Table 1.

Smoking status by baseline characteristics (age standardized)^a

Characteristic	Smoking status
	Never active or passive	Passive only	<5 pack-yrs	≥5 pack-yrs
N	8025	24,172	7,612	11,657
Age, mean	33.9	37.6	39.3	44.7
BMI, %
<25	44.6	37.1	35.5	33.5
25–29	29.5	31.0	32.2	32.0
≥30	25.0	30.5	30.9	33.3
Years of education, %
≤12	11.5	16.9	22.1	27.5
13–15	27.9	34.8	39.0	41.6
≥16	60.5	48.2	38.7	30.6
Family history of breast cancer, %	6.4	6.3	6.7	6.7
Age at menarche, %
≤11	25.7	28.9	27.7	29.3
12–13	54.4	52.2	52.1	51.5
≥14	19.3	18.4	19.7	18.7
Parity, %
Nulliparous	41.9	36.5	30.3	28.7
1–2	41.4	43.9	48.2	48.6
≥3	16.6	19.4	21.2	22.4
Age at first birth among parous,%
<20	24.6	31.4	36.1	42.5
20–24	37.2	37.1	35.4	33.1
≥25	37.5	30.8	27.2	23.4
Premenopausal,%	78.2	77.1	76.4	75.0
Postmenopausal, %	15.5	16.5	17.1	18.6
Years of female hormone use, postmenopausal women
Never used	41.1	40.0	42.9	39.6
<5	30.6	30.9	30.4	31.9
≥5	21.4	23.1	20.3	23.3
Age at menopause, %
<40	27.2	28.3	27.7	30.6
40–44	19.0	19.5	18.5	20.1
45–49	23.3	22.8	24.5	25.1
≥50	26.9	26.6	26.6	22.1
Yrs of oral contraceptive use, %
Never used	26.9	23.5	21.7	23.2
<5	42.4	44.4	48.5	46.8
≥5	30.6	32.1	29.8	30.0
Hr/wk of strenuous exercise, %
None	28.4	31.4	30.0	38.8
<5	53.4	51.8	52.4	46.7
≥5	14.2	13.0	13.5	11.4
Drinks/wk of alcohol, %
<1	85.7	81.7	58.6	58.0
1–3	8.5	10.2	20.2	18.2
≥4	5.2	7.7	20.8	23.4

^aPercentages may not add to 100% because of missing data