Table 1.

Characteristics of the nurses from the DNC at baseline in 1993 or 1999, by incident breast cancer status at follow-up

TotalBreast cancerNo breast cancer
n = 22,877n = 1,145n = 21,732
Age, mean (SD)52.9 (7.8)53.2 (7.3)52.9 (7.8)
Premenopausal, n (%)11,579 (51.0)583 (51.5)10,996 (51.0)
Postmenopausal, n (%)11,120 (49.0)548 (48.5)10,572 (49.0)
Menarche ≤ 12, n (%)5,477 (24.1)290 (25.6)5,187 (24.0)
Menarche > 12, n (%)17,222 (75.9)841 (74.4)16,381 (76.0)
BMI, mean (SD)23.7 (3.5)23.8 (3.5)23.7 (3.5)
Underweight (<18.5 kg/m2), n (%)532 (2.3)22 (1.9)510 (2.4)
Normal (18.5–25 kg/m2), n (%)15,698 (69.2)801 (70.8)14,987 (69.1)
Overweight (25–30 kg/m2), n (%)5,167 (22.8)239 (21.1)4,928 (22.8)
Obese (≥30 kg/m2), n (%)1,302 (5.7)69 (6.1)1,233 (5.7)
Never smoked, n (%)7,956 (35.1)359 (31.7)7,597 (35.2)
Previously smoked, n (%)6,922 (30.5)330 (29.2)6,592 (30.6)
Current smoker, n (%)7,821 (34.5)442 (39.1)7,379 (34.2)
Smoking intensity (g/day), mean (SD)7.8 (9.2)8.4 (8.8)7.8 (9.2)
Never consumed alcohol, n (%)3,894 (17.2)195 (17.2)3,699 (17.2)
Alcohol use in drinkers (g/week), mean (SD)112.5 (127.6)120.7 (119.8)112.1 (127.9)
Low physical activity, n (%)1,496 (6.6)71 (6.3)1,425 (6.6)
Medium physical activity, n (%)6,132 (27.0)299 (26.4)5,833 (27.0)
High physical activity, n (%)15,071 (66.4)761 (67.3)14,310 (66.3)
Nulliparous, n (%)3,163 (10.3)180 (15.9)2,983 (13.8)
Gave birth to 1 child, n (%)2,680 (11.8)140 (12.4)2,540 (11.8)
Gave birth to 2 children, n (%)9,648 (42.5)467 (41.3)9,181 (42.6)
Gave birth to 3 children, n (%)5,499 (24.2)271 (24.0)5,228 (24.2)
Gave birth to 4 children or more, n (%)1,709 (7.5)73 (6.5)1,636 (7.6)
Age at first birth 15–24 years, n (%)8,123 (35.8)383 (33.9)7,740 (35.9)
Age at first birth 25–29 years, n (%)8,350 (36.8)384 (34.0)7,966 (36.9)
Age at first birth 30 years, n (%)3,065 (13.5)184 (16.3)2,881 (13.4)
Never used HT, n (%)16,600 (73.1)743 (65.7)15,857 (73.5)
Previously used HT, n (%)2,193 (9.7)99 (8.8)2,094 (9.7)
Current user of HT, n (%)3,906 (17.2)289 (25.6)3,617 (16.8)
Never used OC, n (%)9,202 (40.5)475 (42.0)8,727 (40.5)
Ever used OC, n (%)13,497 (59.5)656 (58.0)12,841 (59.5)
Rural, n (%)9,784 (43.1)514 (45.4)9,270 (43.0)
Provincial, n (%)3,368 (14.8)163 (14.4)3,205 (14.9)
Urban, n (%)9,547 (42.1)454 (40.1)9,093 (42.2)
Baseline annual PM2.5 (μg/m3), mean (SD)19.7 (3.5)20.2 (3.5)19.6 (3.6)
Baseline annual PM10 (μg/m3), mean (SD)23.5 (3.9)24.1 (3.8)23.5 (3.9)
Baseline annual NO2 (μg/m3), mean (SD)12.5 (8.1)12.7 (8.0)12.5 (8.1)
  • Abbreviation: OC, oral contraceptives.