Table 1.
Risk of skin cancer by rotating night shift work among 68 336 women in the Nurses’ Health Study*
| Years on rotating night shift | Person-years | No. of case subjects | Age-adjusted HR (95% CI) | Multivariable model 1†, HR (95% CI) | Multivariable model 2†, HR (95% CI) |
| Skin cancer | |||||
| Never | 413 727 | 4426 | 1.0 (referent) | 1.0 (referent) | 1.0 (referent) |
| 1–2 y | 246 014 | 2653 | 1.02 (0.97 to 1.07) | 1.01 (0.96 to 1.05) | 1.02 (0.97 to 1.07) |
| 3–5 y | 167 774 | 1875 | 0.99 (0.94 to 1.05) | 0.97 (0.92 to 1.02) | 1.01 (0.96 to 1.07) |
| 6–9 y | 68 474 | 698 | 0.91 (0.84 to 0.99) | 0.89 (0.82 to 0.96) | 0.88 (0.81 to 0.96) |
| ≥10 y | 117 507 | 1147 | 0.84 (0.78 to 0.89) | 0.82 (0.77 to 0.87) | 0.86 (0.81 to 0.92) |
| Ptrend‡ | <.001 | <.001 | <.001 | ||
| Melanoma | |||||
| Never | 416 272 | 147 | 1.0 (referent) | 1.0 (referent) | 1.0 (referent) |
| 1–2 y | 247 545 | 79 | 0.91 (0.69 to 1.19) | 0.89 (0.68 to 1.18) | 0.89 (0.68 to 1.18) |
| 3–5 y | 168 816 | 50 | 0.83 (0.60 to 1.14) | 0.80 (0.58 to 1.10) | 0.83 (0.60 to 1.15) |
| 6–9 y | 68 858 | 18 | 0.72 (0.44 to 1.18) | 0.68 (0.42 to 1.11) | 0.65 (0.40 to 1.06) |
| ≥10 y | 118 144 | 24 | 0.57 (0.37 to 0.87) | 0.54 (0.35 to 0.83) | 0.56 (0.36 to 0.87) |
| Ptrend‡ | .007 | .003 | .005 | ||
| SCC | |||||
| Never | 416 088 | 363 | 1.0 (referent) | 1.0 (referent) | 1.0 (referent) |
| 1–2 y | 247 434 | 207 | 0.97 (0.82 to 1.15) | 0.96 (0.81 to 1.13) | 0.96 (0.81 to 1.14) |
| 3–5 y | 168 744 | 137 | 0.88 (0.72 to 1.07) | 0.85 (0.70 to 1.04) | 0.89 (0.73 to 1.08) |
| 6–9 y | 68 825 | 55 | 0.87 (0.65 to 1.15) | 0.84 (0.63 to 1.12) | 0.84 (0.63 to 1.11) |
| ≥10 y | 118 099 | 87 | 0.76 (0.60 to 0.96) | 0.74 (0.59 to 0.94) | 0.79 (0.63 to 1.00) |
| Ptrend‡ | .02 | .01 | .04 | ||
| BCC | |||||
| Never | 414 194 | 3916 | 1.0 (referent) | 1.0 (referent) | 1.0 (referent) |
| 1–2 y | 246 272 | 2367 | 1.03 (0.98 to 1.08) | 1.01 (0.96 to 1.07) | 1.03 (0.98 to 1.08) |
| 3–5 y | 167 942 | 1688 | 1.01 (0.95 to 1.07) | 0.98 (0.93 to 1.04) | 1.03 (0.97 to 1.09) |
| 6–9 y | 68 537 | 625 | 0.93 (0.85 to 1.01) | 0.90 (0.82 to 0.98) | 0.90 (0.82 to 0.97) |
| ≥10 y | 117 600 | 1036 | 0.86 (0.80 to 0.92) | 0.84 (0.78 to 0.90) | 0.88 (0.82 to 0.94) |
| Ptrend‡ | <.001 | <.001 | .001 | ||
103 613 of the women returned the 1988 questionnaire, which included the question about night work. The population for this study consisted of the 85 197 (82.2%) of the respondents who answered the question on night work. Women who did not answer the shift work question on the 1988 questionnaire did not substantially differ from respondents in terms of their risk profile. A total of 10 799 incident skin cancer case subjects, comprising 9632 BCC, 849 SCC, and 318 melanoma case subjects, were diagnosed in the base population between June 1, 1988, and May 31, 2006. Women who reported having melanoma, SCC, BCC, or any other cancer before 1988 were excluded. The analysis was restricted to non-Hispanic white women because the number of case subjects in the other racial/ethnic categories was small. A total of 68 336 women remained to form the baseline population for this analysis, and 1 013 497 person-years of follow-up were accrued from 1988 to 2006. Skin cancers were defined as having occurred during the period between June 1, 1988, and May 31, 2006. Nurses who reported the occurrence of SCC and melanoma were asked for permission to review their medical records, and skin cancer was confirmed through review of these records. Medical records were not obtained for self-reported BCC. A validation study demonstrated that >90% of self-reported BCC among nurses were confirmed by histopathological findings (24,25). Reviews of medical records were conducted by investigators without knowledge of exposure. BCC = basal cell carcinoma; CI = confidence interval; HR = hazard ratio; SCC = squamous cell carcinoma.
Multivariable model 1: relative risk adjusted for age in 1 year increments, and phenotypic and established risk factors of skin cancer including hair color at 20 years of age (red, blond, light brown, dark brown, and black), family history of skin cancer (yes, no), child and adolescence tendency to burns (some redness, burn, and painful burn/blisters), number of palpable moles on arms and legs (1–2, 3–5, 6–9, and ≥10), and lifetime severe sunburns that blistered (1–2, 3–5, 6–9, and ≥10); to control as finely as possible for confounding by age, calendar time, and any possible two-way interactions between these two time scales, we stratified analyses jointly by age in months at start of follow-up and calendar year of the current questionnaire cycle. Multivariable model 2: additional adjustment for residence ultraviolet exposure level (low, middle, and high) (26) at birth, and at 15 and 30 years of age, average hours of sun exposure per week (2–5, 6–10, and ≥11 h/wk) at 25–35, 36–59, and ≥60 years of age. Complete-subject analyses were used to handle missing values. In 1988, the study participants were asked how many years in total they had worked rotating night shifts, which was defined as at least three nights per month in addition to days or evenings in that month. Data on lifetime years worked on rotating nightshift until 1988 was gathered in eight prespecified categories: never, 1–2, 3–5, 6–9, 10–14, 15–19, 20–29, and ≥30 years.
P value for continuous linear term by two-sided Wald test. Tests of trends across categories of exposure were calculated by treating the levels of exposure as a continuous variable by assigning each category a midpoint and reporting the Wald statistics for the covariate.