. 2021 May 20;42(7):924–930. doi: 10.1093/carcin/bgab041

Table 3.

Association of breast cancer, overall and by ER status,among users of lye and non-lye hair relaxer

	All Breast Cancer				ER+				ER-
	cases	HR^a	95% CI		cases	HR^a	95% CI		cases	HR^a	95% CI
			Most often used lye hair relaxer
Never/light use^b	252	1.00	reference		152	1.00	reference		59	1.00	reference
Ever use	334	1.10	0.94	1.31	214	1.18	0.96	1.16	83	1.09	0.78	1.53
Heavy use^c	105	1.22	0.95	1.56	67	1.32	0.97	1.8	30	1.27	0.79	2.02
Duration of use in years
1–9	44	1.22	0.88	1.69	28	1.29	0.85	1.94	9	0.98	0.48	1.98
10–19	150	1.25	1.00	1.56	101	1.38	1.05	1.83	35	1.12	0.72	1.75
≥20	140	1.00	0.80	1.23	85	1.03	0.78	1.37	39	0.96	0.64	1.47
Frequency of use per year
1–4	86	0.98	0.76	1.26	52	0.99	0.72	1.36	24	1.04	0.65	1.69
5–6	107	1.07	0.84	1.35	70	1.17	0.87	1.57	22	0.79	0.48	1.3
≥7	138	1.26	1.01	1.58	90	1.37	1.04	1.82	36	1.19	0.77	1.83
Number of burns
Never burned	34	0.96	0.67	1.38	19	0.9	0.56	1.46	10	1.06	0.54	2.08
1–9	199	1.07	0.88	1.30	129	1.16	0.9	1.48	49	0.98	0.66	1.45
>10	102	1.23	0.97	1.57	66	1.32	0.97	1.79	25	1.12	0.69	1.82
	Most often used non-lye hair relaxer
	cases^a	HR^a	95% CI		cases^a	HR^a	95% CI		cases^a	HR^a	95% CI
Never/light use	252	1.00	Reference		152	1.00	Reference		59	1.00	Reference
Ever use	1412	1.03	0.90	1.18	866	1.05	0.88	1.26	388	1.14	0.86	1.50
Heavy use^c	328	1.13	0.95	1.35	200	1.18	0.94	1.49	90	1.13	0.8	1.59
Duration of use in years
1–9	198	1.01	0.84	1.22	121	1.02	0.8	1.3	55	1.13	0.78	1.64
10–19	577	1.05	0.90	1.23	347	1.06	0.87	1.29	164	1.17	0.86	1.59
≥20	637	1.04	0.89	1.20	398	1.1	0.91	1.33	169	1.09	0.8	1.47
Frequency of use per year
1–4	581	1.05	0.90	1.21	349	1.05	0.87	1.27	167	1.22	0.91	1.64
5–6	384	0.95	0.81	1.11	237	0.98	0.8	1.21	104	1.01	0.73	1.39
≥7	436	1.11	0.95	1.30	273	1.17	0.96	1.43	115	1.13	0.82	1.55
Number of burns
Never burned	248	0.95	0.79	1.13	147	0.94	0.75	1.18	69	1.08	0.77	1.54
1–9	919	1.05	0.91	1.22	567	1.1	0.91	1.32	253	1.15	0.86	1.53
>10	242	1.04	0.86	1.24	150	1.09	0.86	1.37	66	1.09	0.76	1.56

^aAdjusted for age, questionnaire cycle, region (south, midwest, northeast, west), oral contraceptive use (never, ever, UK), current 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 four years prior breast cancer diagnosis (0, 1–4, 5+).

^bNever used, or used for 1–4 years at a frequency of no more than 2X/yr; Reference for all comparisons.

^cUsed >=15 years, 7+ times/yr.