What Actually Happens to Hair During a Perm

A permanent wave — commonly called a perm — is a two-stage chemical treatment that physically restructures the interior of each hair strand. Unlike heat styling, which temporarily rearranges hydrogen bonds that reform as soon as moisture returns, a perm targets the much stronger disulfide bonds that give hair its natural shape. The result is a change that persists through washing because the bonds themselves have been broken and rebuilt in a new configuration.

The chemistry involved is the same whether the goal is tight coils, loose waves, or straightening (a process sometimes called a chemical relaxer, though the chemistry differs in key ways). Understanding what happens inside the cortex during a perm explains both why the outcome lasts and why the treatment carries real structural consequences for the hair fiber.

How Disulfide Bonds Break and Rebuild Inside the Cortex

Hair's mechanical strength comes largely from keratin, the fibrous protein that makes up the bulk of the cortex — the middle layer of the hair shaft. Keratin chains are held together in part by disulfide bonds, which are covalent links between sulfur atoms on adjacent cysteine amino acid residues. These bonds are among the strongest chemical connections in hair structure, which is why they are responsible for its natural curl pattern or straightness rather than something that shifts with humidity alone.

In the first stage of a perm, a reducing agent — most commonly ammonium thioglycolate, though glyceryl monothioglycolate and cysteamine are also used in certain formulations — is applied to hair that has been wound around rods. The reducing agent donates hydrogen atoms to the disulfide bonds, converting each S–S link into two free sulfhydryl (–SH) groups. This reaction is called reduction, and it effectively "unlocks" the internal scaffolding of the cortex. The hair becomes temporarily pliable and conforms to the shape of the rod it is wrapped around.

After a processing period that allows the reducing agent to penetrate the cortex fully, a neutralizer is applied. Most neutralizers are oxidizing agents — hydrogen peroxide or sodium bromate are the most common. The oxidizer removes the hydrogen atoms that were added during reduction, forcing the sulfhydryl groups back together into disulfide bonds. Because the hair is still wound around the rod, the new bonds form between sulfur atoms that are now physically closer to one another in the rod's curved geometry. The cortex is, in effect, relocked into the new shape.

This two-stage chemistry — reduction followed by oxidation — is what makes the result permanent. The new disulfide bonds are just as covalent and stable as the original ones. They do not relax back when the hair gets wet because they are not hydrogen bonds. The only way the shape changes again is through new growth from the follicle, mechanical damage, or a second chemical treatment. It is worth noting that steam rollers use moisture to reshape hair bonds temporarily by targeting hydrogen bonds rather than disulfide bonds — a fundamentally different and reversible mechanism.

The Chemical Components Involved in a Permanent Wave

Keratin (the substrate). The hair's own keratin protein is both the target and the medium of the reaction. The density of cysteine residues — and therefore the number of disulfide bonds available — varies by individual and by hair type. Hair with a higher density of disulfide bonds generally requires a stronger or longer reduction phase to achieve the same degree of softening. The structural role of genetics in influencing hair thickness and fiber composition also affects how much reducing agent is needed to saturate the cortex.

Reducing agent (the waving lotion). The reducing agent is the active ingredient in the waving lotion applied in stage one. Ammonium thioglycolate is the most widely used compound in alkaline perms; it works at a pH of roughly 8.2–9.6, which causes the cuticle to swell and allows the chemical to penetrate. Acid perms use glyceryl monothioglycolate at a lower pH (around 6.5–7.0) and typically require added heat to accelerate penetration. Cysteamine-based formulations sit at a neutral pH and are marketed as gentler alternatives, though they follow the same reduction chemistry.

Neutralizer (the oxidizing agent). The neutralizer stops the reduction reaction and reforms the disulfide bonds. Hydrogen peroxide is the most common oxidizing agent used in salon perms; sodium bromate, though slower-acting, is sometimes used in at-home kits. Incomplete neutralization — whether from insufficient contact time or uneven application — leaves some sulfhydryl groups unreformed, which weakens the fiber and produces a result that fades faster than expected.

Rods and tension. The physical shape of the rod and the tension applied during winding determine the geometry in which the new disulfide bonds form. Smaller rods produce tighter curls; larger rods produce looser waves. The tension also affects how evenly the waving lotion distributes along the shaft, which is why uneven winding produces uneven results regardless of the chemistry.

