A new study indicates that regular exposure to sunlight - specifically ultraviolet B (UVB) - may reduce the risk of developing rheumatoid arthritis.

But this effect was only evident among older women - perhaps because younger women, being more aware of the hazards of sunlight, tend to cover themselves up with sun block.

The study authors base examined the effects of UVB on participants in two phases of the US Nurses' Health Study (NHS). In the first phase they tracked the health of more than 120,000 nurses between the ages of 30 and 55, while in the second (NHSII) they tracked the health of a further 115,500 nurses between the ages of 25 and 42.

The researchers used a sensitive assessment known as UV-B flux - which is an estimate of UVB radiation based on latitude, altitude and cloud cover - and is measured in R-B units. Exposure was seen to range from an annual average of 93 in Alaska and Oregon to 196 in Hawaii and Arizona.

1314 women developed rheumatoid arthritis over the study period. Among nurses enlisted in the first phase, higher exposure to UVB meant a reduced risk of developing the disease. Significantly, those with the highest levels of exposure were 21% less likely to develop rheumatoid arthritis than those with the least levels of exposure.

Previous studies have already shown a link between geography and the risk of rheumatoid arthritis along with other autoimmune conditions - including type 1 diabetes, inflammatory bowel disease and multiple sclerosis.

No such association for UVB exposure was found among women in the second phase. However, these women were younger and may have been more careful about acquiring a tan.

The study does not reveal at what stage of life UVB’s protective effect happens, but adds to the growing evidence that exposure to UVB light means a lower risk of developing rheumatoid arthritis.