Exposure to ultraviolet-B and risk of developing rheumatoid arthritis among women in the Nurses' Health Study.

Objective: To examine the association between ultraviolet-B (UV-B) light exposure and rheumatoid arthritis (RA) risk among women in two large prospective cohort studies, the Nurses' Health Study (NHS) and the Nurses' Health Study II (NHSII).

Methods: A total of 106 368 women from NHS, aged 30-55 years in 1976, and 115 561 women from NHSII, aged 25-42 in 1989, were included in the analysis. We identified women with incident RA from the start of each cohort until 2008 (NHS) and 2009 (NHSII). Cumulative average UV-B flux, a composite measure of ambient UV exposure based on latitude, altitude and cloud cover, was estimated according to state of residence and categorised as low, medium or high. Estimates of UV-B at birth and age 15 years were also examined. We used multivariable-adjusted Cox proportional hazards models to estimate HR and 95% CI.

Results: 1314 incident RA cases were identified in total. Among NHS participants, higher cumulative average UV-B exposure was associated with decreased RA risk; those in the highest versus lowest category had a 21% decreased RA risk (HR (95% CI); 0.79 (0.66 to 0.94)). UV-B was not associated with RA risk among younger women in NHSII (1.12 (0.87 to 1.44)). Results were similar for UV-B at birth and at age 15.

Conclusions: These results suggest that ambient UV-B exposure is associated with a lower RA risk in NHS, but not NHSII. Differences in sun-protective behaviours (eg, greater use of sun block in younger generations) may explain the disparate results.