Use of chemical hair straighteners is linked with cancer of the uterus

And other harm

A new study links chemical hair straightening with uterine cancer, and it’s definitely worth a discussion. As someone who went through phases of straightening her own hair, I was especially interested. I remember sitting in the salon being promised it was formaldehyde free, and then wondering, so why does it smell like an anatomy lab? For reference, the bodies that we used in gross anatomy lab in medical school were held in embalming solution, which contains formaldehyde. I distinctly remember being warned about the importance of safety gear so it didn’t get on my skin. OHSA also has strict requirements about limiting exposure. And yet here I was putting a supposedly “non formaldehyde” substance on my head that absolutely smelled and irritated like formaldehyde. And I did it more than once.

This new study comes from the National Institutes of Health. It’s from the Sister Study which enrolled over 50,000 women from the US and Puerto Rico who had a sister with breast cancer. The hope is that by choosing sisters, people with  shared genetics and environments, risk factors for breast and other cancers might emerge. 

For this study over 33,000 women ages 35-74 at enrollment were followed for almost 11 years. The researchers found that having used a hair straightening product was a risk factor for cancer of the uterus, but not the use of perms, dyes, bleaching or highlights. 

The researchers included both endometrial cancer and cancer of the uterine muscle (sarcoma) as uterine cancers and they divided hair straightening into never used, infrequent use (less than 4 times in the past year), and frequent use (more than four times in the past year). 

The baseline rate of uterine cancer for women who never used hair straighteners was 1.64%, and for frequent users that rate jumped to 4.05%, so it more than doubled. For those who used straighteners less than four times a year the risk was 2.82%, but that difference wasn’t statistically significant, meaning the increase could have been due to chance. 

I want to be clear, this doesn’t mean everyone who has used hair straighteners will get uterine cancer. Far from it, but a more than doubling of a risk with frequent use is a significant concern and something people should know about. 

This study didn’t find that any woman was at greater risk per se, but as Black women are more likely to use these products they were disproportionately affected. For example, in the study 7.2% of the women were Black, but they accounted for 60% of the people who used hair straighteners. 

What is even more concerning is the rate of endometrial cancer as well as deaths from endometrial cancer are rising, and the rate of death is highest among Black women. While Black women are likely to develop a more aggressive type of cancer, this doesn’t account for the disproportionate rise in mortality. Due to racism Black women are more likely to have their health concerns dismissed, and so that likely–and unacceptably–plays a role in a delay in diagnosis and in under treatment. But even if a Black woman gets timely care, standard screening for endometrial cancer may not be the best option.

If someone is menopausal (meaning after their final period) and we are concerned about endometrial cancer because of bleeding (there should be no bleeding after menopause), there are two testing options: taking a biopsy from the lining or the uterus (called an endometrial biopsy), or evaluating the thickness of the endometrium (uterine lining) by ultrasound. Many people understandable want the ultrasound as it is less invasive and less painful, but one study showed that an ultrasound might miss four times more cases of endometrial cancer for Black women as compared with White women. The theories for this difference include the greater prevalence of fibroids among Black women, which can distort the image on an ultrasound, making the measurements less reliable, and the fact that Black women are more likely to have the more aggressive type of endometrial cancer, which isn’t as well identified by the thickness of the lining of the uterus. 

It’s not possible be definitive about chemical hair straighteners as a cause for an increased risk of cancer from this study, but the association is suggestive and concerning. There is biology to back up the link between the two. Hair straighteners are known to contain several endocrine disrupting chemicals and are also associated with an increased risk of breast cancer (another hormonally responsive cancer), as well as an increased risk of uterine fibroids. Black women are 40% more likely to die from breast cancer than White women and they are disproportionately affected by fibroids.

So which chemicals might be the culprit? We can’t know that from this study. It may one or a combination. The researchers hypothesized that these chemicals might be particularly concerning when used to straighten hair, because the irritation and inflammation from application might increase their absorption and the scalp has an excellent blood supply. As a result, there may be better access to the bloodstream.  

So what might women want to do going forward? First of all, know that while there appears to be an increased risk of uterine cancer, overall, the absolute risk is low. Meaning most people who have used these products don’t get uterine cancer. If you have used hair straighteners and develop abnormal bleeding, tell your health care provider right away. Black women should be aware that if they have bleeding and are in menopause, a “normal” ultrasound ultrasound might be falsely reassuring

For people who want to continue to use hair straighteners, while the absolute risk of cancer is low, it seems using these products less than four times a year is associated with much less of an increased risk.

In summary, the data on straighteners is concerning. It’s not ethical to do a trial and randomize people to use these products or not, so we will have to make do with this kind of indirect data. People need this information, and we definitely need more studies to better understand the risk.

References

C-JChang, KM O’Brien, AP Keil, et. al. Use of Straighteners and Other Hair Products and Incident Uterine Cancer. JNCI: Journal of the National Cancer Institute. 2022, October.

LA Wise, JR Palmer, D Reich, YC Cozier, L Rosenberg. Hair Relaxer Use and Risk of Uterine Leiomyomata in African-American Women. Am Journal Epidem, 2012: 175: 432–440,

MA Clarke, SS Devesa, A Hammer et al. Racial and Ethnic Differences in Hysterectomy-Corrected Uterine Corpus Cancer Mortality by Stage and Histologic Subtype. JAMA Oncol. 2022;8(6):895-903

KM Doll, SS Romano, EE Marsh, et al. Estimated Performance of Transvaginal Ultrasonography for Evaluation of Postmenopausal Bleeding in a Simulated Cohort of Black and White Women in the US. JAMA Oncol. 2021;7(8):1158-1165.