For the more than one-third of U.S. women who regularly use hair dye, the headlines about a National Institutes of Health (NIH) study published late last year may have prompted their hair to stand on end. The research found that women who use permanent hair dye and chemical straighteners were at higher risk for breast cancer compared to women who did not use these products—and ethnicity may play a role in that risk.
The study found that the association between permanent hair dye and breast cancer was significantly higher among black women compared to white women (Int J Ca 2019; doi: 10.1002/ijc.32738). Additionally, the study found that frequent use of chemical straighteners was associated with increased breast cancer risk. Both findings were unexpected.
“The magnitude of the increase in risk in black women surprised us,” said Alexandra White, PhD, corresponding author for the study and an epidemiologist who is the Stadtman Investigator in the Epidemiology Branch of the National Institute of Environmental Health Sciences. “Additionally, we didn't expect to identify a higher risk associated with chemical straightener use.”
These findings contribute new direction to a body of literature that has thus far produced a mixture of findings about a potential association between hair dye and breast cancer.
“A decent amount of studies have shown positive associations, inverse associations, and no association,” White explained. “Most research has been performed in white women, with much less focused on the effect of hair products on black women. Our study is adding to the literature because it is a large prospective cohort and we were able to look at the question separately in black and white women. The results in black women are more concerning and something that we'd like to see validated in other studies.”
Study Design & Findings
This NIH-sponsored research included 46,709 women who are enrolled in the Sister Study, a prospective nationwide study of environmental and genetic risk factors for breast cancer. Between 2003 and 2009, the study enrolled 50,884 aged 35-74. Although concerted efforts were made to achieve a diverse representation in the study, the enrollment result was approximately 84 percent white women and 9 percent black women, White said.
Women enrolled in the study had no history of breast cancer but had at least one sister or half-sister who had been diagnosed with breast cancer. These women have been followed using annual updates documenting health changes, detailed follow-up assessments every 2-3 years, and other measures. Response rates have exceeded 90 percent throughout the study follow-up.
White and her colleagues surveyed Sister Study participants about their use of hair dye and chemical straighteners and followed them for an average of 8.3 years. Participants reported their use of permanent hair dye, semipermanent hair dye, temporary dye, and chemical straighteners in the 12 months prior to enrollment. It is important to note that the women were not asked whether they dyed their hair at home or at a salon.
During the follow-up period, 2,794 of the 46,709 women were diagnosed with breast cancer. Specifically, the researchers discovered the following:
- The use of permanent hair dye in the year before study enrollment was associated with an overall 9 percent increase in breast cancer risk.
- The risk was disproportionately high in black women: 45 percent compared to 7 percent in non-Hispanic white women.
- Among black women who used permanent dyes every 5-8 weeks, breast cancer risk was increased by a rate of 60 percent.
- The frequent use of chemical hair straighteners was associated with a 31 percent increase in breast cancer risk; this risk did not vary by race.
- The use of semipermanent or temporary dyes was not associated with increased breast cancer risk.
The research team noted that their findings were consistent with those recently reported by the Women's Circle of Health Study, “which observed a very similar increase in risk (approximately 50%) for use of dark dyes and higher risk with increased frequency of permanent dye use among African American women.”
Permanent vs. Semipermanent Dye
Concern about an association between hair dye and cancer risk is longstanding—so much so that the National Cancer Institute (NCI) published a fact sheet about it in 2016. This document states that 80 percent of hair dyes in use today are “permanent,” which means they are oxidative and contain aromatic amines (colorless dye intermediates) and dye couplers. In the presence of hydrogen peroxide, the intermediates and couplers react with one another to form pigment molecules. Darker colors are formed via higher concentrations of intermediates. Semipermanent and temporary hair dyes are nonoxidative and consist of colored compounds that directly stain hair.
The NCI fact sheet states that studies have raised questions about potential associations between hair dye and non-Hodgkin lymphoma, leukemia, bladder cancer, and breast cancer. However, the fact sheets note the International Agency for Research on Cancer (IARC) has concluded that, based on its review of evidence, “personal use of hair dyes is ‘not classifiable as to its carcinogenicity to humans.’”
Besides the association between permanent hair dye and increased breast cancer risk, White and colleagues documented an association between hair straighteners and higher cancer risk. Historically, the active ingredients in most straighteners include sodium hydroxide and thioglycolic acid salts, which have not been identified as carcinogenic. A popular form of straightening, introduced in the early 2000s, is known as Brazilian Blowout. This method uses keratin, and many commercially available keratin treatments contain the known carcinogen formaldehyde.
“Our finding is the first estimate of the association between straightener use and breast cancer from a prospective cohort that assessed exposure after the introduction of formaldehyde-containing straighteners to U.S. markets,” the authors wrote.
The authors also noted that the higher breast cancer risk observed in women who used straighteners is consistent with recent findings from two other studies: the Ghana Breast Health Study (Carcinogenesis 2018;39(4):571-579) and the Women's Circle of Health Study (Carcinogenesis 2017;38:883-892).
More research is anticipated to further examine the effects of common hair dyes and straighteners. “We are interested in looking at the impact of early-life use of these products,” White said. “For breast cancer in particular, we hypothesize that there is a window in time during adolescence and early adulthood when the breast may be more susceptible to certain carcinogens.” She noted that her research team is still following the Sisters Study cohort and “will ask more detailed questions on straightener use through the life course.”
Proving a cause-effect relationship between hair dyes or hair straighteners and breast cancer would be challenging, she acknowledged. “I think what is needed is for other studies that have large populations of black women to look at these questions,” she said. “Another important avenue would be for other types of scientists to look at compounds and lab assays. It will take replication in both epidemiologic settings and bench-based approaches.”
For oncologists and other health care professionals who treat women, the growing amount of research that has identified an association between hair products and breast cancer risk is sure to lead to questions from patients. As with most other potential risks, the key is to assess individual risk factors.
“I think that for breast cancer in particular, a lot of different factors influence risk,” White said. “There are many things we do on a daily basis that influence risk, from diet to alcohol to physical activity. At the end of the day, whether to use hair dyes or straighteners is a personal decision. Women need to weigh the risks and benefits for themselves.
“For black women, these findings are more concerning,” she emphasized. “For them, the math might be a little different. But I would caution that we need more studies to validate these findings. These products change a lot over time. We asked these questions about use in the early 2000s, and these formulations may have changed. We know chemical straighteners have.”
Michelle Perron is a contributing writer.