Hair Damage Explained

What hair actually is

A hair is a fibre of dead, keratinised protein, and it has two parts that matter here.

The cuticle is the outside: flat protein scales overlapping like roof tiles, pointing from root to tip. Their job is to lie flat, protect what is underneath, reflect light and keep water out. When they lie flat, hair is shiny, slippery and resists absorbing things. When they lift or break away, hair is dull, catches a comb and absorbs everything instantly.

Bound to the outermost scales is a lipid layer whose principal component is 18-MEA, 18-methyleicosanoic acid. This is the waterproofing. It is attached to the protein beneath through thioester linkages, and it is why virgin hair beads water at the basin and takes its time absorbing anything you put on it. Alkaline oxidative conditions cleave those linkages: bleach, permanent colour, chemical straightening↓DOWN, perms and sustained heat all strip that layer away, and once it is gone it does not return. Conditioners can imitate the feel of it. Nothing replaces it. The loss of 18-MEA is what porosity actually measures.

The cortex is the inside, and it is the majority of the fibre: long keratin chains bundled together, carrying the pigment and, crucially, the cross-links that hold the whole structure in shape. Everything you would call strength, elasticity or bounce lives here.

The four bond types, and which service touches which

Four kinds of bond hold a hair together. Knowing which is which explains almost every question a client has.

Peptide bonds

The backbone: the links joining one amino acid to the next along each keratin chain. They are the strongest, and no service is meant to touch them. When they are broken, the chain itself is being taken apart, which is what over-processing means.

Hydrogen bonds

Weak, individually trivial and enormously numerous. They break with water and heat and re-form as hair dries and cools. This is the whole of temporary styling. A blow-dry, tongs, rollers, a set: all of them break hydrogen bonds and re-form them in a new position, and all of them are undone by the next shower or a damp evening.

Salt bonds

Also called salt bridges or ionic bonds, they depend on pH. They break in strongly acid or strongly alkaline conditions and re-form when the hair is brought back to its normal slightly acidic state. This is why an acidic conditioner after an alkaline service is not a nicety, and why leaving alkalinity in the hair keeps doing damage long after the client has left.

Disulfide bonds

The permanent ones: sulfur-to-sulfur links between two cysteine side chains, forming the amino acid cystine. They do not care about water or heat. Only chemistry moves them, and they are the reason a perm↑UP or a relaxer is permanent while a blow-dry is not. They are also the whole of what this library calls bounce. Bounce is elasticity, and elasticity is disulfide bond health. You can impose a shape on almost any hair. You cannot impose bounce, because bounce has to be earned from bonds that are still there.

Hydrogen bonds are temporary, moved by water and heat. Disulfide bonds are permanent, moved by chemistry. Every permanent texture service in either direction works by reducing disulfide bonds, holding the hair in a new shape and oxidising the bonds back together.

Relaxers are the exception in mechanism: hydroxide chemistry converts the bond by lanthionisation, permanently removing one of the two sulfur atoms, which leaves a single-sulfur link that no perm chemistry can reduce or rebuild. That is why thio chemistry and hydroxide chemistry never meet on the same hair, in either direction. Perm vs relaxer sets it out.

The four kinds of damage, and what each one does

Mechanical

Friction and force: brushing, backcombing, towel rubbing, elastics, extensions, the pillow. It works on the cuticle: scales are chipped, worn away and eventually stripped, exposing the cortex. The visible end point is the small white node along the shaft that clinicians call trichorrhexis nodosa, and then a clean break. Wet hair is at its most fragile, which is why detangling with conditioner still in and a wide-tooth comb, working from the ends upward, is worth more than any product.

Thermal

Heat does two different things depending on how much of it there is. Ordinary styling heat breaks hydrogen bonds so that hair holds a shape, and that is reversible. Sustained high heat drives water out of the cortex, degrades keratin and helps strip the surface lipid layer, and that is not.

Heat also has a chemical consequence that is not widely understood. Glyoxylic acid, the active behind most systems sold as formaldehyde-free straightening↓DOWN, breaks down under flat-iron heat and releases a little formaldehyde. A 2026 study that measured the air during realistic salon treatments found the amount was low, below the WHO indoor limit, but it was not zero. So a treatment can contain no added formaldehyde and still release some in the room. On any acid smoothing service, ask what the active is, and do not sit through one in a room without working extraction.

Chemical

This is where the permanent losses happen, and the clearest case is bleach. Bleach oxidises, and it does not stop at the pigment. It walks the sulfur in a disulfide bond through a series of oxidised stages and leaves it as cysteic acid, a sulfonate group with no partner sulfur to pair with. Each one is a bridge permanently deleted. It also inverts the surface, so that where the water-repellent lipid layer used to be there are now water-loving groups, which is why bleached hair feels dry and swells and softens in water at the same time. Bleach damage has the chemistry in full.

Permanent colour is a milder version of the same oxidative work. Perms, relaxers and structural straightening deliberately break disulfide bonds and rebuild them, and the neutraliser never rebuilds every one. The shortfall is the cost of the service, which is why repeating them is planned in months rather than weeks.

