue of Arthritis & Rheumatism.