title: Exposure to ultraviolet-B and risk of developing rheumatoid arthritis among women in the Nurses' Health Study.
abstract: 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.
authors: Correspondence to Dr Elizabeth V Arkema, Department of Epidemiology, Harvard School of Public Health, 677 Huntington Avenue, 9th Floor, Boston, MA 02115, USA;earkema@post.harvard.edu.
authors: Elizabeth V
authors: EV
authors: Arkema
authors: Jaime E
authors: JE
authors: Hart
authors: Kimberly A
authors: KA
authors: Bertrand
authors: Francine
authors: F
authors: Laden
authors: Francine
authors: F
authors: Grodstein
authors: Bernard A
authors: BA
authors: Rosner
authors: Elizabeth W
authors: EW
authors: Karlson
authors: Karen H
authors: KH
authors: Costenbader
mesh_terms: D000328:Adult; D001172:Arthritis, Rheumatoid / Q000453:epidemiology*; D015331:Cohort Studies; D004307:Dose-Response Relationship, Radiation; D004781:Environmental Exposure / Q000009:adverse effects* / Q000706:statistics & numerical data*; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D015994:Incidence; D008297:Male; D008875:Middle Aged; D015999:Multivariate Analysis; D009726:Nurses / Q000706:statistics & numerical data; D015995:Prevalence; D016016:Proportional Hazards Models; D011446:Prospective Studies; D012307:Risk Factors; D040242:Risk Reduction Behavior; D014466:Ultraviolet Rays / Q000009:adverse effects*; D014481:United States / Q000453:epidemiology; D014887:Weather
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