Table 4.

Adjusted HRs and 95% CIs for incident breast cancer among women who reported spending <30 or 30 min to 2 h compared with >2 h outside in daylight hours in the summer, in the other seasons, and all year: the WHIOS

Average reported time spent outside
In summer
In other seasons
Year round*
Total at risk (breast cancer cases)63,622 (2, 316)
63,496 (2, 315)
46,926 (1, 753)
#cases/nHR (95% CI)#cases/nHR (95% CI)#cases/nHR (95% CI)
>2 h407/12,0351.0230/7,2371.0203/6,0291.0
30 min to 2 h1,095/31,5900.99 (0.89, 1.12)1,117/32,1681.05 (0.91, 1.22)840/23,9620.99 (0.85, 1.16)
<30 min814/19,9971.18 (1.05, 1.34)968/24,0911.23 (1.06, 1.42)710/16,9351.20 (1.02, 1.41)
  • NOTE: HRs adjusted for age, race/ethnicity, education, weight, family history, age at menarche, age at menopause, parity, age at first birth, hormone therapy duration of use, alcohol consumption, and physical activity.

  • * Reported average time spent outside in daylight hours per day year round is defined as those women who responded spending the same average amount of time outside in daylight hours per day in summer and in other seasons at year four follow-up.

  • The sample sizes are less than the starting sample of 71,662. Not all women answered questions regarding time spent outdoors at year 4 or women were lost to follow-up.

  • A test for trend was conducted for average time per day spent outside year round using ordinal variables assigned to <30, 30 min to 2 h, and >2 h.