Table 5

HR of incident rheumatoid arthritis according to cumulative average UV-B exposure in NHS and NHS II stratified by age

 Cumulative average UV-B*
LowMediumHighp Trend
NHS
 <52 years old
  RA cases/person-years125/407 699109/438 47544/211 690
  Multivariable-adjusted HR† (95% CI)1.0 (ref)0.80 (0.62 to 1.04)0.67 (0.47 to 0.95)0.01
 ≥52 years old
  RA cases/person-years249/630 562231/682 490175/527 517
  Multivariable-adjusted HR† (95% CI)1.0 (ref)0.87 (0.72 to 1.04)0.85 (0.69 to 1.04)0.08
NHS II
 <52 years old
  RA cases/person-years74/532 44988/488 495139/768 973
  Multivariable-adjusted HR† (95% CI)1.0 (ref)1.17 (0.85 to 1.61)1.24 (0.93 to 1.65)0.15
 ≥52 years old
  RA cases/person-years27/67 68620/70 06333/114 608
  Multivariable-adjusted HR† (95% CI)1.0 (ref)0.67 (0.37 to 1.21)0.79 (0.47 to 1.33)0.43

  • *Low: ≤109 Robertson–Berger units×10−4 (R-B), medium: >109 to ≤117 R-B, high: >117 R-B.

  • †Adjusted for age, pack-years smoking, parity, breast feeding, physical activity, alcohol consumption, race, husband's education, census-tract median income, postmenopausal status and hormone use, mean number of rheumatologists per zip code.

  • UV-B, ultraviolet-B.

  • HR, hazard ratio; RA, rheumatoid arthritis; UV-B, ultraviolet-B.