Where the Chemistry Produces Unexpected or Damaging Results

Over-processing and cortex degradation. If the reducing agent is left on too long, or if the concentration is too high for the hair's existing condition, the reaction does not stop at breaking disulfide bonds. Thioglycolate can also cleave peptide bonds — the backbone links of the keratin protein itself. When peptide bonds break, the structural protein is irreversibly degraded. Hair that has been over-processed becomes weak, gummy when wet, and prone to breakage. This is not a cosmetic issue that conditioning can resolve; the protein structure has been chemically destroyed, not merely depleted.

It is important to note that a perm's keratin chemistry is entirely distinct from what is marketed as a "keratin treatment." A keratin smoothing treatment applies an external keratin-based coating — often cross-linked with formaldehyde or a formaldehyde-releasing compound — to the surface and outer cortex of the hair to temporarily reduce frizz. It does not break disulfide bonds. A perm, by contrast, breaks and reforms those bonds from within. Confusing the two leads to misunderstanding both their mechanisms and their risks. Similarly, a deep-conditioning treatment works by temporarily filling voids in the cortex with humectants and proteins — it does not re-form covalent bonds of any kind.

Pre-existing chemical damage. Hair that has already been bleached, colored with a high-lift process, or previously permed has a compromised cuticle and a cortex with fewer intact disulfide bonds available. The reducing agent penetrates faster and less evenly, making it difficult to predict processing time. The neutralizer may also be less effective because there are fewer sulfhydryl groups available to re-oxidize into stable bonds.

Uneven neutralization. The neutralizer must reach every part of the cortex that the reducing agent reached. If rinsing removes the neutralizer too early, or if application is uneven, some sections of the hair are left with unreformed sulfhydryl groups. Those sections are structurally weaker and may not hold the curl pattern as long as fully neutralized sections.

Scalp and skin exposure. The reducing agent and the neutralizer are both capable of causing chemical burns if they contact the scalp for extended periods. Ammonium thioglycolate is a known skin sensitizer, and hydrogen peroxide at the concentrations used in neutralizers can cause irritation. The FDA has received adverse event reports related to scalp burns and hair breakage from both professional and at-home perm products.

What a Perm Product Label Discloses — and What It Does Not

Perm products sold in the United States are regulated as cosmetics under the Federal Food, Drug, and Cosmetic Act, which means they are subject to ingredient labeling requirements under the Fair Packaging and Labeling Act. The ingredient list on a perm kit will identify the reducing agent (e.g., ammonium thioglycolate, cysteamine hydrochloride) and the neutralizer (e.g., hydrogen peroxide, sodium bromate), along with conditioning agents, fragrance, and pH adjusters.

What the label does not disclose is the concentration of the active reducing agent, which is the single most important variable in predicting how aggressively the product will act on a given hair fiber. Concentration is considered a trade formulation detail and is not required to appear on consumer packaging. A product labeled "gentle" or "for damaged hair" signals a lower pH or a milder reducing agent, but without concentration data, the label alone cannot fully characterize the treatment's chemical intensity.

Labels are also not required to disclose the formaldehyde content of any conditioning agents added to the waving lotion — a relevant detail given that some perm formulations include smoothing or anti-frizz additives that may release formaldehyde. The FDA has noted concerns about formaldehyde-releasing ingredients in hair straightening products, and some of that chemistry overlaps with conditioning components found in combination perm-and-smooth formulations. Consumers and professionals relying solely on the label cannot always distinguish a straightforward thioglycolate perm from a hybrid formulation with additional reactive ingredients.

The label also says nothing about the condition of the hair being treated, which is the other half of the chemical equation. A perm product that is appropriately matched to virgin hair may be significantly over-strength for hair that has been bleached or previously chemically processed. No regulatory framework currently requires manufacturers to characterize their products relative to pre-existing hair damage states.

A perm is one of the more chemically precise processes applied to hair — it targets a specific class of covalent bond, breaks it under controlled conditions, and reforms it in a new geometry. The permanence of the result is a direct consequence of the strength of disulfide bonds, and the risks of the process are a direct consequence of the same chemistry applied unevenly or to a fiber that cannot absorb it without structural loss.

Note: This explains how hair products and treatments work. It is not a hair-care routine, it does not diagnose or treat hair loss, and it is not a substitute for a dermatologist or licensed stylist. Check the cited sources for current guidance.