Environmental

Slow, cumulative and consistently underestimated. Ultraviolet light oxidises the surface and the pigment. Sea salt and sun strip the lipid layer. Chlorine at mains-water concentrations is not a real threat, but regular swimming is, particularly where copper is present from pool water or old plumbing; a client who swims several times a week arrives with hair that behaves as though it has been mildly bleached.

Hard water is the London problem: dissolved calcium and magnesium settle into the cuticle, stiffening the hair, dulling it and weighting a soft wave flat, and trace metals in the water speed up oxidation in ways nobody intends. A chelating wash once a month is the answer, and it is not the same thing as a clarifying shampoo, which removes product but not bound minerals.

Why it adds up, and why most of it stays

Living tissue repairs itself. A hair does not. It is dead keratinised protein with no metabolism, so every change it undergoes is filed and kept. Bleach at eighteen is still in the ends at twenty-two, unless they have been cut off.

Damage does not make hair harder to process. It makes it dangerously easy.

Hair that has lost its 18-MEA absorbs liquid much faster than the hair growing out of your scalp today, so the most fragile section on your head is also the section that takes chemical fastest. One lotion at one strength across a virgin root and a four-year-old end finishes the end while the root has barely started. Every barrier cream, dual-lotion technique and protective pre-wrap in professional practice exists to manage that one gap.

It also explains why history matters more than condition. A stylist is not asking about your last four years of colour to be nosy. Hair that is porous from weathering and sun may still have most of its disulfide bonds intact and can often be processed with care, while hair that is porous because it was bleached has lost the bonds themselves. Those two heads look similar and can need opposite handling.

What “repair” actually does

The category is genuinely useful and routinely oversold. Taking it honestly, one type at a time:

Bond builders

The best-known active is bis-aminopropyl diglycol dimaleate, the molecule in the Olaplex system, a maleate diester. When a disulfide bond splits it leaves free thiol groups, and the claimed mechanism is that the two maleate sites capture those thiols before oxidation can carry them on to cysteic acid, bridging a pair so that it behaves like a cross-link. That is the maker’s claim, and it should be read as one.

An independent study by Taddei and colleagues, published in the International Journal of Biological Macromolecules in 2021, tested commercial reconstructive agents on bleached hair by infrared, Raman and scanning electron microscopy. It found clear benefit to the surface, a more regular fibre and better-lying scales, but no increase in the disulfide content of the cortex and no direct evidence of the proposed cross-linking. What nobody disputes is the part downstream: nothing proposed by any manufacturer turns cysteic acid back into cystine. Bond builders protect bonds you still have. They do not return bonds you have lost. The bond-protected perm shows where they genuinely earn their place.

Protein and keratin treatments

These deposit protein fragments into and onto the fibre and stiffen it. Hair feels stronger and combs better for a few washes, and on porous hair the improvement is real and worth having. It is plaster over a crack. Overuse produces hair that is rigid and snappy, the classic protein overload, and the answer to that is moisture, not more protein.

Conditioners, silicones and lipid-replacement products

These work on the surface: they smooth the cuticle down, cut friction, restore slip and imitate the behaviour of the lipid layer that has gone. That is exactly what damaged hair needs day to day, and it is exactly what it is not being sold as. Better feel, better combing, less breakage in handling: a good product doing a good job under a misleading headline.

Porosity equalisers

Professional products used before a chemical service to even out absorption across a head with virgin roots and processed ends. Genuinely useful, and useful in the salon rather than at home.

Everything on that list improves the hair you have. Nothing on it rebuilds a disulfide bond that has been oxidised to cysteic acid.

How to assess your own hair

Do this on clean, product-free hair, at three places: root, mid-length and end. The answers will differ, and the ends govern.

The wet stretch

Take one wet strand, hold it between the fingers of both hands about five centimetres apart and pull slowly. Healthy hair stretches appreciably and springs back. Damaged hair stretches and stays stretched, or feels gummy, or parts under very little load. That is your elasticity, and elasticity is bounce, and bounce is disulfide bond health. It is the test that decides whether a chemical service is available to you.

The friction test

Run your fingers up a damp strand, from end to root. Smooth means the cuticle is lying flat. Rough, catching or squeaky means it is lifted and the lipid layer is diminished. That is porosity, and it tells you how fast chemical will go in, not whether there is anything for it to work with.

The dry signs

A matte surface that catches a comb through the mid-lengths, ends that mesh together instead of separating, small white nodes along the shaft, short broken hairs along a parting where foils have been placed year after year. These are suggestive rather than conclusive; the diagnosis is made wet.

The history question, asked properly

Ask yourself about lightener specifically, not about colour generally: bleach, highlights, foils, balayage, a bleach bath, colour remover, a home kit. “Have I coloured my hair” is not the question, and answering the wrong question is how people end up in trouble at the basin.

The float test, in which a shed hair is dropped into a glass of water, is widely repeated and largely unreliable. Surface tension and any residue on the strand affect the result more than the hair’s actual condition does.

Questions