Table 3.

Total pack years and age started active smoking in relation to breast cancer incidence overall and by menopausal status^a

Pack yrs - Age started	All women			Premenopausal		Postmenopausal
	Cases	P-yrsb	IRR (95% CI)	Cases	IRR (95 % CI)	Cases	IRR (95 % CI)
Never active or passive	141	100,184	1.00	70	1.00	52	1.00
<10 pk-yr	256	126,703	1.05 (0.85–1.30)	98	1.11 (0.81–1.54)	115	0.86 (0.61–1.20)
10–19 pk-yr	150	61,009	1.03 (0.81–1.31)	53	1.37 (0.94–2.00)	83	0.81 (0.57–1.56)
≥20 pk-yr	173	52,214	1.20 (0.95–1.52)	33	1.33 (0.86–2.05)	115	0.92 (0.66–1.30)
Started age <18	219	96,680	1.13 (0.90–1.41)	87	1.35 (0.97–1.88)	98	0.83 (0.58–1.17)
Started age ≥18	379	150,537	1.06 (0.86–1.30)	102	1.11(0.81–1.53)	226	0.88 (0.64–1.20)
<10 pk-yr
Started age <18	59	39,757	0.93 (0.68–1.27)	28	0.97 (0.62–1.52)	18	0.69 (0.40–1.19)
Started age ≥18	195	85,663	1.11 (0.88–1.38)	69	1.19 (0.84–1.67)	96	0.90 (0.64–1.28)
10–19 pk-yr
Started age <18	67	27,485	1.17 (0.86–1.57)	31	1.64 (1.05–2.54)	29	0.81 (0.51–1.29)
Started age ≥18	83	33,171	0.96 (0.72–1.27)	22	1.13 (0.69–1.85)	54	0.81 (0.55–1.20)
≥20 pk-yr
Started age <18	83	26,140	1.28 (0.96–1.70)	24	1.70 (1.05–2.75)	45	0.87 (0.58–1.31)
Started age ≥18	89	25,725	1.14 (0.87–1.61)	9	0.85 (0.42–1.72)	69	0.95 (0.66–1.38)

^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

^bPerson-years