Table 3.

Age started smoking, duration of smoking, years since quitting and pack-years of smoking in relation to the incidence of breast cancer among 111,140 participants of NHS with follow-up from 1976 to 2006.

Current amount of smoking	Person-years	Cases	Age-adjusted HR(95% CI)	Covariate-adjusted HR(95% CI)*
Never	1341442	3788	1.0	1.0
1–14 cigs/day	189721	484	1.00 (0.91 – 1.10)	1.04 (0.94 – 1.15)
15–24 cigs/day	235079	545	1.00 (0.92 – 1.10)	1.06 (0.96 – 1.17)
≥25 cigs/day	143503	343	1.11 (0.99 – 1.24)	1.14 (1.02 – 1.29)
P for trend			0.18	0.02
Past amount of smoking
Never	1341442	3788	1.0	1.0
1–14 cigs/day	519339	1659	1.05 (0.99 – 1.11)	1.02 (0.97 – 1.09)
15–24 cigs/day	355624	1212	1.14 (1.06 – 1.21)	1.11 (1.04 – 1.18)
≥25 cigs/day	172765	583	1.12 (1.03 – 1.22)	1.08 (0.99 – 1.18)
P for trend			<0.0001	0.003
Age at which started smoking
Never smoker	1341442	3788	1.0	1.0
≥20 years	341835	968	1.07 (0.99 – 1.15)	1.07 (0.99 – 1.15)
18–19 years	633381	1826	1.06 (1.00 – 1.12)	1.08 (1.01 – 1.14)
≤17 years	721330	2199	1.04 (0.98 – 1.09)	1.04 (0.99 – 1.11)
P for trend			0.02	0.01
Duration of smoking:
Never smoked	1341442	3788	1.0	1.0
<20 years	617370	1580	1.08 (1.01 – 1.14)	1.04 (0.98 – 1.11)
20–39 years	753013	2092	1.06 (1.00 – 1.12)	1.07 (1.00 – 1.14)
≥40 years	253225	902	1.08 (0.99 – 1.17)	1.15 (1.04 – 1.27)
P for trend			0.02	0.01
Years since quitting:
Never smoker	1341442	3788	1.0	1.0
20+ years	427489	1613	1.09 (1.03 – 1.16)	1.05 (0.99 – 1.12)
10–19 years	309881	935	1.07 (1.00 – 1.15)	1.03 (0.96 – 1.11)
<10 years	332579	995	1.11 (1.04 – 1.19)	1.09 (1.02 – 1.17)
Current smoker	586952	1422	1.02 (0.96 – 1.09)	1.05 (0.99 – 1.12)
P for trend			0.14	0.08

^*Adjusted for age(continuous), family history of breast cancer (yes, no), age at menarche (≤11, 12, 13, 14, ≥15 years), height (continuous), BMI at age 18 (continuous), OC use (never, ever for <1 year, ever for 1–4 years, ever for 5–10 years, ever for >10 years), history of benign breast disease (yes, no), leisure time physical activity (<1, 1–<2, 2–<4, 4–<7, ≥7 hours/week), alcohol consumption (0, 0.1–4.9, 5–14.9, ≥15 grams/day), passive smoking at home and at work (never, occasional, regular), age at first birth (nulliparous, ≤24 years, 25–29 years, 30–34 years, ≥35 years), parity (0, 1, 2, 3, ≥4), and current BMI (continuous). Current smoking status was also adjusted for in the analysis of age started smoking, duration of smoking, and years since quitting.