In November 2022, the National Institutes of Health (NIH) published a paper in the Journal of the National Cancer Institute that identified a possible association between the frequent use of chemical hair straighteners and uterine cancers, such as endometrial cancer.

The study used data from the Sister Study, an 11-year prospective cohort study of environmental and genetic risk factors for breast cancer and other diseases among more than 50,000 sisters of women who have had breast cancer. The researchers found that 1.64% of women who never used hair straighteners developed uterine cancer by the age of 70. For users who got four or more hair straightening treatments a year, the risk increased to 4.05%—more than twice as likely.

Previous papers from the Sister Study reported an association between other hair products, such as hair dyes, and breast and ovarian cancer.

Women and especially Black women, who comprise 60% of hair straightener users in the U.S., are naturally concerned about these cancers and want to know about any risk factors that may be under their control. But incomplete answers from studies that were not designed to determine causation do not help.

Yes, observational studies should point out the direction for future research, and we should follow them up with studies that can provide quality evidence of cause-and-effect and mechanisms. But the current way things are done leads to “messy” reporting by journalists who do not understand the nuances of epidemiology, as well as class-action recruitment campaigns looking for people who want to sue for damages before cause is scientifically established.

Not surprisingly, there has been pushback from the Personal Care Products Council regarding the lawsuits.

This study raised many questions and sufficiently answered none. The women in the study already have a first-degree relative with an estrogen-dependent cancer (breast cancer) and are likely more at risk. The study did not collect any information on brand or type or chemical content of hair straightener, and only asked once at the beginning of the 11-year follow-up how often the participant used the product in the past year. It is possible that frequency of use could have changed at any point during the study, or been different in the time before the study began.

In March 2023, two U.S. Congresswomen, Ayanna Pressley and Shontel Brown, asked the FDA to look into the safety of hair straighteners. This past October, the FDA announced a proposal for a new rule to be finalized in the spring of 2024 that would ban formaldehyde and formaldehyde-releasing products in hair straighteners.

Why formaldehyde?

It’s a great question. Why was formaldehyde singled out by the FDA, when other chemicals are also used in hair straighteners? The Sister Study did not collect any information on the products used by the women in the study, and other chemicals found in hair straighteners that have also been considered as ‘possibly carcinogenic’ were not mentioned in the proposed FDA rule, among them: parabens, phthalates, and heavy metals. The type and amount of chemicals vary by product, and the study cannot tell us which exposures or combination of exposures may have been involved in the cancer cases, or in what amounts.

Where is the quality science that was reviewed by the FDA as it crafted this proposed new rule? The FDA does keep track of public complaints about beauty products, and there is at least one recent standard literature review available from the National Library of Medicine—but rigorous, transparent, replicable state-of-the-science systematic reviews of current research, including accepted criteria for review of quality, have not been reported. These are needed for balanced policy decisions, and in this case, they are nowhere in sight.

Notably, USA Today, CNN, NBC and several other outlets reporting on the proposed rule wrote that the FDA did not respond to requests for comment.

We need better research on chemical hair straighteners

Endometrial cancer, a type of uterine cancer, is the most common reproductive cancer diagnosed in women in the U.S., and the fourth most common cancer in women overall. But it is still rare. Only 0.1% of women in the Sisters Study got this cancer in the 11-year period of the study.

However, as estrogen-dependent cancer rates rise, it is important to identify risk factors that can be acted on. Currently, unhealthy weight and physical inactivity head the list, according to the American Cancer Society. Screening and early detection can also help survival rates.

Banning formaldehyde from beauty products is the easier thing to do and can look like progress. But we do not currently know if it will prevent cancer. We need better answers on the safety of chemical hair straighteners, and that starts with asking better questions:

  • When will NIH and others lay out a comprehensive research agenda that will determine evidence for cause?
  • What is the quality of that evidence?
  • What further dedicated research is needed on this issue?
  • Is the patient population being studied adequately diverse?

These are the questions that journalists and researchers should be pursuing, instead of raising women’s fears prematurely.