Chemical hair straighteners and relaxers are linked to higher rates of several cancers, including uterine, breast, and ovarian cancer. The risk increases with frequent use. Women who used chemical straightening products more than four times a year were more than twice as likely to develop uterine cancer compared to women who never used them, based on a large NIH-funded study that followed over 33,000 women for roughly 11 years.
These findings apply specifically to chemical straightening and relaxing products, not necessarily to heat-only flat irons. The distinction matters, and the details below explain why.
Chemical Straighteners and Uterine Cancer
The strongest evidence connects chemical hair straighteners to uterine cancer. A study published in the Journal of the National Cancer Institute tracked women enrolled in the Sister Study, a large research project run by the National Institutes of Health. Women who reported any use of straightening products in the past year had an 80% higher rate of uterine cancer than non-users. For frequent users (more than four times per year), the risk jumped to about 2.5 times higher.
To put that in perspective, uterine cancer is still relatively uncommon. The baseline lifetime risk is around 3%. Even a doubling of that risk brings it to roughly 6%. So the absolute risk for any individual remains low, but the relative increase is striking and was statistically significant. These were the first epidemiologic findings linking straightening products to uterine cancer.
Breast and Ovarian Cancer Risks
Chemical straighteners have also been tied to breast cancer. Women who used these products at least every five to eight weeks were about 30% more likely to develop breast cancer, according to data from the same NIH cohort. That association held for both Black and white women, though straightener use was far more common among Black participants, meaning a larger share of that population is exposed to the risk.
For ovarian cancer, occasional use didn’t show a clear link. But frequent use, again defined as more than four times per year, was associated with roughly double the risk of ovarian cancer. More recent analyses from the same cohort also found suggestive links to pancreatic cancer, thyroid cancer, and possibly non-Hodgkin lymphoma, though these findings are newer and less established.
What’s in These Products
Chemical straighteners contain ingredients that can disrupt your body’s hormonal system. Three categories of concern stand out: phthalates, parabens, and cyclosiloxanes. These compounds can mimic or interfere with estrogen and other hormones, which is particularly relevant for cancers of the uterus, breast, and ovaries, all of which are hormone-sensitive tissues.
Many straightening and smoothing products also contain formaldehyde or chemicals that release formaldehyde when heated. The International Agency for Research on Cancer classifies formaldehyde as a known human carcinogen. When a stylist applies these products and then uses heat to seal them, formaldehyde gas is released into the air. If the salon isn’t well ventilated, everyone in the room inhales it. At concentrations above 0.1 parts per million, formaldehyde irritates the eyes, nose, and lungs. Chronic exposure is associated with increased rates of headaches, asthma, skin reactions, and possibly cancer.
The FDA has acknowledged this problem. In response to a citizen petition filed in 2021, the agency moved toward proposing a rule that would ban formaldehyde and formaldehyde-releasing chemicals in hair straightening and smoothing products sold in the United States. As of 2024, this rule was in the proposed stage and had not yet been finalized.
How These Chemicals Enter Your Body
Your scalp is highly absorbent compared to skin on other parts of your body. Chemical straighteners are applied directly to the hair and scalp, often for extended periods, and the process frequently causes irritation, burns, or small lesions that make absorption even easier. Studies from Kenya found that 67% of women with Afro-textured hair reported both local and systemic effects after using various relaxer brands. Scalp reactions including eczema, inflammation, pain, and chemical burns have been widely documented.
Many users dismiss these reactions as a normal part of the process. That normalization may be masking the extent of chemical exposure. Irritant contact dermatitis from formaldehyde, for instance, is often treated as expected rather than as a sign that a harmful substance is penetrating the skin. In severe cases, the chemical damage can progress to permanent scarring and hair loss in affected areas.
Heat Styling vs. Chemical Straightening
The cancer risks identified in research are tied to chemical products, not to the heat itself from a flat iron or blow dryer. A plain flat iron used on dry hair without any chemical product doesn’t expose you to formaldehyde, phthalates, or parabens. However, the research has a notable limitation: the study questionnaires asked participants about “straightening, relaxing, or pressing” their hair, and some women may have interpreted “pressing” as using a flat iron without chemicals. This means some non-chemical users could have been grouped with chemical users, which would actually dilute the observed risk rather than inflate it.
If you want straighter hair without the chemical exposure, heat-only tools are a lower-risk option. The trade-off is that heat styling is temporary and can cause hair damage over time, but it doesn’t carry the same hormonal disruption or carcinogen exposure as chemical treatments.
Who Is Most Affected
The health impact of chemical straighteners falls disproportionately on Black women. In the Sister Study, straightener use was far more prevalent among Black participants. Cultural expectations around hair texture and workplace appearance standards contribute to more frequent and earlier use of these products, often starting in childhood or adolescence. Because the cancer risk scales with frequency of use, populations that use these products more often face a compounded burden.
This isn’t just a matter of individual choice. The availability and marketing of chemical straighteners has historically targeted Black communities, and the regulatory gap around formaldehyde in these products has persisted for decades. The proposed FDA ban, if finalized, would represent the first federal restriction on these ingredients in hair products.