Hair straightening chemicals and uterine cancer

Briefing Notes
How to cite: Hair straightening chemicals and uterine cancer. Beauty SciComm Group. August 20, 2023. Accessed February 26, 2024. https://beautyscicomm.com/hair-straightening-chemicals-and-uterine-cancer/

This briefing note is in response to the October 2022 paper titled “Use of straighteners and other hair products and incident uterine cancer” published to the JNCI: Journal of the National Cancer Institute, featuring perspectives from Tony Simion, Ph.D.

Tony Simion, PhD

Toxicologist and Adjunct Professor at the University of Cincinnati College of Pharmacy Cosmetic Sciences program

In this paper Chang et al. looked at the occurrence of uterine cancer in a population of women from a “Sister Study”. These women have had at least one sister diagnosed with breast cancer and live in the USA (including Puerto Rico). At baseline (between 2003 and 2009) women completed an interview and a self-administered questionnaire including questions on hair product usage. Their height and weight were determined.

Of the panel of approximately 50,000 women, 378 reported having uterine cancer. Of these, 262 cases were confirmed either by medical records or from a death certificate. The rest were self-reported or from next of kin information.

The comparison group were women who had never been diagnosed with uterine cancer. From this cohort, those women that had a hysterectomy, had died, or were otherwise lost to follow up were removed from consideration. Statistical adjustments were made when more than one family member was enrolled in the study. In this paper, 33,947 women were used as the control group.

Results indicate a hazard ratio significantly greater than 1 for women who had used a hair straightener, relaxer or pressing product in the last 12 months before baseline (inclusion in the Sisters Study). The Hazard Ratio (HR) was determined by comparing the number of women (with or without uterine cancer) who had never used that category of product (Table 3 in paper). The Hazard Ratio was dependent on whether the women used the product occasionally or frequently, implying a dose dependence (Table 2).

Other results showed a correlation between uterine cancer and factors such as BMI and activity levels. This is not unexpected from previous epidemiological studies of different cancers.

Comments About the Paper

Correlation does not equal causation. Because two factors are correlated does not mean that one causes the other.  In this paper the authors pointed to different cosmetic ingredients as causes of the uterine cancer. These include parabens, cyclosiloxanes, diethanolamine (DEA), metals and formaldehyde. There is data suggesting these ingredients have the potential to effect reproductive organs (hazard), but a risk assessment is needed to determine if there is any risk to consumers. Some of the ingredients and contaminants are present at very low levels in hair straighteners and relaxers which are washed away soon (minutes or hours) after application. Some ingredients such as DEA are no longer used in cosmetic products. However, formaldehyde which is used in “Brazilian Blow-out” is known to be a carcinogen. Instead, ingredients more likely to cause problems when the scalp skin is exposed to a chemical straightener are caustic soda and sulfur containing products (thioglycolates).  However, these are not usually thought of as being carcinogens.

Information on hair care product usage was collected at baseline (between 2003 and 2009), a decade or more before the data on cancer was collected (September 2019). Although there is often a long lag between carcinogen exposure and a cancer diagnosis, it is unclear what panelists were exposed to in the interim. For instance, did a panelist stop using hair straightener after she entered the study?  What else did she use during the intervening years?

Relatively few women (38) used hair straightener in the 12-month baseline and then were diagnosed with uterine cancer. Such low numbers make it difficult to draw rigorous conclusions. 

Women in the “Sister Study” may be more susceptible to the development of cancer. After all, they were selected based on having a sibling with breast cancer. This raises the question as to whether the results of the Sister Study can be extrapolated to the entire female population.

Main takeaway: you probably don’t need to worry about hair straighteners and uterine cancer.

The study by Chang et. al. was part of a larger project called the Sister Study, which involved recruiting a large group of US based women between 2003-2009 who had a sister with breast cancer, inquiring about their health with a follow up over time. The purpose of this project was to see if future health patterns existed to show risks and benefits in terms of cancer. The Chang et al. (2022) looked at about 50,000 women and their reported use of hair products, which included permanent, semi permanent, and temporary hair dyes; bleach; highlights; straighteners; and relaxers, along with future uterine cancer risk. Women who underwent hysterectomies were excluded from the dataset, and associations between product use and uterine cancer were analyzed an average of roughly 10 years after the women enrolled.

