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Table 3 Association between active smoking and breast cancer in 21,831 women in the Danish Nurse Cohort

From: Active smoking and risk of breast cancer in a Danish nurse cohort study

  N Person-years No. of cases Age adjusted Fullya adjusted
HR (95% CI) HR (95% CI)
Never smoked (ref.) 7923 126,950 379 1.00 1.00
 Former smoker 6557 102,190 338 1.11 (0.96–1.29) 1.08 (0.94–1.26)
 Current smoker 7351 113,400 445 1.32 (1.15–1.51) 1.27 (1.11–1.46)
 Ever smokers 13,908 215,589 783 1.22 (1.08–1.38) 1.18 (1.04–1.34)
Smoking duration among ever smokers
  ≤ 10 years 2925 47,335 156 1.10 (0.92–1.33) 1.10 (0.91–1.33)
 11–20 years 3175 50,151 175 1.17 (0.98–1,40) 1.15 (0.96–1.38)
 21–30 years 4456 68,818 259 1.26 (1.08–1.48) 1.24 (1.06–1.46)
  > 30 years 3295 49,285 193 1.32 (1.11–1.57) 1.21 (1.01–1.46)
p-value for trend      0.004
 Every increase of 10 years     1.08 (1.04–1.12) 1.06 (1.02–1-11)
Smoking (Tobacco) Intensity among ever smokers
  > 6 g/day 3098 49,245 161 1.10 (0.91–1.32) 1.07 (0.89–1.29)
 6–10 g/day 4317 67,410 235 1.17 (0.99–1.38) 1.14 (0.97–1.34)
 11–15 g/day 3050 46,962 175 1.25 (1.05–1.50) 1.22 (1.02–1.46)
  > 15 g/day 3443 51,973 212 1.37 (1.16–1.62) 1.31 (1.11–1.56)
p-value for trend      0.001
 Every increase of 10 g/day     1.05 (0.97–1.14) 1.04 (0.95–1.13)
Lifetime pack-years
  ≤ 10 pack-years 6082 96,640 327 1.13 (0.98–1.31) 1.12 (0.97–1.30)
 11–20 pack-years 3536 54,993 192 1.17 (0.99–1.39) 1.14 (0.96–1.36)
  > 20 pack-years 4290 63,956 264 1.39 (1.19–1.63) 1.32 (1.12–1.55)
p-value for trend      0.002
 Every increase of 20 pack-years     1.10 (1.01–1.20) 1.07 (0.98–1.18)
Initiation of smoking in 18,661 parous women
 Parous never smoker (ref.) 6938 111,811 326 1.00 1.00
 Parous, around first birth or later 1457 23,553 88 1.28 (1.01–1.62) 1.28 (1.00–1.62)
 Parous, before first childbirth, ≤ 10 pack-years 9367 146,965 493 1.15 (1.00–1.33) 1.13 (0.98–1.31)
 Parous, before first childbirth, > 10 pack-years 899 12,901 66 1.77 (1.36–2-31) 1.58 (1.20–2.10)
Initiation of smoking in 3170 nulliparous women
 Nulliparous never smoker (ref.) 985 15,138 53 1.00 1.00
 Nulliparous, before age 21b years 1264 18,737 74 1.14 (0.80–1.62) 1.10 (0.76–1.58)
 Nulliparous, age 21 years or later 921 13,434 62 1.31 (0.91–1.90) 1.29 (0.89–1.88)
Initiation of smoking in 12,376 postmenopausal women
 Postmenopausal never smoker (ref.) 3988 66,617 202 1.00 1.00
 Before menopause 8301 130,987 488 1.23 (1.05–1.45) 1.20 (1.01–1.41)
 After menopause 87 1252 6 1.60 (0.71–3.59) 1.66 (0.74–3.75)
Pack-years in 8347 parous postmenopausal womenc
 Parous postmenopausal never smoker (ref.) 2712   129 1.00 1.00
 Pack-years before 1st childbirthd
   ≤ 10 pack-years 4349 70,627 227 1.16 (0.93–1.44) 1.14 (0.92–1.42)
  11–20 pack-years 366 5233 34 2.36 (1.62–3.45) 2.16 (1.45–3.20)
   > 20 pack-years 61 854 4 1.69 (0.63–4.59) 1.38 (0.50–3.78)
   p-value for trend      0.006
 Pack-years from 1st childbirth until menopaused
   ≤ 10 pack-years 2572 42,484 128 1.08 (0.85–1.38) 1.03 (0.81–1.33)
  11–20 pack-years 1606 25,391 79 1.12 (0.85–1.49) 1.13 (0.85–1.50)
   > 20 pack-years 817 12,471 60 1.74 (1.28–2.36) 1.84 (1.34–2.53)
   p-value for trend      0.029
 Pack-years after menopaused
   ≤ 10 pack-years 2294 37,270 124 1.20 (0.94–1.53) 1.19 (0.93–1.53)
  11–20 pack-years 453 6695 20 1.09 (0.68–1.74) 1.03 (0.64–1.66)
   > 20 pack-years 1966 32,094 118 1.32 (1.03–1.70) 1.29 (0.70–4.98)
   p-value for trend      0.412
  1. HR hazard ratio, CI confidence interval aAdjusted for age, no of births, Body Mass Index, physical activity, alcohol use, oral contraceptive use, age at menarche, menopause, age at 1st birth, parity, number of birth, hormone therapy use, and night shift work. b21 is mean age at 1st childbirth in this cohort; c with information on age at cessation of menstrual bleeding; d every increase of 20 pack-years