Expanding the Conversation on Hair Straighteners and Cancer

Environmental exposures are increasingly recognized as important contributors to cancer risk. The authors of a recent analysis from the National Institutes of Health (NIH)–supported Sister Study frame the issue starkly, noting that “approximately 2 million incident cancer cases are estimated to occur in the United States in 2025,”and that “70% [to] 90% of lifetime cancer risk can be attributed to exogenous drivers such as environmental exposures.” Personal care products, including chemical hair straighteners and relaxers, have emerged as a focus of this concern due to their widespread use and repeated application over the life course.¹

This new publication builds on earlier work by the same investigative group, which previously reported associations between frequent hair straightener use and increased incidence of breast, ovarian, and uterine cancers.² In the current analysis, the authors extend their inquiry to a range of nonreproductive malignancies, including pancreatic, thyroid, hematologic, colorectal, and kidney cancers. Although the findings suggest elevated risks for certain cancer types, the expert review highlights important methodological limitations that complicate clinical interpretation.

Study Design and Cohort Characteristics

The analysis uses data from the Sister Study, a large prospective cohort conducted by the National Institute of Environmental Health Sciences. Between 2003 and 2009, more than 50,000 women aged 35 to 74 years were enrolled across the United States and Puerto Rico. All participants were cancer free at baseline but had a sister diagnosed with breast cancer, placing them at elevated baseline cancer risk. After exclusions, 46,287 women were included and followed for a mean of 13.1 years.

Exposure to hair straighteners or chemical relaxers was assessed through a baseline questionnaire, focusing on use within the 12 months preceding enrollment. Participants were grouped into 3 categories: nonusers, those using products 4 times or fewer per year, and those using them more than 4 times per year. Cancer outcomes were self-reported and validated using pathology reports and national registries.

Findings and Initial Interpretation

Compared with nonusers, women reporting hair straightener use had a higher incidence of pancreatic cancer (HR, 2.66), thyroid cancer (HR, 1.71), and possibly non-Hodgkin lymphoma. Associations with kidney cancer were positive but imprecise, and no clear associations were observed for several other cancer types. These findings suggest that the potential health effects of straighteners may extend beyond hormonally mediated cancers.

The authors propose several biologically plausible mechanisms. Formaldehyde and formaldehyde-releasing agents may induce DNA damage and oxidative stress, and phthalates and parabens may contribute to inflammation and endocrine disruption, including effects on thyroid signaling.

Expert Perspectives and Concerns

Despite these findings, experts caution against overinterpretation. According to hair disorders specialist Amy McMichael, MD, a professor of dermatology at Wake Forest University School of Medicine, a key concern is the misclassification of exposure. The study groups together multiple straightening modalities that differ fundamentally in chemistry and biological plausibility.

Chemical relaxers containing sodium or guanidine hydroxide permanently break disulfide bonds within the hair shaft, whereas formaldehyde-based products act through heat-activated cross-linking, and flat-iron straightening may involve heat alone.³ These exposures vary dramatically in scalp penetration, systemic absorption, and tissue injury. Lumping these distinct modalities together, without specifying product type or chemical constituents, makes it difficult to identify which exposures, if any, are driving observed associations.

McMichael notes that the absence of a dermatologist or hair expert as an author likely contributed to this limitation, resulting in a definition of “straightening” that lacks clinical precision. From a dermatologic standpoint, differentiating between chemical, thermal, and combination approaches is essential for meaningful risk assessment.

Another concern relates to how the researchers categorized exposure frequency. The threshold of greater than 4 uses per year does not reflect real-world patterns for many patients. Weekly or even daily heat straightening is common, particularly among women with textured hair, and may represent a substantially different exposure profile than 5 applications per year. Grouping these patterns together risks diluting true associations and complicates dose–response interpretation.

Race, Representation, and Power

The cohort included approximately 8% Black women, despite a substantially higher reported use of hair straighteners in this group. The authors acknowledge that the limited sample size restricted race-stratified analyses. As a result, the study cannot adequately address whether risks differ in populations that are most heavily exposed. This limitation is particularly important given the known racial disparities in both cancer incidence and environmental exposures.

Clinical Implications in Practice

This study adds to a growing body of literature suggesting potential associations between chemical hair straighteners and cancer. However, significant methodological limitations preclude the provision of definitive clinical guidance. As McMichael emphasizes, the current evidence does not support advising patients that use of hair straighteners alone places them at increased risk for a specific cancer diagnosis.

Looking Ahead to Future Steps

This NIH-supported analysis yields noteworthy findings that warrant further investigation into the health effects of hair straighteners, extending beyond reproductive cancers. At the same time, expert critique highlights the need for more precise exposure definitions, improved frequency stratification, greater inclusion of highly exposed populations, and interdisciplinary collaboration. Until such data are available, these findings should be interpreted as hypothesis generating rather than practice changing, underscoring the need for more rigorous, clinically informed research in this area.

References

1. Bailey JT, Chang CJ, Gaston SA, et al. Use of hair straighteners and chemical relaxers and incidence of non-reproductive cancers. J Natl Cancer Inst. Published online September 30,2025. doi:10.1093/jnci/djaf280
2. Chang CJ, O'Brien KM, Keil AP, et al. Use of straighteners and other hair products and incident uterine cancer. J Natl Cancer Inst. 2022;114(12):1636-1645. doi:10.1093/jnci/djac165
3. Hatsbach de Paula JN, Basílio FMA, Mulinari-Brenner FA. Effects of chemical straighteners on the hair shaft and scalp. An Bras Dermatol. 2022;97(2):193-203. doi:10.1016/j.abd.2021.02.010