Several common hair products contain chemicals linked to increased cancer risk, including chemical hair straighteners and relaxers, keratin smoothing treatments, permanent hair dyes, and some aerosol dry shampoos. The strongest evidence connects chemical hair relaxers to uterine cancer and formaldehyde-releasing smoothing treatments to cancers of the nose and throat. Here’s what the research shows for each product type and what to look for on labels.
Chemical Hair Relaxers and Straighteners
Chemical hair relaxers carry the most alarming recent findings. A large NIH-funded study tracking tens of thousands of women found that those who used chemical straighteners more than four times in the previous year had roughly 2.5 times the risk of developing uterine cancer compared to women who never used them. To put that in perspective, about 1.6% of women who never used straighteners were predicted to develop uterine cancer by age 70. For frequent users, the estimated risk was about 4%, more than double.
A separate study from Boston University’s Black Women’s Health Study confirmed these findings in a population where relaxer use is especially common. Among postmenopausal Black women, those who used chemical hair relaxers more than twice a year or for more than five years had a greater than 50% increased risk of uterine cancer, even after researchers adjusted for other risk factors. Black women already face higher rates of aggressive uterine cancer subtypes and are nearly twice as likely to die from the disease compared to non-Hispanic white women, making this exposure particularly concerning.
The cancer link likely involves endocrine-disrupting chemicals found in relaxer formulas, including phthalates and other compounds that mimic or interfere with the body’s hormones. These chemicals can bind to estrogen receptors, disrupt hormone production, or alter the way hormone signals are processed. Because the uterus is highly sensitive to estrogen, repeated chemical exposure through scalp absorption may fuel the growth of hormone-responsive cancers over time.
Keratin Smoothing Treatments
Many professional keratin or “Brazilian blowout” treatments release formaldehyde gas when heated with a flat iron. The National Toxicology Program classifies formaldehyde as “known to be a human carcinogen,” a designation it upgraded in 2011 based on strong evidence linking the chemical to cancers of the nasal passages, sinuses, and a type of blood cancer. The International Agency for Research on Cancer has reached a similar conclusion.
Some product labels avoid the word “formaldehyde” entirely. Instead, they list methylene glycol, formalin, or methanediol. These are essentially the same substance in a different chemical state. At room and body temperatures, methylene glycol is the dominant form of formaldehyde in liquid solutions, and it converts back to formaldehyde gas when the product is heated during application. There is no separate, safer cancer classification for methylene glycol. A label that says “formaldehyde-free” but lists methylene glycol still poses the same risk.
The FDA has proposed a rule to ban formaldehyde and formaldehyde-releasing chemicals in hair smoothing and straightening products sold in the United States. As of late 2024, this rule was still in the proposed stage and had not yet taken effect, meaning these products remain on the market.
Permanent Hair Dyes
Permanent and semi-permanent hair dyes contain aromatic amines, a class of chemicals that can be absorbed through the scalp. The cancer risk from personal hair dye use at home appears to be small for most people, but occupational exposure tells a clearer story. A major analysis of 42 studies found that hairdressers and barbers had a 34% higher overall risk of bladder cancer. The risk climbed with years on the job: those who had worked as hairdressers for 10 years or more were 70% more likely to develop bladder cancer than people who had never worked in the profession.
Male hairdressers showed a higher risk (52% increase) than female hairdressers (25% increase), possibly because male barbers historically used darker, more concentrated dye formulations. Importantly, these elevated risks held up even after adjusting for smoking, which is a major bladder cancer risk factor on its own. For consumers who dye their hair at home a few times a year, the exposure is far lower than what a professional stylist accumulates over a career, but the chemicals involved are the same.
Aerosol Dry Shampoos
In 2022, independent laboratory testing of 148 batches of dry shampoo from 34 brands found widespread benzene contamination. Benzene is a known carcinogen linked to leukemia and other blood cancers. Three lots from one brand contained more than 100 parts per million (ppm) of benzene in their spray. Eleven lots from three brands exceeded 20 ppm. For reference, the FDA’s limit for benzene in certain drug products is 2 ppm, based on a daily lifetime exposure threshold of 20 micrograms per day.
The contamination is not an intentional ingredient. Benzene appears to form as a byproduct of the propellant systems used in aerosol cans, which means it won’t show up on the ingredient label. Several major brands issued voluntary recalls following these findings. If you have older cans of aerosol dry shampoo, checking the brand’s recall list is worth your time. Non-aerosol powder dry shampoos don’t carry this same risk.
Endocrine Disruptors Across Products
Beyond the products above, many everyday shampoos, conditioners, and styling products contain low levels of endocrine-disrupting chemicals. Phthalates, often hidden under the word “fragrance” on labels, can act as estrogen mimics or interfere with the hormonal feedback loop that regulates reproductive function. Research has linked phthalate exposure to endometriosis, early puberty, and disruptions to the reproductive system, all of which can influence long-term cancer risk in hormone-sensitive tissues like the breast and uterus.
Metals found in some personal care products can similarly activate estrogen pathways and trigger stress responses in cells. The concern with these chemicals is not that a single use causes cancer. It’s that daily, repeated exposure over years creates a cumulative hormonal burden the body was never designed to handle. Products you leave on your skin or scalp, like styling creams and leave-in conditioners, deliver more sustained exposure than rinse-off products like shampoo.
How to Identify Risky Ingredients
Labels on hair products can be difficult to decode. Here are the key terms to watch for:
- Formaldehyde and its aliases: methylene glycol, formalin, methanediol, methanal. Any of these in a smoothing or straightening product means formaldehyde will be released during heat styling.
- Fragrance: This single word can legally cover dozens of undisclosed chemicals, including phthalates. Products labeled “phthalate-free” or that list individual scent components are more transparent.
- Lye and no-lye relaxers: Both types of chemical relaxers have been associated with uterine cancer risk. “No-lye” does not mean chemical-free. It simply uses a different base ingredient to break down hair bonds.
For aerosol products, there is no way to tell from the label whether benzene contamination is present, since it’s a manufacturing byproduct rather than an ingredient. Choosing non-aerosol alternatives eliminates that particular risk. For hair straightening, heat-based tools without chemical treatments, or products verified to be free of formaldehyde-releasing compounds, are safer options.