Table 2.

Associations of hair relaxer use with breast cancer, overall and by ER status, 1997–2017

	Person-years	All Breast Cancer (n=2311)				ER positive (n=1420)				ER negative (n=601)
		cases	HRa	95% CI		cases	HRa	95% CI		cases	HRa	95% CI
Hair relaxer use
Never/light useb	94,599	252	1.00	reference		152	1.00	reference		59	1.00	reference
Ever use	848,793	2059	1.05	0.92	1.20	1268	1.08	0.91	1.28	542	1.11	0.84	1.46
Heavy usec	191,248	491	1.13	0.96	1.33	298	1.15	0.93	1.42	138	1.21	0.88	1.67
Duration of use, years
1–4	37,933	117	1.21	0.97	1.51	74	1.26	0.95	1.66	29	1.26	0.81	1.97
5–9	96,574	192	0.95	0.78	1.14	111	0.90	0.70	1.15	53	1.08	0.74	1.56
10–14	193,471	361	1.04	0.88	1.22	225	1.06	0.86	1.31	93	1.06	0.76	1.48
15–19	211,224	477	1.14	0.97	1.33	292	1.16	0.94	1.42	133	1.24	0.91	1.70
≥20	309,591	912	1.02	0.89	1.18	566	1.06	0.89	1.28	234	1.06	0.79	1.41
Frequency of use/yeard
1–2	63,514	160	0.96	0.79	1.17	93	0.94	0.73	1.21	46	1.11	0.76	1.62
3–4	245,745	645	1.05	0.91	1.22	390	1.06	0.88	1.27	175	1.13	0.85	1.50
5–6	254,345	582	0.99	0.86	1.15	369	1.06	0.88	1.28	140	0.92	0.68	1.24
≥7	278,863	655	1.12	0.97	1.30	407	1.17	0.97	1.42	175	1.13	0.85	1.52
Age at first use
<10	48,035	73	1.02	0.77	1.35	42	0.96	0.67	1.38	21	1.05	0.62	1.79
10–19	501,839	1,029	1.11	0.9	1.31	632	1.14	0.93	1.42	277	1.06	0.77	1.45
20–29	240,349	740	1.08	0.9	1.28	460	1.14	0.92	1.40	184	1.02	0.74	1.40
≥30	57,339	207	0.95	0.78	1.16	128	0.97	0.75	1.25	59	1.12	0.76	1.65
Number of burns
Never burned	135,310	352	1.00	0.84	1.19	205	0.97	0.77	1.22	95	1.02	0.72	1.44
1–2	234,523	583	1.06	0.90	1.26	361	1.11	0.90	1.38	154	1.03	0.74	1.53
3–4	184,092	449	1.11	0.93	1.32	279	1.16	0.93	1.45	119	1.07	0.76	1.50
5–9	119,302	266	1.07	0.88	1.29	170	1.16	0.91	1.47	64	0.93	0.63	1.36
>10	173,767	400	1.12	0.94	1.34	248	1.16	0.93	1.46	108	1.08	0.76	1.54

^aAdjusted for age, questionnaire cycle, region (south, midwest, northeast, west), oral contraceptive use (never, ever, UK),

body mass index (<25, 25–29, 30–34, 35+), age menarche (<11, 11, 12–13, 14+), body mass index at age 18 (<20, 20–24, 25+, UK), and number of mammograms received up to 4 years prior to breast cancer diagnoses (0, 1–4, 5+).

^bReference group includes never users and those who used hair relaxers for <4 years at a frequency no more than 2 times/year; it is the reference group for all exposures in the table.

^cA subset of ever use: used >=15 years, 7+ times/year.

^dP for trend over frequency of use for ER+ cancer was 0.03. All other P for trend were >0.05.