Keratin itself is a structural protein that makes up your hair, skin, and nails, and it is not harmful. The real concern is what gets mixed into “keratin treatments,” the popular salon and at-home hair-straightening products that borrow the protein’s name. These formulations often contain formaldehyde or chemicals that release it when heated, along with newer ingredients like glyoxylic acid that carry their own risks. The distinction between the protein and the product matters enormously, because the two get conflated in ways that leave people either unnecessarily worried about a basic ingredient or dangerously unconcerned about a chemical procedure.
The Protein Versus the Treatment
Keratin is the most abundant protein in human hair, accounting for the bulk of each strand’s structure. When cosmetic companies sell “keratin treatments,” they are really selling a chemical straightening procedure that coats or restructures hair fibers. The keratin protein in the formula is there mostly for conditioning and marketing. The heavy lifting, the part that actually straightens curly or frizzy hair, is done by reactive chemicals that alter keratin’s internal bonds. Those chemicals are where virtually all the health concerns originate.
An expert panel convened by the cosmetic industry reviewed eight keratin-derived ingredients used in personal care products and concluded they are safe at the concentrations typically found in shampoos, conditioners, and skin creams.1PubMed. Safety Assessment of Keratin and Keratin-Derived Ingredients as Used in Cosmetics A pilot study even tested a keratin-based topical cream on breast cancer patients undergoing radiation therapy and reported no adverse events.2PubMed Central. Randomized Pilot Study of a Keratin-based Topical Cream for Radiation Dermatitis in Breast Cancer Patients So when you see keratin listed as an ingredient in a regular shampoo or lotion, there is no cause for alarm. The danger lives in the straightening treatments, not in the protein they are named after.
Formaldehyde and Why It Shows Up
Formaldehyde is a known carcinogen and respiratory irritant, and it has been a workhorse ingredient in professional hair-straightening products for years. It works by cross-linking proteins in the hair shaft, locking strands into a smoother shape. The problem is that when a stylist blow-dries and flat-irons treated hair, the heat releases formaldehyde vapor into the air at concentrations that can be startlingly high. One study measuring airborne formaldehyde during four professional straightening treatments found mean levels ranging from 0.08 to about 3.5 parts per million during blow-drying and up to about 1 ppm during flat-ironing.3PubMed. Characterization of formaldehyde exposure resulting from the use of four professional hair straightening products To put that in context, the U.S. occupational exposure limit for formaldehyde is 0.75 ppm averaged over an eight-hour workday. A single treatment can blow past that ceiling in minutes.
Many products are labeled “formaldehyde-free,” but that label has historically been unreliable. Some formulations contain methylene glycol or other compounds that are chemically stable in the bottle but release formaldehyde the moment they are heated. A scoping review of chemical hair-straightening products found that keratin treatments were more frequently associated with formaldehyde or formaldehyde-releasing preservatives than other types of straighteners like relaxers.4PubMed Central. Chemicals of Concern in Chemical Hair Straightening Products: A Scoping Review The takeaway is that a “formaldehyde-free” claim on a keratin treatment bottle does not guarantee you will not be breathing in formaldehyde during the application.
What Formaldehyde Does to Your Lungs
The most immediate health risk from keratin treatments is respiratory. Formaldehyde vapor irritates the lining of the nose, throat, and lungs. Short-term exposure during a single salon visit can cause burning eyes, a sore throat, coughing, and headaches. Repeated exposure over months or years is where the damage gets more serious.
A cross-sectional study of salon workers who regularly performed keratin treatments found that their lung function, measured by standard breathing tests, was significantly lower than predicted norms.5PubMed Central. Impact of occupational exposure to formaldehyde in Keratin hair treatments on lung function among salon workers: A cross-sectional study These were not people with pre-existing lung conditions; the deficits were linked to their occupational chemical exposure. Another study looking at hairdressers’ respiratory complaints found that workers who had been in the profession for 12 or more years reported shortness of breath, wheezing, coughing, and excess mucus at notably higher rates, with four hairdressers developing asthma after entering the profession. The researchers noted that keratin application was among the procedures most strongly associated with worsening symptoms.6PubMed Central. Breathing the Job: Impaired Pulmonary Function in Hairdressers due to Occupational Chemical Exposure
If you are a customer getting a treatment once or twice a year, your cumulative exposure is far lower than a stylist who performs these treatments daily. But even a single session in a poorly ventilated salon can leave you with irritated airways for hours, and people with asthma or other respiratory conditions should be especially cautious.
Scalp and Hair Damage
The irony of keratin treatments is that they are marketed as restorative, yet the process itself can damage the very hair it claims to repair. The chemicals used to restructure hair bonds, combined with the high heat from flat irons that often operate above 200°C, can degrade hair keratin. Research using thermal analysis techniques has shown that flat-ironing at these temperatures converts the protein’s natural structure into a less resilient form and causes outright protein degradation.7PubMed. The effect of various cosmetic pretreatments on protecting hair from thermal damage by hot flat ironing Hair may look and feel smoother immediately after treatment, but repeated sessions can leave it increasingly brittle and prone to breakage.
