Chemical hair straighteners and relaxers, not heat-based flat irons, are the products linked to cancer. A major NIH study found that 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. The risk extends to ovarian cancer, breast cancer, and potentially several other types.
Chemical Straighteners, Not Flat Irons
This is an important distinction. The cancer risk comes from chemical hair straightening and relaxing treatments, the kind that use formaldehyde, lye, phthalates, parabens, and other compounds to permanently or semi-permanently alter hair texture. A standard plug-in flat iron that uses heat to temporarily smooth hair is not associated with cancer. Flat irons work by breaking hydrogen bonds in the hair shaft, which revert back as soon as moisture returns. No chemicals enter your body during that process.
Chemical straighteners and relaxers work differently. They use strong chemicals to permanently restructure hair, and those chemicals can be absorbed through the scalp, which absorbs substances more readily than other areas of skin. Repeated treatments can also damage the scalp, making it even more permeable. Inhalation is another route of exposure, particularly with formaldehyde-containing products, both for the person getting the treatment and for salon workers nearby.
Which Cancers Are Linked
The strongest evidence involves uterine cancer. The NIH’s Sister Study, which followed tens of thousands of women, estimated that 1.64% of women who never used chemical straighteners would develop uterine cancer by age 70. For frequent users (more than four times per year), that number jumped to 4.05%. That roughly 2.5 times higher risk makes uterine cancer the most clearly elevated risk from these products.
Ovarian cancer showed a similarly large increase. Women who frequently used straighteners or relaxers had about 2.2 times the risk of ovarian cancer compared to non-users. Breast cancer risk was also elevated, though by a smaller margin: about 30% higher among frequent users. Lye-based relaxers used seven or more times per year were specifically tied to a 37% increase in estrogen receptor-positive breast cancer.
Newer research from the same study cohort has also flagged possible links to pancreatic cancer, thyroid cancer, and non-Hodgkin lymphoma among frequent users, though these associations are less certain and need further confirmation.
The Chemicals Driving the Risk
Several ingredients in chemical straighteners act as endocrine disruptors, meaning they interfere with the body’s hormone signaling. This is likely why the cancers most strongly linked to these products (uterine, ovarian, breast) are hormone-sensitive cancers. The key offenders include formaldehyde and formaldehyde-releasing compounds like methylene glycol, which are common in keratin treatments and smoothing products. Phthalates and parabens, often hidden under the catch-all label “fragrance,” are also found in many relaxers.
The FDA has proposed a rule to ban formaldehyde and formaldehyde-releasing chemicals from hair straightening and smoothing products sold in the United States. As of 2024, this rule was still in the proposed stage and had not yet taken effect, meaning these products remain available on the market.
How Frequency Affects Risk
The threshold that consistently appears across studies is more than four uses per year, roughly every two to three months or more often. Below that frequency, the associations weaken considerably. But for many people who use chemical relaxers, four times a year is on the low end. Touch-ups every six to eight weeks are common, which puts users well into the high-frequency category.
Duration of use matters too. Data from the Black Women’s Health Study found that women who used straighteners for 20 or more years had about 71% higher uterine cancer risk compared to non-users. Even moderate use over long periods showed elevated risk.
Why Black Women Are Disproportionately Affected
Chemical relaxers are overwhelmingly marketed to and used by Black women and girls, often starting in childhood. Societal pressure to conform to straight-hair standards has driven widespread, long-term use of these products in Black communities for decades. This means Black women bear a disproportionate share of the health consequences.
Researchers at Harvard’s T.H. Chan School of Public Health have spent over 20 years documenting links between chemicals in hair products marketed to Black women and serious health problems, including uterine fibroids, preterm birth, infertility, and the cancers described above. Many of the researchers leading this work are themselves Black women who grew up using relaxers, a personal connection that has driven two decades of advocacy and study. The cancer risk from chemical straighteners is not just a consumer safety issue. It is a health equity issue shaped by cultural pressure and the cosmetics industry’s targeting of specific communities.
Lower-Risk Alternatives
If you want straight hair without the chemical exposure, a heat-based flat iron is the simplest swap. It carries no cancer risk from chemical absorption. The tradeoff is that results are temporary and humidity will undo them, but it is genuinely chemical-free.
Some products marketed as “formaldehyde-free” keratin treatments still contain related compounds. One product called Liquid Keratin, for example, lists glyoxal (a formaldehyde relative) on its ingredient label despite advertising itself as a safe alternative. If you’re evaluating salon treatments, ask specifically whether the product contains formaldehyde, methylene glycol, or any formaldehyde-releasing ingredient. Reading the full ingredient list yourself is more reliable than trusting front-of-package marketing claims.