Table 3.

Multivariate RRs with 95% CI of breast cancer according to various measures of smoking exposures among current and ever smokers smoking at least 20 years compared with never smokers (n = 12,743) and the corresponding figures restricted to nondrinkers, with 3,834 never smokers as reference group, Norwegian-Swedish Cohort Study, 1991-2000

Exposure measuresStudy population
Nondrinkers
Current smokers (n = 28,624)
Ever smokers 20+ y (n = 26,724)
Current smokers (n = 4,043)
Ever smokers 20+ y (n = 3,778)
Multivariate*Multivariate*Multivariate*Multivariate*
Cases (n)RR (95% CI)Cases (n)RR (95% CI)Cases (n)RR§ (95% CI)Cases (n)RR (95% CI)
Never smokers1371.00 (ref.)1371.00 (ref.)341.00 (ref.)341.00 (ref.)
Smoking initiation
Age (y)
    20+1081.02 (0.78-1.35)841.16 (0.86-1.57)161.14 (0.59-2.19)131.35 (0.67-2.71)
    15-192161.15 (0.91-1.47)2591.23 (0.97-1.55)261.30 (0.72-2.33)341.35 (0.78-2.32)
    10-14361.51 (1.00-2.28)491.48 (1.03-2.13)52.46 (0.90-6.68)72.20 (0.92-5.30)
P = 0.07P = 0.03P = 0.15P = 0.11
Before/around menarche
    No2821.10 (0.88-1.38)2941.20 (0.96-1.51)381.26 (0.75-2.12)411.37 (0.82-2.29)
    Yes721.30 (0.94-1.80)921.39 (1.03-1.87)71.52 (0.63-3.66)111.66 (0.79-3.49)
P = 0.13P = 0.03P = 0.28P = 0.13
Before first birth*
    No910.99 (0.72-1.37)780.98 (0.70-1.62)171.36 (0.66-2.79)120.97 (0.44-2.12)
    Yes2131.18 (0.92-1.52)2571.27 (1.00-1.62)261.46 (0.81-2.64)371.75 (1.02-2.99)
P = 0.14P = 0.03P = 0.20P = 0.03
No. of cigarettes per day
    1-91350.96 (0.74-1.25)1381.10 (0.85-1.43)231.33 (0.74-2.67)281.70 (1.03-3.12)
    10+2251.28 (1.01-1.63)2541.34 (1.06-1.70)241.25 (0.69-2.26)261.15 (0.65-2.05)
P = 0.03P = 0.01P = 0.60P = 0.89
No. of years smoked
    1-19680.93 (0.68-1.28)-101.07 (0.49-2.32)-
    20-24961.09 (0.81-1.45)1581.13 (0.88-1.45)100.96 (0.44-2.10)221.32 (0.73-2.38)
    25+1961.26 (0.98-1.63)2341.36 (1.06-1.74)271.63 (0.90-2.94)321.53 (0.87-2.68)
P = 0.05P = 0.02P = 0.15P = 0.13
No. of pack-years
    0-141620.95 (0.74-1.20)1531.01 (0.85-1.40)271.28 (0.73-2.26)291.61 (0.93-2.78)
    15-19901.28 (0.96-1.72)1141.35 (1.02-1.77)81.11 (0.50-2.49)121.26 (0.63-2.53)
    20+1081.48 (1.14-1.96)1251.46 (1.11-1.93)121.54 (0.72-3.29)131.22 (0.59-2.53)
P = 0.001††P < 0.02††P = 0.31††P = 0.57††
Latency (y)
    1-19480.75 (0.52-1.08)120.88 (0.47-1.66)101.33 (0.62-2.87)21.27 (0.29-5.49)
    20-241161.20 (0.91-1.58)1251.13 (0.86-1.47)100.70 (0.30-1.63)151.02 (0.52-2.02)
    25+1961.27 (0.98-1.64)2551.37 (1.07-1.76)271.67 (0.93-3.02)371.74 (1.01-3.02)
P = 0.02††P < 0.05††P = 0.41††P = 013††
  • * Adjusted for age at enrollment, menopausal status, number of children, age at first birth, hormonal contraceptive use, BMI (and alcohol consumption for the analysis among all women).

  • Based upon a model with 446 cases.

  • Based upon a model with 478 cases.

  • § Based upon a model with 74 cases.

  • Based upon a model with 82 cases.

  • Trend test between levels of smoking categories including never smokers.

  • ** Among parous women.

  • Trend test between levels of smoking categories excluding never smokers.