The scalp does not always escape unscathed either. Dermatologists have documented cases of psoriasis-like eczema on the scalp, face, neck, and upper trunk following Brazilian keratin treatments. In some cases, the reactions included pustules. Skin biopsies showed inflammatory patterns suggesting that formaldehyde can provoke reactions beyond simple allergic contact dermatitis, possibly triggering the skin through other immune pathways.8Anais Brasileiros de Dermatologia. Effects of chemical straighteners on the hair shaft and scalp If you notice redness, flaking, or itching on your scalp or around your hairline after a keratin treatment, it is worth seeing a dermatologist rather than assuming it will resolve on its own.
The Glyoxylic Acid Problem
As regulators and consumers pushed back against formaldehyde, the hair-straightening industry pivoted to glyoxylic acid as a supposedly safer alternative. These products are now widely marketed as “formaldehyde-free” or “acid-based” keratin treatments and have gained popularity fast. But a new and alarming risk has emerged: kidney injury.
A case series from a medical toxicology service documented 13 patients who had been exposed to glyoxylic acid-containing hair straightening products. Twelve of the thirteen developed acute kidney injury. In about a quarter of cases, urinalysis revealed calcium oxalate crystals, suggesting that glyoxylic acid was being metabolized into oxalate, which then crystallized in the kidneys and caused damage.9PubMed. Hair straightening and acute kidney injury: a case series from a medical toxicology service A separate case report described a teenager who developed severe kidney injury requiring dialysis shortly after using a product labeled formaldehyde-free. A kidney biopsy in that case showed acute tubular damage and microcalcifications consistent with oxalate deposition.10Clinical Kidney Journal. Recent use of formaldehyde-‘free’ hair straightening product and severe acute kidney injury
These reports are still emerging, and the total number of documented cases is small relative to how many people use these products. But the severity of the outcomes, some patients needed dialysis, makes this a risk worth knowing about. The route of exposure appears to be inhalation or possibly skin absorption of glyoxylic acid during the heat-activation step. If you are considering a “formaldehyde-free” keratin treatment, it is worth asking what the active straightening ingredient actually is, because swapping one hazard for another is not an improvement.
Cancer Concerns
The most unsettling research on chemical hair straighteners involves cancer. A large prospective study followed tens of thousands of women over roughly a decade and found that those who reported using hair straighteners in the previous year had about 80% higher risk of developing uterine cancer compared to women who did not use them. The relationship followed a dose-response pattern: infrequent use, four times a year or fewer, carried a modest and statistically uncertain increase in risk, while frequent use, more than four times per year, was associated with roughly two and a half times the risk.11JNCI: Journal of the National Cancer Institute. Use of Straighteners and Other Hair Products and Incident Uterine Cancer In absolute terms, among women who never used straighteners, about 1.6% were predicted to develop uterine cancer by age 70. For frequent users, that estimate rose by roughly 2.4 percentage points.
Research on this topic has since expanded beyond uterine cancer. Hair straighteners and chemical relaxers have also been linked to increased incidence of breast and ovarian cancers in observational studies.12PubMed Central. Use of hair straighteners and chemical relaxers and incidence of non-reproductive cancers These findings do not prove causation on their own. They are observational, meaning the researchers tracked what people did and what happened to them rather than randomly assigning treatments. But the consistency of the association across multiple cancer types, plus the known presence of formaldehyde and endocrine-disrupting chemicals in these products, gives the findings biological plausibility that is hard to dismiss.
One important nuance: these studies measured the effects of chemical straightening products broadly, not keratin-branded treatments specifically. The chemical profiles overlap substantially, though. Keratin treatments tend to contain formaldehyde or its precursors, while relaxers more often contain endocrine disruptors like phthalates and parabens. Both product categories appear to carry risk, but possibly through different chemical mechanisms.
Salon Workers Face Higher Exposure
Most of the discussion so far has focused on consumers, but salon professionals face a different risk calculus entirely. A consumer might get a keratin treatment a few times a year. A stylist might perform several treatments per week, standing in a cloud of formaldehyde vapor for hours at a time. The scoping review mentioned earlier highlighted that several studies found the potential for considerably higher chemical exposures in salon workers compared to their clients.4PubMed Central. Chemicals of Concern in Chemical Hair Straightening Products: A Scoping Review
The lung function data bear this out. Stylists who performed keratin treatments as a regular part of their job showed measurably worse breathing capacity than what would be expected for their age and size.5PubMed Central. Impact of occupational exposure to formaldehyde in Keratin hair treatments on lung function among salon workers: A cross-sectional study Respiratory complaints among hairdressers generally accumulated with years in the profession, and the symptoms were worst among those who had been working 12 years or more.6PubMed Central. Breathing the Job: Impaired Pulmonary Function in Hairdressers due to Occupational Chemical Exposure Ventilation matters enormously. A well-ventilated salon with local exhaust systems near the styling station will have much lower airborne chemical concentrations than a cramped, closed room. If you are a stylist, investing in proper ventilation and wearing a respirator rated for organic vapors during heat-activation steps are meaningful protective measures.
