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. 2011 Aug 29;174(8):919–928. doi: 10.1093/aje/kwr192

Table 2.

Hazard Ratios and 95% Confidence Intervals for Invasive Breast Cancer Incidence Associated With Smoking Status Among 76,628 Postmenopausal Women by Obesity Status, the Women's Health Initiative Observational Study, United States, 1993–2009a

Exposure Nonobese
 Obese
 Pinteraction
No. of Cases Multi-adjusted HRb 95% CI No. of Cases Multi-adjusted HRb 95% CI
Smoking history
    Never smokers 1,210 1.00 Referent 458 1.00 Referent
    Ever smokers 1,296 1.16 1.07, 1.26 414 0.96 0.84, 1.10 0.01
    Smoking status 0.047
        Former smokers 1,133 1.15 1.05, 1.25 374 0.96 0.83, 1.11
        Current smokers 163 1.25 1.05, 1.47 40 0.96 0.69, 1.34
Age at smoking initiation, years 0.10
    <20 747 1.19 1.08, 1.31 245 1.00 0.85, 1.18
    20–24 422 1.15 1.03, 1.29 119 0.92 0.75, 1.14
    ≥25 127 1.03 0.86, 1.24 50 0.88 0.66, 1.18
            Ptrend 0.0002 0.73
Average no. of cigarettes/day 0.03
    <15 708 1.13 1.03, 1.25 212 0.97 0.82, 1.15
    ≥15 588 1.20 1.08, 1.33 202 0.95 0.80, 1.13
            Ptrend 0.0003 0.54
Total no. of smoking years 0.03
    <10 288 1.02 0.90, 1.16 96 0.96 0.77, 1.20
    10–29 588 1.16 1.05, 1.28 174 0.91 0.76, 1.09
    30–49 370 1.25 1.11, 1.41 138 1.07 0.88, 1.30
    ≥50 50 1.62 1.22, 2.17 6 0.62 0.28, 1.40
            Ptrend <0.0001 0.76
No. of smoking pack-years 0.005
    <10 517 1.05 0.95, 1.17 170 1.01 0.84, 1.21
    10–<30 463 1.31 1.17, 1.46 112 0.86 0.69, 1.06
    30–<50 174 1.14 0.97, 1.34 60 0.87 0.66, 1.15
    ≥50 142 1.20 1.00, 1.43 72 1.15 0.89, 1.48
            Ptrend 0.0001 0.84
Years since quit smoking (former smokers) 0.6
    <10 201 1.25 1.08, 1.46 77 0.94 0.73, 1.20
    10–<20 284 1.18 1.03, 1.35 108 1.02 0.82, 1.26
    20–<30 278 1.11 0.97, 1.27 83 0.94 0.74, 1.19
    ≥30 303 1.14 1.00, 1.29 86 0.95 0.75, 1.20
            Ptrend 0.0005 0.69

Abbreviations: CI, confidence interval; HR, hazard ratio.

a

“Obese” here was defined as body mass index ≥30.

b

The adjusted variables in all multi-adjusted models included age (<55, 55–59, 60–64, 65–69, 70–74, ≥75 years), race (American Indian or Alaska Native, Asian or Pacific Islander, black or African American, Hispanic/Latino, non-Hispanic white, and other), education (high school or less, some college/technical training, college or some postcollege, and master's degree or higher), family history of cancer (yes/no), age at menarche (<12, 12–13, 14–15, ≥16 years), age at first livebirth (never had term pregnancy, <20, 20–29, ≥30 years), hormone use (no, estrogen alone, estrogen and progestin, mixed), parity (never pregnant, never had term pregnancy, 1, 2, 3, 4, ≥5), alcohol intake (nondrinker, past drinker, <1 drink/month, 1 drink/month–<1 drink/week, 1–<7 drinks/week, ≥7 drinks/week), body mass index (<18.5, 18.5–24.9, 25.0–29.9, 30.0–34.9, 35.0–39.9, ≥40), physical activity (metabolic equivalent tasks/week: <5, 5–<10, 10–<20, 20–<30, ≥30), and mammography during the last 2 years (yes/no). The Ptrend test included the reference group.