Hair Products Under Scrutiny: Major NIH Sister Study Links Permanent Hair Dye and Chemical Straighteners to Elevated Breast Cancer Risk – With Stronger Effects in Black Women

A large prospective study led by researchers at the National Institute of Environmental Health Sciences (NIEHS), part of the National Institutes of Health (NIH), has found that personal use of permanent hair dye and chemical straighteners is associated with a modestly increased risk of breast cancer. The investigation, which followed nearly 47,000 U.S. women for an average of more than eight years, highlights potentially greater risks among Black women and frequent users of these products. The findings point to chemicals in certain hair products as possible contributors to breast carcinogenesis, though the authors emphasize that additional research is needed to confirm causality.

Study Design and Methods

The analysis used data from the Sister Study, a prospective cohort of 50,884 women aged 35–74 years who were enrolled between 2003 and 2009. All participants had a sister previously diagnosed with breast cancer but were themselves breast cancer-free at enrollment. After exclusions, the analytic sample included 46,709 women who completed the enrollment questionnaire and provided informed consent.Hair product use was self-reported for the 12 months prior to enrollment. Participants answered detailed questions about personal use of permanent hair dye, semi-permanent hair dye, temporary dye, and chemical straighteners/relaxers, including frequency (“Did not use,” “1–2 times per year,” “Every 3–4 months,” “Every 5–8 weeks,” “Once a month,” or “More than once a month”). Non-professional application of these products to others was also assessed. Use was categorized as “any use” versus “no use” in the past year, with frequency grouped into low (1–4 times/year) or high (every 5–8 weeks or more).Incident breast cancers (invasive and ductal carcinoma in situ) were ascertained through annual health updates and biennial follow-up surveys, with medical record confirmation for more than 80% of cases. Follow-up extended through September 15, 2016 (mean 8.3 years; 386,338 person-years), during which 2,794 breast cancers were identified. Women were censored at age of last contact, death, or other events.Researchers used Cox proportional hazards models with age as the time scale to estimate hazard ratios (HRs) and 95% confidence intervals (CIs). Models adjusted for established breast cancer risk factors including age at menarche, menopausal status, race/ethnicity, education, body mass index, smoking history, oral contraceptive use, parity, and age at first birth. Additional sensitivity analyses excluded hair salon workers, mutually adjusted for dye and straightener use, and tested effect modification by race (Non-Hispanic White vs. Black). Proportional hazards assumptions were met, and complete-case analysis was used (<2% missing data).

Key Findings

Over the follow-up period, 55% of participants reported using permanent hair dye in the past year. Any personal use of permanent dye was associated with a 9% higher breast cancer risk overall (HR = 1.09, 95% CI 1.01–1.17). The association was significantly stronger among Black women (HR = 1.45, 95% CI 1.10–1.90) than among non-Hispanic White women (HR = 1.07, 95% CI 0.99–1.16; p for heterogeneity = 0.04). Risk did not vary meaningfully by dye color (light vs. dark) or duration of use.Personal use of semi-permanent or temporary dyes showed no association with breast cancer risk (semi-permanent: HR = 0.96, 95% CI 0.87–1.06). However, non-professional application of semi-permanent dye to others was linked to a 28% higher risk (HR = 1.28, 95% CI 1.05–1.56).Chemical straightener/relaxer use was far more common among Black women (74% vs. 3% in White women). Any personal straightener use was associated with an 18% higher breast cancer risk overall (HR = 1.18, 95% CI 0.99–1.41), with a significant dose-response trend by frequency (p for trend = 0.02). High-frequency use (every 5–8 weeks or more) was associated with a 31% higher risk (HR = 1.31, 95% CI 1.05–1.63). Non-professional application of straighteners to others showed a similar pattern (HR = 1.27, 95% CI 0.99–1.62). The straightener association did not differ significantly by race.Results were robust in sensitivity analyses and did not vary substantially by estrogen-receptor status of the tumor or menopausal status at diagnosis. The authors noted that products used predominantly by Black women may contain higher levels of hormonally active compounds, potentially contributing to the observed disparities.

Interpretation and Implications

The researchers conclude that personal use of permanent hair dye and chemical straighteners is associated with higher breast cancer risk, with particularly pronounced effects for permanent dye among Black women and for frequent straightener use across groups. They describe these findings as consistent with the presence of endocrine-disrupting and carcinogenic compounds in many hair products and suggest that such chemicals may play a role in breast cancer development.Strengths of the study include its large, prospective design, detailed exposure assessment, long follow-up, high response rates, and ability to examine differences by race. The authors caution that the results are observational and cannot prove causation; residual confounding or other biases remain possible. They call for replication in other cohorts and further investigation into specific chemical constituents.

Citation
Eberle CE, Sandler DP, Taylor KW, White AJ. Hair dye and chemical straightener use and breast cancer risk in a large U.S. population of black and white women. International Journal of Cancer. 2020;147(2):383-391. doi:10.1002/ijc.32738 (Epub 2019 Dec 4).