Are Newer “Acid-Based” Products Actually Safer?
Glyoxylic acid was introduced as a direct replacement for formaldehyde, and in one narrow sense it delivered: one study measuring airborne formaldehyde release from glyoxylic acid-based products during treatment found levels well below regulatory limits.13Toxicology and Applied Pharmacology. Quantification of the exposure of consumers and hairdressers to formaldehyde and oxalic acid after application of hair straightening products containing glyoxylic acid So you may breathe in less formaldehyde. But as the kidney injury cases make clear, lower formaldehyde does not mean lower overall risk. The glyoxylic acid itself, once absorbed and metabolized, can produce oxalate that damages the kidneys. The trade-off is not straightforward, and calling these products “safe” based solely on reduced formaldehyde vapor is misleading.
Research into genuinely safer alternatives is ongoing. Some groups are exploring marine-derived proteins like fish collagen peptides as bio-based hair repair materials. Lab testing of one such composite showed it could improve damaged hair’s protein structure and reduce oxidative damage without harsh chemicals.14PubMed Central. Composite Fish Collagen Peptide-Based Biopolymer Emulsion for Keratin Structure Stabilization and Hair Fiber Repair These are early-stage products, not yet widely available in salons, but they represent a different philosophy: conditioning and repairing hair rather than chemically restructuring it. Whether they can deliver the dramatic smoothing that consumers associate with keratin treatments remains to be seen.
What About Keratin Supplements?
Keratin also shows up in oral supplement form, typically marketed for stronger hair, skin, and nails. The evidence here is thin and unexciting. A controlled study of keratin supplementation in physically active individuals found no significant changes in blood parameters, oxygen consumption, or exercise performance.15PubMed Central. The effect of chronic soluble keratin supplementation in physically active individuals on body composition, blood parameters and cycling performance No serious adverse effects were reported either, which is consistent with what you would expect from ingesting a common dietary protein. Keratin supplements are unlikely to hurt you, but the evidence that they meaningfully improve hair or nail quality is limited. Your body breaks down ingested keratin into its component amino acids just like any other protein; it does not shuttle intact keratin molecules to your hair follicles.
Racial Disparities in Exposure
The health risks of chemical hair straightening are not distributed equally. Black women in the United States use chemical straighteners and relaxers at substantially higher rates than women of other racial groups, which means they bear a disproportionate share of any health consequences. The uterine cancer study noted that the association between straightener use and cancer was consistent across racial groups, but because Black women were more likely to be frequent users, the absolute burden of excess risk falls more heavily on them.11JNCI: Journal of the National Cancer Institute. Use of Straighteners and Other Hair Products and Incident Uterine Cancer This pattern has prompted calls from public health researchers and advocacy organizations for stronger regulation of these products and better labeling of their chemical contents.
The issue is compounded by the fact that many of these products are marketed specifically to Black consumers with claims of safety and “natural” formulations. When a product labeled “formaldehyde-free” turns out to release formaldehyde upon heating, or when a glyoxylic acid product causes kidney injury, the gap between marketing and reality has real consequences for the communities most likely to use it.
Practical Steps If You Still Want the Treatment
Not everyone reading about these risks will decide to stop getting keratin treatments, and that is a personal decision. If you choose to continue, a few things can meaningfully reduce your exposure:
- Ask about ingredients: Request the Safety Data Sheet for the specific product. Look for formaldehyde, methylene glycol, formalin, and glyoxylic acid. If the salon cannot or will not tell you what is in the product, that is a red flag.
- Ventilation matters: Choose a salon with good airflow. An open window is better than nothing, but a proper local exhaust ventilation system near the styling chair is far more effective at pulling chemical vapors away from your breathing zone.
- Frequency counts: The cancer data showed a clear dose-response relationship: more frequent use meant higher risk. If you space treatments further apart, you reduce your cumulative exposure.
- Watch for symptoms: Eye irritation, throat burning, or headaches during the treatment are signs that formaldehyde levels are high. Persistent scalp irritation, flaking, or unusual rashes afterward warrant a visit to a dermatologist.
- Consider alternatives: Keratin-infused conditioners and leave-in products deliver some smoothing benefit without the heat-activated chemical straightening step. They will not give you the same dramatic results, but they also will not fill the room with formaldehyde vapor.
Why Regulation Has Been Slow
In many countries, cosmetic products receive far less regulatory scrutiny than drugs or food additives. In the United States, the FDA has limited authority to require pre-market safety testing of cosmetics. The agency has issued warnings about formaldehyde in hair-straightening products and has conducted its own testing, but it cannot pull a product from shelves as easily as it can recall a contaminated food. The European Union has set stricter limits on formaldehyde concentrations in cosmetic products, but enforcement varies, and products purchased online may not comply with any particular jurisdiction’s rules. This regulatory patchwork means that consumers often have to be their own safety investigators, reading ingredient lists and watching for the chemical synonyms that disguise formaldehyde’s presence. It is an unreasonable burden to place on someone who just wants smoother hair, but until regulation catches up, it is the reality.