Table III.
Hazard ratios [HR] and 95% CI for endometriosis risk by sunburns, recreational sun exposure and sunscreen use, Nurses’ Health Study II cohort questionnaire cycles 1989–2015 (n = 95 080).
| Cases | Person-years | Age-adjusted HR 1 (95% CI) | Multivariable-adjusted HR 2 (95% CI) | |
|---|---|---|---|---|
| Number of sunburns at ages 15–20 | ||||
| Never | 1565 | 420 397 | 1.00 (Reference) | 1.00 (Reference) |
| 1–2 | 1880 | 501 375 | 1.00 (0.93–1.07) | 1.00 (0.93–1.07) |
| 3–4 | 835 | 209 511 | 1.05 (0.97–1.15) | 1.05 (0.96–1.14) |
| ≥5 | 498 | 116 777 | 1.13 (1.02–1.25) | 1.12 (1.01–1.24) |
| P trend | 0.02 | 0.03 | ||
| Times per week spent outdoors in a swimsuit as a teenager | ||||
| <1 per week | 471 | 117 197 | 1.00 (Reference) | 1.00 (Reference) |
| 1 per week | 342 | 96 975 | 0.87 (0.75–0.99) | 0.91 (0.79–1.04) |
| 2 per week | 660 | 170 688 | 0.94 (0.83–1.06) | 1.01 (0.89–1.14) |
| Several times per week | 1972 | 508 644 | 0.93 (0.84–1.03) | 1.02 (0.92–1.13) |
| Daily | 702 | 175 342 | 0.95 (0.84–1.07) | 1.05 (0.93–1.18) |
| P trend | 0.73 | 0.12 | ||
| Times per week spent outdoors in a swimsuit in the past summer | ||||
| <1 per week | 2040 | 499 168 | 1.00 (Reference) | 1.00 (Reference) |
| 1 per week | 609 | 151 936 | 0.97 (0.88–1.06) | 1.05 (0.96–1.15) |
| 2 per week | 700 | 187 426 | 0.89 (0.82–0.98) | 1.00 (0.92–1.09) |
| Several times per week | 682 | 201 933 | 0.81 (0.74–0.88) | 0.96 (0.88–1.05) |
| Daily | 70 | 20 532 | 0.81 (0.64–1.03) | 0.99 (0.78–1.26) |
| P trend | <0.0001 | 0.47 | ||
| Percentage of time using sunscreen as a teenager | ||||
| Not in sun | 61 | 16 457 | 0.99 (0.77–1.28) | 0.94 (0.72–1.21) |
| 0% | 2317 | 618 073 | 1.00 (Reference) | 1.00 (Reference) |
| 25% | 1040 | 263 151 | 1.02 (0.94–1.10) | 1.02 (0.94–1.10) |
| 50% | 459 | 117 102 | 1.00 (0.91–1.11) | 0.96 (0.87–1.06) |
| 75% | 205 | 47 202 | 1.12 (0.97–1.29) | 1.03 (0.89–1.19) |
| 100% | 70 | 15 045 | 1.21 (0.95–1.53) | 0.99 (0.79–1.26) |
| P trend | 0.11 | 0.96 | ||
| Percentage of time using sunscreen in the past summer (<1993) | ||||
| Not in sun | 242 | 65 422 | 1.04 (0.89–1.23) | 0.88 (0.74–1.04) |
| 0% | 353 | 98 474 | 1.00 (Reference) | 1.00 (Reference) |
| 25% | 493 | 138 443 | 0.98 (0.85–1.12) | 0.99 (0.86–1.13) |
| 50% | 589 | 160 115 | 1.02 (0.89–1.16) | 1.04 (0.91–1.19) |
| 75% | 1088 | 292 021 | 1.03 (0.91–1.16) | 1.02 (0.90–1.15) |
| 100% | 1367 | 319 629 | 1.19 (1.06–1.34) | 1.10 (0.97–1.24) |
| P trend | 0.0004 | 0.002 |
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), 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), and smoking status (never, past or current smoking), childhood skin’s reaction to sun exposure (except for number of sunburns at ages 15–20), number of moles on leg, natural hair colour, annual income (<75k, 75 to <100k, ≥100k), self-reported social standing in community (high, medium, low), predicted vitamin D score and cumulative average UV.