The results? The study found no increased risk of uterine cancer associated with the use of hair dye or bleach. Interestingly, they found a reduced risk of uterine cancer for those who regularly got highlights. There was a modest increase in risk for those who reported regularly (4+ times yearly) who used chemical straightening products. The scary media headlines you may have seen in the media blew this risk out of proportion, where an 84% increase risk has been often cited; this is the relative risk in the study. Put in the context of absolute risk however, the increased risk year over year was 0.03%. In other words, for every 10,000 women who never straightened their hair, about 10 got uterine cancer yearly, while for that same number of women who straightener their hair 4+ times yearly, about 17 got uterine cancer yearly during the study. To summarize, despite the scary headlines about this study, they found that most hair products had mostly no impact on cancer risk, while chemical straightening products had a very small increase in risk.

I’m skeptical of the results of this study for a number of reasons, not least because the study had some large issues that are hard to reconcile with. For example, the increased cancer risk almost entirely disappeared in women who reported frequent physical activity, which may indicate this is simply the result of systematic differences between women who do or do not regularly straighten their hair, rather than their product use habits. The risks were also fairly small, and mostly for women who very regularly used chemical straightening products. If you chemically straighten your hair 2x per year, the associated risk is far lower. Finally, I’m not sure you can actually assess the risk in a population where nearly 1/3rd of the women had already had a hysterectomy when the study started. Excluding these women from the dataset is necessary, but also introduces biases in the study design that are hard to overcome. Given that the women who used straighteners were on average much younger than those who didn’t, it’s plausible that this study is simply measuring survivorship bias, because older women who were prone to uterine cancer had already had hysterectomies and had been removed from the sample. At the end of the day, while the findings may be real, it’s also entirely plausible that the study is simply measuring differences between people who do and do not report getting their hair straightened a lot.

Should you as an individual stop using chemical straighteners if this is something you do? It’s plausible there’s a fairly small risk, but there’s also a lot of uncertainty. If you’re worried about uterine cancer, or any cancer for that matter, it might be useful to talk to a medical professional about it. Beyond that, based on the study and evidence available, it’s hard to make any definitive statements at all. Ultimately, you probably don’t need to worry about hair straighteners and uterine cancer.

Entry by Gideon Meyerowitz-Katz

Epidemiologist and PhD candidate

Main takeaway: you probably don’t need to worry about hair straighteners and uterine cancer.

The study by Chang et. al. was part of a larger project called the Sister Study, which involved recruiting a large group of US based women between 2003-2009 who had a sister with breast cancer, inquiring about their health with a follow up over time. The purpose of this project was to see if future health patterns existed to show risks and benefits in terms of cancer. The Chang et al. (2022) looked at about 50,000 women and their reported use of hair products, which included permanent, semi permanent, and temporary hair dyes; bleach; highlights; straighteners; and relaxers, along with future uterine cancer risk. Women who underwent hysterectomies were excluded from the dataset, and associations between product use and uterine cancer were analyzed an average of roughly 10 years after the women enrolled.

The results? The study found no increased risk of uterine cancer associated with the use of hair dye or bleach. Interestingly, they found a reduced risk of uterine cancer for those who regularly got highlights. There was a modest increase in risk for those who reported regularly (4+ times yearly) who used chemical straightening products. The scary media headlines you may have seen in the media blew this risk out of proportion, where an 84% increase risk has been often cited; this is the relative risk in the study. Put in the context of absolute risk however, the increased risk year over year was 0.03%. In other words, for every 10,000 women who never straightened their hair, about 10 got uterine cancer yearly, while for that same number of women who straightener their hair 4+ times yearly, about 17 got uterine cancer yearly during the study. To summarize, despite the scary headlines about this study, they found that most hair products had mostly no impact on cancer risk, while chemical straightening products had a very small increase in risk.

