This article is part of our October 2020 special issue. Download the full issue here.
Eberle CE, Sandler DP, Taylor KW, White AJ. Hair dye and chemical straightener use and breast cancer risk in a large US population of black and white women. Int J Cancer. 2020;147(2):383-391.
To examine the association between use of hair dyes and chemical hair treatments and risk of breast cancer and to examine the differences in risk based on race/ethnicity
Observational, prospective cohort study
Participants from the Sister Study cohort, which was originally a group of 50,884 women aged 35 to 74 years from the United States who had been recruited between 2003 to 2009 and who had no personal history of breast cancer but had a sister who had been diagnosed with breast cancer. Total cohort used for analysis was 46,709, with the following subgroups:
- White, non-Hispanic=39,261
Study Parameters Assessed
The study assessed whether individuals had been exposed to personal or nonprofessional application of permanent and semipermanent hair dye, as well as chemical relaxer/straightener treatments, and with what frequency. Participants answered questionnaires regarding their hair product use over the past 12 months from the time of enrollment. They were asked whether they had used dark colors (black, brown, auburn/dark red); light colors (blonde, light red); or both. Researchers collapsed data for duration of use into categories of less than 5 years or greater than 5 years, with the latter considered long duration use.
Primary Outcome Measures
The incidence of breast cancer diagnoses among the participants were reported up until 2016 and measured against their exposure to various hair treatments.
The majority of participants reported using hair dye at the time of enrollment (55%, or 25,887 participants). There were 2,794 cases of breast cancer identified over a mean follow-up period of 8.3 years. Use of permanent hair dye was associated with a 45% increase in breast cancer risk in Black women (HR=1.45, 95% CI: 1.10–1.90), and a 7% increase in White women (HR=1.07, 95% CI: 0.99–1.16; heterogeneity P=0.04). Hair straightener use across the entire cohort was associated with an 18% increased risk, and the frequency of self-use of straightener correlated with increasing risk (P for trend=0.02).
For those administering dyes and straighteners to others (non-professionals), the application of semipermanent dye was associated with a 28% increased risk (HR=1.28, 95% CI: 1.05–1.56), and application of straightener was associated with a 27% increased risk (HR=1.27, 95% CI: 0.99–1.62).
This study suggests that the use of chemical hair treatments increases cancer risk—specifically breast cancer risk—in women, regardless of ethnicity. With the number of known endocrine disruptors (along with other toxic ingredients) in both salon and at-home hair treatment products,1 environmental medicine advocates have been warning of the carcinogenicity of chemical hair treatments for many years. And while some integrative health practitioners may have been telling their patients to avoid these beauty practices or switch to more natural alternatives, the evidence in support of that claim has not necessarily “matched colors” with a number of previous studies finding no associated cancer risks from conventional hair treatments.2,3
However, the large sample size of this study, combined with the reasonable length of follow-up (7-13 years), allows for better statistical analysis than previously possible with small cohorts and case-control studies. The results of this study suggest that women increase their risk of breast cancer when they are exposed to permanent hair dye or chemical straightener compared to women who do not use these treatments at all. These results, combined with other case-controlled studies,4,5 provide clinicians reason to counsel patients about the use of these agents and the possible increased breast cancer risk.
The results of this study suggest that women increase their risk of breast cancer when they are exposed to permanent hair dye or chemical straightener compared to women who do not use these treatments at all.
The study also tried to address whether there was a “dose-response” effect with exposure—that is, whether the increased risk is greater among women who use these treatments more often (for example, more than once a month) compared to those who use them less frequently (for example, 1-2 times a year). The answer to this question does not seem to be as clear. From the study results, use of permanent hair dye is associated with an increased risk of breast cancer, but the frequency of use does not seem to impact the risk. However, for chemical straightener use, increased frequency very clearly seems to increase risk. Furthermore, counterintuitively, while personal use of semipermanent hair dye was not associated with any increased risk of breast cancer, the nonprofessional application of semipermanent hair dye was, and that risk increased with more frequent applications. A serious limitation of the study is that it only recorded chemical hair treatment usage at enrollment, but not throughout the follow-up period, so the long-term usage of chemical hair treatments may not have been accurately assessed.
