Table 5.

Total pack years of active smoking and started smoking before age 18 in relation to premenopausal breast cancer incidence by body mass index, alcohol consumption, and estrogen receptor status of the tumor^a,^b

Subgroup	Pack yrs and age started	Cases	IRR (95% CI)
BMI <25	Never active or passive	24	1.00
	<20 total	41	1.26 (0.73–2.17)
	≥20 total	14	2.22 (1.09–4.54)
	≥20, started before age 18	12	3.34(1.58–7.05)
BMI ≥25	Never active or passive	46	1.00
	<20 total	110	1.16 (0.81–1.67)
	≥20 total	19	1.01 (0.58–1.77)
	≥20, started at age < 18	12	1.14 (0.59–2.19)
Alcohol drinker	Never active or passive	22	1.00
	<20 total	106	1.29 (0.80–2.06)
	≥20 total	23	1.40 (0.76–2.58)
	≥20, started at age < 18	18	1.85(0.97–3.53)
Nondrinker	Never active or passive	48	1.00
	<20 total	45	1.20 (0.79–1.83)
	≥20 total	10	1.41 (0.70–2.83)
	≥20, started at age < 18	6	1.63 (0.68–3.83)
ER+ cancer	Never active or passive	26	1.00
	<20 total	60	1.37 (0.84–2.23)
	≥20 total	16	1.85 (0.96–3.57)
	≥20, started at age < 18	12	2.58 (1.26–5.29)
ER− cancer	Never active or passive	19	1.00
	<20 total	29	0.96(0.52–1.77)
	≥20 total	4	0.69( 0.23–2.09)
	≥20, started at age <18	2	0.59 (0.13–2.58)

^aIRRs adjusted for age, questionnaire cycle, BMI, years of education, family history of breast cancer, age at menarche, parity, age at first birth, oral contraceptive use, vigorous exercise, alcoholic beverage consumption, menopausal status, age at menopause, and menopausal female hormone use

^bER status was obtained for 854 of the 1,377 cases