Soaking up the sun may reduce a woman's risk of rheumatoid arthritis, a study has shown.

Over-exposure to the sun's ultraviolet (UV) rays can damage the skin and trigger skin cancer. But new research involving more than 200,000 women suggests sunshine can also cut the risk of rheumatoid arthritis (RA) by more than a fifth.

The findings are based on data from the Nurses' Health Study, a major US investigation looking at lifestyle factors affecting women's health.

Scientists looked at conditions where the women lived to work out their likely exposure to UV-B rays.

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The researchers used a sensitive measurement method known as UV-B flux which takes into account latitude, altitude and cloud cover.

It is expressed in R-B units which relate to the impact of exposure over time. A count of 440 R-B units in 30 minutes, for example, would produce slight redness in untanned white skin.

Study participants from US states with different climates had different R-B ratings, the research published in the journal Annals of the Rheumatic Diseases found.

Exposure ranged from an annual average of 93 R-B units in the northern states of Alaska and Oregon, to 196 in Hawaii and Arizona.

During the study period, which lasted up to three decades, a total of 1,314 women developed RA, an auto-immune condition that attacks the joints.

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Those with the highest levels of exposure were 21% less likely to develop the disease than those with the lowest.

However, the association was only seen in older women enrolled at the start of the study in the 1970s. This might be because in their youth they were less likely to protect themselves from the sun than future generations, the researchers believe.

Previous studies have shown that people living in northern latitudes, where the sun is less intense, have a higher risk of RA, as well as other auto-immune conditions including Type 1 diabetes, inflammatory bowel disease and multiple sclerosis.

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The scientists, led by Dr Elizabeth Arkema, from the Harvard School of Public Health in Boston, wrote: "Our study adds to the growing evidence that exposure to UV-B light is associated with decreased risk of rheumatoid arthritis. The mechanisms are not yet understood, but could be mediated by the cutaneous (skin) production of vitamin D and attenuated by use of sunscreen or sun-avoidant behaviour."

It is unclear whether RA risk is affected by sun exposure at birth, in adolescence or throughout adulthood, said the researchers.

Many study participants did not move home during their childhood and were still living in the same place they were born in at the age of 15.

The scientists acknowledged a lack of individual data on time spent outdoors, travel to sunny destinations, and sunscreen use. But they pointed out that UV-B flux had been shown to reflect the risk of non-melanoma skin cancer among the study participants. It could therefore be considered a "reasonable proxy" for sun exposure.

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