I’m skeptical of the results of this study for a number of reasons, not least because the study had some large issues that are hard to reconcile with. For example, the increased cancer risk almost entirely disappeared in women who reported frequent physical activity, which may indicate this is simply the result of systematic differences between women who do or do not regularly straighten their hair, rather than their product use habits. The risks were also fairly small, and mostly for women who very regularly used chemical straightening products. If you chemically straighten your hair 2x per year, the associated risk is far lower. Finally, I’m not sure you can actually assess the risk in a population where nearly 1/3rd of the women had already had a hysterectomy when the study started. Excluding these women from the dataset is necessary, but also introduces biases in the study design that are hard to overcome. Given that the women who used straighteners were on average much younger than those who didn’t, it’s plausible that this study is simply measuring survivorship bias, because older women who were prone to uterine cancer had already had hysterectomies and had been removed from the sample. At the end of the day, while the findings may be real, it’s also entirely plausible that the study is simply measuring differences between people who do and do not report getting their hair straightened a lot. 

Should you as an individual stop using chemical straighteners if this is something you do? It’s plausible there’s a fairly small risk, but there’s also a lot of uncertainty. If you’re worried about uterine cancer, or any cancer for that matter, it might be useful to talk to a medical professional about it. Beyond that, based on the study and evidence available, it’s hard to make any definitive statements at all. Ultimately, you probably don’t need to worry about hair straighteners and uterine cancer.

Entry by Crystal Porter, PhD

Hair Scientist at Maine Insights

Takeaways of the study from Chang et al. (2022).

The authors state there is an increased risk of uterine cancer with hair straightener usage. This does not mean that hair straighteners cause uterine cancer. For the population studied, they saw an increase in risk for those who had:

  1. ever used straightening products,
  2. an increased frequency of use (over 4 times per year), and
  3. lower physical activity levels.

Would this study be considered strong evidence? What were the limitations?

The use of the word “strong” is quite subjective; so, what I will do is refer back to the study and state that the findings were statistically significant in showing an association or an increased risk of uterine cancer with the use of hair straighteners. The analyses used were standard in the industry and seemed to be robust in considering covariates and confounders which can influence the study findings and conclusions.

There are always limitations to scientific studies, especially those that involve human subjects. But there are some specific ones that should be considered when interpreting the research findings:

  1. This is an observational study that enrolled sisters of women who had breast cancer but did not have breast cancer themselves. Therefore, the population was not representative of all women, but the results were sound within the population of women who were enrolled in the study.
  2. The researchers had to rely on the subjects’ abilities to accurately recall lifestyle scenarios over the course of a year when answering the questionnaire. Therefore, it is possible that there is recall bias – especially when participants reported on the frequency of hair straightening usage.
  3. The observational study was not designed to control lifestyle variables that can influence outcomes. Therefore, it’s not possible to know if there is more than a risk factor. These types of studies are oftentimes the catalyst to expand efforts and have a more controlled experimental design to understand if there are causative effects of lifestyle and/or environmental conditions.
  4. Information about the type of hair straightener was not available so it is not possible to know the specific products or ingredients that are associated with increased risks.

Should consumers be concerned?

Concerned is a strong word so I will say that consumers should be more cautious. They should be aware of the results and consider what it means for them on an individualized basis. One can err on the side of caution and choose to avoid all types of hair straighteners – especially if there is a family history of cancer. Or one could still use hair straighteners but mitigate risks based on the results of the study by decreasing the frequency of use and making sure one does not have a sedentary lifestyle.

One other consideration for consumers involves them being proactive in understanding how manufacturers test products to ensure safety. That way, they can make informed decisions about the products they use. It also encourages transparency within the cosmetics industry.

Overall thoughts on what this means for cosmetics?

In light of this study’s findings and other collective research that suggests that there are environmental and lifestyle variables that can affect health conditions and outcomes, the cosmetics industry has a legal and ethical obligation to continue to substantiate the safety of ingredients and finished formulations which go beyond the status quo of irritation and inflammation responses. The evolution of study methodologies should incorporate technological advancements that have increased testing sensitivity and research participants must be representative of diverse populations in different environments with varying lifestyles. Cosmetic companies should also consider the use of 3rd party assessors to determine possible health impacts.

Chang, C. J., O’Brien, K. M., Keil, A. P., Gaston, S. A., Jackson, C. L., Sandler, D. P., & White, A. J. (2022). Use of straighteners and other hair products and incident uterine cancer. JNCI: Journal of the National Cancer Institute, 114(12), 1636-1645. https://doi.org/10.1093/jnci/djac165