Table 3.
Cases of incident invasive breast cancer and related hazard ratios associated with smoking status among 79 990 postmenopausal women
| Smoking status | No of cases | Hazard ratio (95% Cl) | |
|---|---|---|---|
| Multivariable adjusted* | Multivariable adjusted† | ||
| Never smoked: | 1692 | 1.00 (reference) | — |
| With no exposure to passive smoking | 145 | — | 1.00 (reference) |
| With exposure to passive smoking | 1515 | — | 1.08 (0.91 to 1.28) |
| Former smokers | 1619 | 1.09 (1.02 to 1.17) | 1.16 (0.98 to 1.38) |
| Current smokers | 209 | 1.16 (1.00 to 1.34) | 1.24 (1.00 to 1.54) |
| Age at starting smoking (years): | |||
| <15 | 112 | 1.12 (0.92 to 1.36) | 1.19 (0.93 to 1.53) |
| 15–19 | 940 | 1.13 (1.04 to 1.23) | 1.21 (1.01 to 1.44) |
| 20–24 | 581 | 1.09 (0.99 to 1.20) | 1.16 (0.97 to 1.40) |
| 25–29 | 110 | 1.02 (0.84 to 1.24) | 1.09 (0.85 to 1.40) |
| ≥30 | 80 | 0.94 (0.75 to 1.18) | 1.00 (0.76 to 1.32) |
| P value for trend | 0.002 | 0.04 | |
| Smoking started before first term pregnancy‡: | |||
| Yes | 833 | 1.21 (1.11 to 1.33) | 1.28 (1.06 to 1.55) |
| No | 116 | 1.12 (0.92 to 1.36) | 1.17 (0.90 to 1.52) |
| Uncertain | 618 | 1.07 (0.97 to 1.18) | 1.13 (0.93 to 1.37) |
| Average No of cigarettes smoked/day: | |||
| <5 | 383 | 1.05 (0.94 to 1.17) | 1.12 (0.92 to 1.35) |
| 5–14 | 560 | 1.11 (1.01 to 1.23) | 1.19 (0.99 to 1.43) |
| 15–24 | 503 | 1.14 (1.03 to 1.27) | 1.22 (1.01 to 1.47) |
| ≥25 | 301 | 1.08 (0.95 to 1.22) | 1.15 (0.94 to 1.41) |
| P value for trend | 0.01 | 0.16 | |
| Total No of smoking years: | |||
| <5 | 220 | 0.97 (0.84 to 1.11) | 1.03 (0.86 to 1.28) |
| 5–9 | 182 | 1.05 (0.90 to 1.22) | 1.12 (0.90 to 1.40) |
| 10–19 | 401 | 1.09 (0.98 to 1.22) | 1.17 (0.96 to 1.42) |
| 20–29 | 387 | 1.09 (0.97 to 1.22) | 1.16 (0.96 to 1.41) |
| 30–39 | 341 | 1.21 (1.07 to 1.36) | 1.29 (1.06 to 1.58) |
| 40–49 | 178 | 1.14 (0.98 to 1.34) | 1.22 (0.98 to 1.53) |
| ≥50 | 56 | 1.35 (1.03 to 1.77) | 1.45 (1.06 to 1.98) |
| P value for trend | 0.0002 | 0.002 | |
| No of smoking pack years: | |||
| <10 | 702 | 1.04 (0.95 to 1.14) | 1.11 (0.93 to 1.33) |
| 10–<20 | 359 | 1.21 (1.08 to 1.36) | 1.29 (1.06 to 1.58) |
| 20–<30 | 225 | 1.13 (0.98 to 1.30) | 1.21 (0.98 to 1.50) |
| 30–<40 | 168 | 1.01 (0.86 to 1.19) | 1.08 (0.86 to 1.36) |
| 40–<50 | 69 | 1.16 (0.91 to 1.47) | 1.24 (0.92 to 1.65) |
| ≥50 | 218 | 1.18 (1.02 to 1.37) | 1.26 (1.02 to 1.57) |
| P value for trend | 0.005 | 0.08 | |
| Years since quitting smoking (former smokers): | |||
| <10 | 295 | 1.15 (1.01 to 1.31) | 1.19 (0.96 to 1.47) |
| 10–<20 | 416 | 1.14 (1.02 to 1.27) | 1.17 (0.96 to 1.43) |
| 20–<30 | 391 | 1.08 (0.96 to 1.21) | 1.11 (0.91 to 1.36) |
| ≥30 | 414 | 1.06 (0.95 to 1.18) | 1.08 (0.89 to 1.32) |
| P value for trend | 0.005 | 0.08 | |
*Reference category was lifetime non-smokers.
†Reference category was lifetime non-smokers with no history of exposure to passive smoking.
‡Among 69 533 women who had at least one full term pregnancy, 1429 cases were among never smokers and 125 cases in no exposure group.