The most significant finding from this study is not that these treatments increase breast cancer risk, but that the increased risk seems to be especially high for Black women. Whenever medical research reports outcomes that differ based on race or ethnicity, it is extremely important to critically evaluate the study design and context of the research. Accordingly, it is important to summarize the following:
Of the almost 47,000 participants in this study, 39,261 were White and 4,087 were Black. Not only does this mean that the sample size of White participants was almost 10 times larger than the sample size of Black participants, it also means that, demographically, the total sample population did not accurately reflect the North American population (US census data reports an African-American/Black population of approximately 13%, whereas in this study Black women made up only 8% of the study population). Even with the study adjusting for many different demographic differences, one cannot ignore the fact that there was a much more robust sample size from which to draw conclusions (and eliminate confounding biases) for White women than Black women. This means that in the statistical analysis of the data, there is a higher likelihood for chance to influence results in the Black cohort versus the White, simply due to the smaller sample size of the former. Since this study concluded that the overall sample (N=46,706) showed a statistically significant increase in associated breast cancer incidence with the use of dyes and straighteners, none of this negates the strong evidence of harm from these agents. However, there are clearly differences in the amount of risk for Black women (45% increase) versus White women (7% increase), and it is essential that we bring better awareness to this common issue in research of study cohorts not aligning with the ethnicities found in the overall population.
Among the study participants, White women were more likely to have been exposed to permanent hair dye than not at all (56% versus 44%) compared with Black women. Black women were more likely to never have used permanent hair dye (58%) than to have ever used it (42%). So, based on these results, it means that even though Black women, in general, are more likely to not use permanent hair dye at all compared to White women, they are at a greater increased risk of breast cancer if they do use permanent hair dye compared to White women. Conversely, Black women were much more likely to have ever used chemical straighteners (72%) versus White women (3%). However, risk of breast cancer was not found to be different between ethnicities when it was related to chemical straightener use (increased risk of 31%). So, while Black women use chemical straighteners significantly more than White women, the increased risk of breast cancer with usage did not appear to differ between races in this study (granted, the straightener usage of 3% in the White cohort may have contributed to this finding).
The authors of this paper acknowledge the obvious social injustice implications of these findings, writing, “The strength of association [with increased breast cancer risk] observed for permanent dye use among black women is consistent with toxicological assessments that report higher concentrations of estrogens and endocrine-disrupting compounds in hair products that are marketed to black women”; and “previous studies on hair dye use and breast cancer risk, showing little or no association, have largely been limited to white women.” Not only are hair treatments specifically marketed toward Black women more toxic and carcinogenic,6,7 but the information and research regarding their potential risk has failed to properly represent the Black population; therefore the data misrepresents the true risk posed to Black women in particular.
Black women are at an increased risk of being diagnosed with more advanced breast cancers and having poorer cancer-related outcomes,8 so potentially modifiable or preventative behaviors that can reduce risk deserve attention. Hopefully more awareness of such data will empower more patients when it comes to making personal choices regarding their health. Of most importance is ensuring that this information is delivered in a socially conscious way and that all clinicians are sensitive to the complex history of discrimination based on hair texture that may affect their patients. For those unaware of the contextual history of the discrimination of natural hair, the NAACP has a free resource that can offer guidance before having these conversations with patients.
Lastly, this study also accounted for personal versus the nonprofessional application of these products, which would inherently mean that women who work with these products are at increased risk of breast cancer through occupational exposure. This may imply that women who are hair treatment specialists should undergo increased breast cancer screening measures and/or should prioritize implementing preventative medicine strategies for lowering breast cancer risk.
Ultimately, this research helps to inform the clinical recommendation of avoidance of permanent hair dye and chemical straightening products when counseling women on the ways they can reduce their personal risk of breast cancer. It is important for this information to be made available and accessible to all women of all ethnicities and racial backgrounds, and as more attention is being brought to the systemic injustices affecting marginalized people in all aspects of life (including healthcare), the medical community must make representation of all ethnic populations in clinical research a priority to begin to create healthcare equity for all.