Environmental factors may be behind a reported rise in cases of rheumatoid arthritis (RA) in women, according to a new study.

Mayo Clinic researchers have found that after a four-decade period of decline, the incidence of RA cases in women has risen.

Researchers followed 1,761 participants over the age of 18 who had received more than one diagnosis of arthritis, excluding degenerative arthritis or osteoarthritis, between 1995 and 2007.

Results show that RA incidence in women increased by 2.5 percent per year from 1995 to 2007, while a decrease of 0.5 percent was noted for men. Researchers did not find a disproportionate increase in RA incidence in any particular age group over the study period.

Study authors speculate environmental factors such as cigarette smoking, vitamin D deficiency, and lower dose synthetic estrogens in oral contraceptives may be the source of the increase.

RA is an auto-immune disease that leads to inflammation of the joints and surrounding tissues. It is estimated to affect as many as 2.1 million people.

Exercise and relaxation as well as hot or cold joint therapy are believed to help manage the symptoms of the disease. According to the Mayo Clinic, alternative treatments include thunder god vine, certain plant oils, fish oil and tai chi.