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Table 1 Description of the Danish Nurse Cohort (n = 21,831) at the time of recruitment in 1993 or 1999 and by breast cancer status during follow-up until 2013

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

  Total N = 21,831 Breast Cancer N = 1162 No Breast Cancer N = 20,669
Age
 Mean (SD) age at baseline (years) 53.2 (8.0) 53.7 (7.6) 53.2 (8.1)
Birth Cohort
 Born 1900–1934, n (%) 5179 (23.7) 295 (25.4) 4884 (23.6)
 Born 1935–1944, n (%) 6707 (30.7) 428 (36.8) 6279 (30.4)
 Born 1945–1949, n (%) 4564 (20.9) 244 (21.0) 4320 (20.9)
 Born 1950–1955, n (%) 5381 (24.6) 195 (16.8) 5186 (25.1)
Reproductive Factors
 Mean (SD) age at menarche 13.5 (1.5) 13.5 (1.5) 13.5 (1.5)
 Nulliparous, n (%) 3170 (14.5) 189 (16.3) 2981 (14.4)
 Mean (SD) number of children in parous women 2.3 (0.9) 2.3 (0.9) 2.3 (0.9)
 Mean (SD) age at first birth in parous women 25.9 (4.0) 25.9 (4.0) 26.3 (4.1)
 Postmenopausal, n (%) 12,376 (56.7) 696 (59.9) 11,680 (56.5)
Body Mass Index (BMI)
 Mean (SD) BMI (kg/m2) 23.7 (3.5) 23.8 (3.5) 23.7 (3.5)
 BMI < 18.5 kg/m2, n (%) 529 (2.4) 22 (1.9) 507 (2.5)
 BMI 18.5–24.9 kg/m2, n (%) 15,070 (69.0) 819 (70.5) 14,251 (68.9)
 BMI 25–29.9 kg/m2, n (%) 4979 (22.8) 245 (21.1) 4734 (22.9)
 BMI ≥ 30 kg/m2, n (%) 1253 (5.7) 76 (6.5) 1177 (5.7)
Alcohol consumption
 Does not drink alcohol, n (%) 3320 (15.2) 183 (15.7) 3137 (15.2)
 Moderate drinker (1–14 drinks/week), n (%) 13,533 (62.0) 670 (57.7) 12,863 (62.2)
 Heavy drinker (> 14 drinks/week), n (%) 4978 (22.8) 309 (26.6) 4669 (22.6)
Leisure time physical activity
 Low physical activity, n (%) 1428 (6.5) 79 (6.8) 1349 (6.5)
 Medium physical activity, n (%) 5905 (27.0) 298 (25.6) 5607 (27.1)
 High physical activity, n (%) 14,498 (66.4) 785 (67.6) 13,713 (66.3)
Shift Work
 Working night shifts, n (%) 967 (5.5) 45 (4.9) 922 (5.5)
Hormone Use
 Ever used oral contraceptives, n (%) 12,701 (58.2) 663 (57.1) 12,038 (58.2)
 Ever used hormone therapy, n (%) 5951 (27.3) 407 (35.0) 5544 (26.8)
Smoking Status
 Never smoker, n (%) 7923 (36.3) 379 (32.6) 7544 (36.5)
 Former smoker, n (%) 6557 (30.0) 338 (29.1) 6219 (30.1)
 Current smoker, n (%) 7351 (33.7) 445 (38.3) 6906 (33.4)
 Ever smoker, n (%) 13,908 (63.7) 783 (67.4) 13,125 (63.5)
Smoking Intensity
 Mean (SD) smoking durationa (years) 23.3 (12.2) 23.9 (11.7) 23.3 (12.2)
 Mean (SD) smoking intensitya (g/day) 12.4 (8.3) 12.7 (8.0) 12.4 (8.3)
 Mean (SD) pack-yearsa 16.0 (14.8) 16.6 (14.8) 15.9 (14.8)
Initiation of smoking
 Mean age (SD) at smoking initiationa (years) 20.4 (5.5) 20.7 (5.7) 20.4 (5.5)
Initiation of smoking in 18,661 parous women
 Parous never smoker 6938 (37.2) 326 (33.5) 6612 (37.4)
 Parous ever smoker 11,723 (62.8) 647 (66.5) 11,076 (62.6)
 Parous/around 1st birth or later, n (%) 1457 (6.7) 88 (7.6) 1369 (6.6)
 Parous/before 1st childbirth, ≤ 10 pack-years, n (%) 9367 (42.9) 493 (42.4) 8874 (42.9)
 Parous/before 1st childbirth, > 10 pack-years, n (%) 899 (4.1) 66 (5.7) 833 (4.0)
Initiation of smoking in 3170 nulliparous women
 Nulliparous never smoker 985 (31.1) 53 (28.0) 932 (31.3)
 Nulliparous ever smoker 2185 (68.9) 136 (72.0) 2049 (68.7)
 Nulliparous, before age 21b years, n (%) 1264 (5.8) 74 (6.4) 1190 (5.8)
 Nulliparous, age 21 years or later, n (%) 921 (4.2) 62 (5.3) 859 (4.2)
Initiation of smoking in 12,376 postmenopausal women
 Postmenopausal never smoker, n (%) 3988 (32.2) 202 (29.0) 3786 (32.4)
 Before menopause, n (%) 8301 (67.1) 488 (70.1) 7813 (66.9)
 After menopause, n (%) 87 (0.7) 6 (0.9) 81 (0.7)
  1. SD standard deviation; ain ever smokers. b21 is mean age of smoking initiation in the cohort