Table II.
Hazard ratios [HR] and 95% CI for endometriosis risk by frequency of tanning bed use, Nurses’ Health Study II cohort questionnaire cycles 1989–2015 (n = 95 080).
| Tanning bed use | Cases | Person-years | Age-adjusted HR 1 (95% CI) | Multivariable-adjusted HR 2 (95% CI) |
|---|---|---|---|---|
| During high school (HS)/college | ||||
| None | 3257 | 876 553 | 1.00 (Reference) | 1.00 (Reference) |
| 1–2 times/year | 171 | 44 890 | 0.98 (0.84–1.15) | 0.96 (0.82–1.12) |
| 3–5 times/year | 104 | 22 708 | 1.19 (0.98–1.45) | 1.13 (0.93–1.38) |
| ≥6 times/year | 156 | 31 547 | 1.26 (1.07–1.48) | 1.19 (1.01–1.40) |
| P trend | 0.003 | 0.04 | ||
| At ages 25–35 years | ||||
| None | 2750 | 773 472 | 1.00 (Reference) | 1.00 (Reference) |
| 1–2 times/year | 336 | 82 201 | 1.11 (0.99–1.24) | 1.02 (0.91–1.14) |
| 3–5 times/year | 205 | 43 101 | 1.26 (1.09–1.45) | 1.13 (0.98–1.31) |
| ≥6 times/year | 397 | 77 122 | 1.37 (1.23–1.52) | 1.24 (1.12–1.39) |
| P trend | <0.0001 | <0.0001 | ||
| Combined HS/college and at ages 25–35 | ||||
| None | 2559 | 722 280 | 1.00 (Reference) | 1.00 (Reference) |
| <2 times a year ever | 405 | 104 351 | 1.06 (0.95–1.17) | 0.99 (0.89–1.10) |
| ≥3 times a year in HS/college only | 118 | 27 935 | 1.17 (0.97–1.41) | 1.10 (0.91–1.32) |
| ≥3 times a year at ages 25–35 years only | 454 | 93 048 | 1.30 (1.18–1.44) | 1.17 (1.06–1.30) |
| ≥3 times a year both in HS/college and at 25–35 years | 142 | 26 093 | 1.43 (1.20–1.70) | 1.30 (1.09–1.54) |
| P trend | <0.0001 | 0.0001 |
Adjusted for current age (continuous months) and calendar time (2-year questionnaire period).
Additionally adjusted for BMI at age 18 (<18.5, 18.5–22.4, 22.5–24.9 or ≥25 kg/m2), age at menarche (≤11, 12–13 or ≥14 years), menstrual cycle length (≤25, 26–31, 32–39 or ≥40 days), current menstrual cycle pattern (regular, usually irregular, always irregular, no menses), parity (nulliparous, or 1, 2, 3 or ≥4 pregnancies lasting ≥6 months), oral contraceptive use (never, past or current use), smoking status (never, past or current smoking), childhood skin’s reaction to sun exposure, number of moles on leg, natural hair colour (black, dark brown, light brown, blonde, red), annual income (<75k, 75 to <100k, ≥100k), self-reported social standing in community (high, medium, low), predicted vitamin D score and cumulative average UV.