While white women are slightly more likely to get breast cancer than black women, black women are more likely to be diagnosed with aggressive breast cancers and also have a higher mortality rate. A large prospective cohort study examining "Hair dye and chemical straightener use and breast cancer risk in a large US population of black and white women" is making headlines with its findings that purport to explain at least some of this difference.
The researchers examined data from over 46,000 U.S. women with at least one sister with breast cancer who are participants in the Sister Study, a project supported by the National Institutes for Health. The Sister Study enrolled these women between 2003 and 2009, tracking any new onset of breast cancer among them along with possible associations of a wide array of variables. Some of these variables centered on hair products, specifically hair dye and chemical straighteners. Enrollees completed a questionnaire regarding their use of these products in the year prior to their enrollment; 55% reported the use of permanent hair dye. The participants were followed for an average of 8.3 years; during this time, nearly 2,800 of the Sister Study women developed breast cancer.
After controlling for menopausal status, age at menarche, educational attainment, smoking history, and age at first birth, the researchers found that the risk of developing breast cancer was higher in women who had reported hair dye use than those who had not, with a disparity in the effect based on ethnicity. Black women had a 45% increase in risk with permanent hair dye use (hazard ratio 1.45, 95% confidence interval 1.10-1.90) while the 7% higher risk in white women was not statistically significant (HR 1.07, 95% CI 0.99-1.16). The use of chemical straightener was also associated with an increased risk of developing breast cancer, though this risk was also not statistically significant (HR 1.18, 95% CI 0.99-1.41). The use of semi-permanent dye ("highlights") and temporary dyes were not associated with an increased risk of cancer. The researchers note the consistency of their findings with earlier, smaller studies.
When asked whether women should stop using these projects, the co-lead investigator of the study responded:
"We are exposed to many things that could potentially contribute to breast cancer, and it is unlikely that any single factor explains a woman’s risk. While it is too early to make a firm recommendation, avoiding these chemicals might be one more thing women can do to reduce their risk of breast cancer."
It's important to note that an observational study, like this one, can only determine correlation, not causality, between these hair products' use and the development of breast cancer. It also may not be appropriate to generalize this study's findings to women who don't have a sister with breast cancer; the women enrolled in the Sister Study may have been at higher risk of developing breast cancer to begin with since they had a positive family history. Discussing the study design, findings, and limitations with our patients can help them make an informed choice regarding their use of these products. There's an AFP By Topic on Cancer with a Breast Cancer subheading which includes information about risk reduction strategies, screening and diagnosis, and care of survivors if you'd like to read more.