Table 4.
Multivariate adjusted hazard ratios (HR) and 95 % CIs for breast cancer according to various measures of smoking exposures at enrollment, with never smokers as the reference group, for 302,865 Norwegian women (1974–2003)
| Subjects | Cases | Multivariate adjusted HR (95 % CI)a | |
|---|---|---|---|
| Smoking exposures | |||
| Smoking status | |||
| Never | 123,503 | 3,028 | Ref. |
| Former | 64,021 | 1,581 | 1.17 (1.10–1.24) |
| Current | 115,341 | 2,881 | 1.14 (1.08–1.20) |
| Sum | 302,865 | 7,490 | |
| Ever | 179,362 | 4,462 | 1.15 (1.10–1.21) |
| Ever smokers | |||
| Age at smoking initiation for nulliparous women (years) | |||
| ≥25 | 2,788 | 91 | 1.01 (0.80–1.26) |
| 20–24 | 5,001 | 140 | 1.03 (0.85–1.25) |
| 15–19 | 8,087 | 153 | 1.01 (0.83–1.22) |
| <15 | 960 | 16 | 1.33 (0.81–2.21) |
| Sum | 16,836 | 400 | |
| p trend* | <0.001 | ||
| Age at smoking initiation for parous women (years) | |||
| ≥25 | 20,362 | 540 | 0.99 (0.90–1.09) |
| 20–24 | 35,822 | 987 | 1.20 (1.11–1.29) |
| 15–19 | 65,131 | 1,348 | 1.33 (1.24–1.42) |
| <15 | 6,442 | 88 | 1.30 (1.05–1.61) |
| Sum | 127,757 | 2,963 | |
| p trend* | <0.001 | ||
| Smoking duration (years)b | |||
| <11 | 48,446 | 1,167 | 1.08 (1.02−1.15) |
| 11–20 | 70,384 | 1,822 | 1.12 (1.05−1.35) |
| 21–30 | 53,420 | 1,313 | 1.26 (1.18−1.35) |
| ≥31 | 5,379 | 131 | 1.31 (1.09−1.57) |
| Sum | 177,629 | 4,433 | |
| p trend* | <0.001 | ||
| Number of cigarettes smoked per day | |||
| ≤5 | 32,591 | 786 | 1.08 (0.99–1.17) |
| 6–10 | 77,405 | 1,844 | 1.09 (1.03–1.16) |
| 11–15 | 41,266 | 1,087 | 1.26 (1.17–1.35) |
| ≥16 | 26,567 | 725 | 1.30 (1.20–1.41) |
| Sum | 177,829 | 4,442 | |
| p trend* | <0.001 | ||
| Number of pack-yearsc | |||
| ≤5 | 56,837 | 1,360 | 1.07 (1.00–1.14) |
| 6–10 | 46,041 | 1,150 | 1.11 (1.04–1.19) |
| 11–15 | 35,149 | 889 | 1.18 (1.09–1.27) |
| ≥16 | 35,152 | 942 | 1.34 (1.25–1.45) |
| Sum | 173,179 | 4,341 | |
| p trend* | <0.001 | ||
| Smoking initiation in relation to first childbirth for parous women (years) | |||
| After first childbirth (>1 year) | 23,261 | 589 | 0.93 (0.86–1.02) |
| Around childbirthd | 18,574 | 482 | 1.09 (0.99–1.20) |
| 1–3 | 21,29 | 524 | 1.21 (1.10–1.33) |
| 4–6 | 28,367 | 588 | 1.14 (1.03–1.24) |
| 7–10 | 21,256 | 450 | 1.34 (1.21–1.48) |
| ≥11 | 14,970 | 330 | 1.60 (1.42–1.80) |
| Sum | 127,757 | 2,963 | |
| p trend** | <0.001 | ||
* Trend test between four levels of smoking categories excluding never smokers
** Trend test between five levels of smoking categories excluding never smokers and those smoking >1 year after first childbirth
aAdjusted for age, education level, number of children, age at first childbirth, BMI, age at enrollment and physical activity when applicable
bTotal number of years smoked
cPack years: Number of cigarettes smoked per day multiplied by number of years smoked, divided by 20. One pack has 20 cigarettes
d1 year before to 1 year after first childbirth