Table 1.

Distribution of study population characteristics^a by smoking status

	Never smoker/no SHSb		Never smoker/with SHSb		Former smoker		Current smoker		p-value comparing never/no SHS to ever smokers
n (%)	344 (43)		203 (25)		179 (22)		73 (9)
Mean percent mammographic density (95% CI)c	48.5 (46.3, 50.6)		43.1 (40.3, 45.8)		40.7 (37.7, 43.6)		38.4 (33.7, 43.0)		<0.001
	n	%	n	%	n	%	n	%
Characteristics
Age (years)									0.63
40–44	90	26	53	26	44	25	23	32
45–49	198	58	115	57	96	54	41	56
50–55	56	16	35	17	39	22	9	12
Race/ethnicity									<0.001
Non-Hispanic white	132	38	109	54	114	64	36	49
African-American	11	3	18	9	16	9	15	21
Chinese	131	38	37	18	9	5	2	3
Japanese	70	20	39	19	40	22	20	27
Highest education level									0.04
High school graduate or less	56	16	35	17	19	11	19	26
Some college	87	25	61	30	57	32	31	43
College graduate or more	201	58	107	53	103	58	23	32
Body mass index, kg/m2									<0.001
<25.0	228	67	100	50	84	48	37	52
25.0–29.9	71	21	51	25	48	27	18	25
≥30	40	12	51	25	43	25	16	23
Alcohol, drinks/week									<0.001
Abstainer	277	81	133	67	110	62	37	52
≥1	66	19	67	34	68	38	34	48
Age at menarche, years									0.06
<12	61	18	47	23	42	24	15	21
12	89	26	58	29	56	31	25	35
13	112	33	60	30	47	26	15	21
>13	79	23	38	19	34	19	17	24
Parity
Nulliparous	51	15	40	20	42	23	14	19
Parous	293	85	163	80	137	77	58	81	0.02
1–2 full-term births	213	62	113	56	100	56	39	54	0.06
≥3 full-term births	80	23	50	25	37	21	19	26
Menopausal status									0.45
Early perimenopausal	179	52	118	58	97	54	42	58
Premenopausal	165	48	85	42	82	46	31	42
Ever used oral contraceptives									<0.001
No	111	32	53	26	29	16	16	22
Yes	232	67	148	73	148	83	57	78
Breast cancer family history									0.07
None	251	74	143	71	120	68	47	64
≥1 first degree relative	57	17	35	17	42	24	19	26
≥1 second degree relative	32	9	24	12	15	8	7	10

^aData was from baseline (race/ethnicity, education, age at menarche, parity, oral contraceptive use, family history, secondhand smoke exposure) or from the closest annual follow-up visit closest to the woman’s index mammogram (age, body mass index, alcohol, menopausal status, active smoking)

^bSecondhand smoke (SHS) exposure is based on self-reported person-hours (#people x #days × hours) of exposure in past 7 days at home, work and other sources

^cConfidence interval, CI