Table 1. Relative risk of breast cancer in relation to current and past smoking duration, intensity and initiation.

Smoking measure	Cases/controls	ORa (95% CI)	Cases/controls	ORa (95% CI)
Never smoker	1526/1453	1.0 (ref)	1526/1453	1.0 (ref)
	Current smokers		Past smokers
Ever smoker	657/589	1.0 (0.9–1.2)	601/541	1.0 (0.9–1.1)
Duration of smoking, years
1–10	22/28	0.7 (0.4–1.3)	255/216	1.0 (0.8–1.3)
11–30	173/147	1.0 (0.8–1.3)	282/257	1.0 (0.8–1.2)
>30	457/406	1.1 (0.9–1.2)	61/65	0.9 (0.7–1.4)
P for linear trendb		0.58		0.95
Cigarettes per day
1–10	282/259	1.0 (0.9–1.3)	373/349	1.0 (0.8–1.2)
11–20	347/297	1.1 (0.9–1.3)	196/170	1.0 (0.8–1.3)
>20	23/25	0.8 (0.4–1.4)	29/19	1.2 (0.7–2.2)
P for linear trendb		0.71		0.73
Pack-years
1–10	114/114	0.9 (0.7–1.2)	391/359	1.0 (0.8–1.1)
11–20	193/187	1.0 (0.8–1.2)	118/103	1.0 (0.8–1.3)
21–30	229/153	1.4 (1.1–1.7)	61/59	0.9 (0.6–1.3)
>30	116/127	0.9 (0.7–1.2)	28/17	1.5 (0.8–2.8)
P for linear trendb		0.77		0.69
Age at initiation, years
<20	359/281	1.2 (1.0–1.4)	300/265	1.0 (0.8–1.2)
20–29	196/204	0.9 (0.7–1.1)	221/202	1.0 (0.8–1.2)
⩾30	99/99	1.0 (0.7–1.3)	77/71	1.1 (0.8–1.5)
P for linear trendb		0.73		0.99
Initiation in relation to birth
Prior	284/223	1.2 (0.9–1.4)	289/260	1.0 (0.8–1.2)
At around the same time	196/204	1.0 (0.8–1.2)	157/159	1.0 (0.8–1.3)
Following	89/99	1.0 (0.7–1.4)	72/63	1.3 (0.9–1.9)

^aAdjusted for age (5-year categories), age at first birth (nulliparous, 20–24, 25–29, ⩾30 years), recent body mass index (in quintiles) and recent alcohol consumption (no intake, <5, 5<−10, >10 g day⁻¹).

^bP for Wald χ², test of linear effect using the original continuous variable.