Table 4. Relative risks of breast cancer by passive smoking exposure among never smokers by different characteristics.

	Cases	Controls
Subgroup	Exposed/unexposed		RR (95% CI)a
Menopausal status
Premenopausal	112/170	126/167	0.83 (0.59–1.17)
Peri/postmenopausal	7/4	7/5	1.51 (0.19–12.2)
			P-valueb=0.35
Alcohol consumption
Never drinkers	42/66	42/64	0.93 (0.51–1.69)
Drinkers	79/107	89/108	0.86 (0.56–1.30)
			P-value=0.98
Oral contraceptive use
Never user	11/35	17/39	0.68 (0.25–1.91)
Last use within 5 years	19/22	12/30	2.51 (0.90–6.99)
Last use greater than 5 years ago	85/111	91/88	0.74 (0.49–1.12)
			P-value=0.11
Family history of breast cancer in mother or sister
No	108/154	122/161	0.89 (0.62–1.26)
Yes	8/9	9/7	1.12 (0.20–6.41)
			P-value=0.57
Parity and age at first giving birth
Nulliparous	10/33	11/30	0.64 (0.21–1.91)
Parous and age at first giving birth <25 years	66/54	66/68	1.06 (0.63–1.78)
Parous and age at first giving birth 25+ years	45/78	57/74	0.68 (0.40–1.16)
			P-value=0.21
Socioeconomic status
Professional	25/54	26/47	0.81 (0.40–1.63)
Nonmanual	40/55	50/61	0.80 (0.45–1.43)
Manual/not employed	50/52	57/61	1.03 (0.58–1.85)
			P-value=0.82
Body mass index (kg m−2)
<25	89/137	95/116	0.72 (0.48–1.07)
25+	31/38	36/51	1.07 (0.54–2.14)
			P-value=0.18
Age at menarche (years)
<12	61/82	56/81	1.09 (0.66–1.79)
13+	59/90	74/86	0.67 (0.42–1.09)
			P-value=0.15

^aRelative risks are stratified by age and region and additionally adjusted for parity and oral contraceptive use.

^bP-values are for tests of heterogeneity in effect between subgroup characteristics.