A chemical hair straightener is a product that uses strong alkaline or acidic compounds to permanently break and reshape the internal structure of your hair, turning curls or waves into a straighter pattern. These products work at a molecular level, altering the protein bonds that give hair its natural shape. The results last until the treated hair is cut off or grows out, which is why they’re often called “permanent” straighteners.
How Chemical Straighteners Change Hair Structure
Hair gets its shape from bonds between sulfur-containing amino acids in the protein keratin. These connections, called disulfide bonds, act like rungs on a twisted ladder, holding each strand in its natural curl pattern. Chemical straighteners break those rungs apart, and when the hair is smoothed into a straight position during processing, the bonds either reform in the new shape or are permanently converted into a different type of bond altogether.
The specific chemical reaction depends on which type of straightener you use. Hydroxide-based products (the traditional “relaxers”) trigger a reaction called lanthionization, which irreversibly converts the original bonds into new ones. This is a one-way change. Thioglycolate-based products break the same bonds but in a reversible way, then use an oxidizing agent like hydrogen peroxide to lock the hair into its new straight shape. Both approaches are permanent on the treated hair, but they achieve that permanence through different chemistry.
Types of Chemical Straighteners
Lye Relaxers
These contain sodium hydroxide, one of the strongest alkaline substances used in personal care. They operate at a pH of 12 to 13, which is extremely basic. Lye relaxers work fast and produce the straightest results, often described as “bone straight,” but they’re also the most likely to irritate the scalp. They’re typically used in professional settings where the stylist can control timing precisely.
No-Lye Relaxers
No-lye formulas use calcium hydroxide mixed with a liquid activator containing guanidine carbonate. When combined, these two ingredients react to form guanidine hydroxide, which is the actual straightening agent. The pH sits around 11, lower than lye relaxers, making them gentler on sensitive scalps. They’re the more common choice for at-home kits. The tradeoff is that calcium deposits can build up on the hair over time, leaving it feeling dry or dull without proper clarifying.
Thioglycolate Straighteners
Ammonium thioglycolate works at a much milder pH of 9 to 9.3. It breaks the same disulfide bonds as hydroxide relaxers but without the lanthionization reaction, meaning the chemistry is fundamentally different. One practical advantage: thioglycolate-based straighteners can be used on hair that’s already been colored or permed. Hydroxide and thioglycolate products are not compatible with each other, and using both can cause severe breakage.
Formaldehyde-Based Smoothing Treatments
Often marketed as “keratin treatments” or “Brazilian blowouts,” these products rely on formaldehyde or chemicals that release formaldehyde when heated. They coat and seal the hair shaft rather than breaking bonds in the same way relaxers do, and results typically last a few months rather than being truly permanent. Formaldehyde is a known carcinogen, and salon workers in particular face exposure risks from the fumes released during flat-ironing. Advocacy groups petitioned the FDA to ban formaldehyde in hair straightening products back in 2021, but as of early 2025, the agency has repeatedly pushed back its deadline for even proposing a rule. The target date has shifted from October 2023 to April 2024, then to November 2024, and most recently to December 2025, with no formal consequences for missing it.
What Happens During a Relaxer Application
The hair is not shampooed beforehand, because washing can create micro-irritation on the scalp that makes chemical burns more likely. A protective cream is applied along the hairline and on the scalp. The stylist then sections the hair and applies the relaxer using a brush, staying about half an inch away from the scalp to minimize direct skin contact.
Processing time depends on your hair type. Fine or color-treated hair may need only 10 minutes, while coarse hair can take up to 18 minutes. The stylist checks progress every few minutes by isolating small sections and testing how much the curl has relaxed. Going too long risks serious damage.
Once the desired straightness is reached, the hair is rinsed with warm water for at least five minutes. A neutralizing shampoo follows, which lowers the pH back to a safe range and stops the chemical reaction. Some neutralizing shampoos change color (turning pink, for instance) in areas where relaxer residue remains, signaling the need for more rinsing. The process is repeated until the lather runs clear.
How Long Results Last
The straightened portion of your hair stays straight permanently. It won’t revert to curls or waves on its own. What changes over time is the new growth at your roots, which comes in with your natural texture. Most people notice enough new growth to need a touch-up within four to six months, depending on how fast their hair grows. Fine to medium hair tends to show root contrast sooner (around four months), while thicker hair may hold the appearance longer.
Touch-ups should only be applied to the new growth. Overlapping relaxer onto previously treated hair is one of the most common causes of breakage and damage, since that hair has already had its bonds permanently altered.
Scalp and Hair Damage
Chemical straighteners carry real risks for both your scalp and your hair. In studies on relaxer users, the two most frequently reported problems were burns and hair loss. About 25% of users in one study reported burning pain on the scalp during processing. Other documented effects include flaking, allergic reactions, skin thinning, and contact dermatitis that can progress to infection in severe cases.
Hair loss from relaxers takes several forms. Some people develop scarring alopecia, particularly in the crown area, where the hair follicles are permanently destroyed and the hair does not grow back. In African American girls, prior relaxer use doubled the risk of traction alopecia, and when combined with tight braiding styles, the risk increased more than fivefold. Repeated relaxer treatments have also been linked to a condition where the hair shaft becomes so fragile it snaps near the root, sometimes resulting in widespread hair loss.
Concerns Beyond Hair Damage
Chemical straighteners contain more than just the active straightening ingredient. Testing of relaxer kits has detected several types of parabens and phthalates, both of which are endocrine disruptors, meaning they can interfere with hormone signaling in the body. These chemicals are absorbed through the skin during application.
A large study from the National Institute of Environmental Health Sciences found that women who used hair straightening products more than four times in the previous year were more than twice as likely to develop uterine cancer compared to women who never used them. To put the numbers in perspective: 1.64% of women who never used straighteners were estimated to develop uterine cancer by age 70, compared to 4.05% of frequent users. That’s a significant jump in absolute terms, and the doubling of risk drew considerable attention when the findings were published. The study did not distinguish between specific product types, so it’s not yet clear which chemicals are driving the association.
Choosing Between Straightener Types
Your choice depends on your hair’s current condition, your scalp sensitivity, and what other chemical treatments you’ve had. If your hair has been previously colored or permed, thioglycolate-based products are the only compatible option. If you have a sensitive scalp, no-lye relaxers are generally better tolerated than lye formulas, though they can leave mineral buildup that needs regular clarifying. Lye relaxers deliver the most dramatic straightening but demand the most careful application.
Mixing chemical systems is dangerous. Applying a hydroxide relaxer to hair previously treated with thioglycolate, or vice versa, causes breakage that no amount of conditioning can fix. If you don’t know what was last used on your hair, a stylist can perform a strand test to check compatibility before proceeding.