Bleach damaged hair, explained at the level of the fibre

Bleach is the most aggressive thing most people voluntarily do to their hair, and it is worth understanding at the level of the fibre — not to talk anyone out of it, but because every rule about caring for bleached hair follows directly from the chemistry.

The three-step demolition

Step one: swelling. An alkaline agent, usually ammonia or a substitute, raises the pH from hair's natural 4.5–5.5 to around 9 or 10. This lifts the cuticle scales and swells the fibre, opening a path inward. Nothing can lighten hair without this step, and this step alone accounts for a good deal of the roughness afterwards.

Step two: oxidation of pigment. Hydrogen peroxide enters the cortex and oxidises melanin. The pigment is not removed so much as broken into smaller, colourless fragments. This is the goal, and it is why lightening is a one-way process: you cannot reassemble melanin.

Step three: collateral damage. Peroxide is not selective. Three things happen to the fibre alongside the pigment.

• Disulfide bonds break. The cross-links between keratin chains — the things that give hair its strength and elasticity — are oxidised. Many convert irreversibly into cysteic acid, which cannot be reformed into a bond by anything.
• Keratin degrades. Protein is lost from the fibre outright, leaving gaps in the cortex.
• 18-MEA is stripped. This is the one people never hear about and the one that explains the most. 18-methyleicosanoic acid is a fatty acid layer covalently bound to the cuticle surface, and it is what makes virgin hair hydrophobic — water beads on it, it feels smooth, strands slide past each other. Bleach removes it, and it does not come back. The fibre goes from water-repellent to water-attracting, permanently.

Why bleach damaged hair behaves the way it does

Every characteristic follows from that list.

It tangles. With 18-MEA gone, the surface friction between strands rises substantially. Hair that used to slide now catches.

It is much weaker wet than dry. All hair loses strength when wet; bleached hair takes on more water because the surface no longer repels it, swells further, and loses more. This is why the shower is where bleached hair breaks, and why detangling it wet with a brush is so destructive.

It feels dry no matter what. The fibre cannot hold water the way it did, so moisture is a maintenance activity rather than a fix.

It stretches and stays stretched. Fewer intact cross-links means less elastic recovery. This shows up immediately on the stretch test, and it is the classic bleached-hair result.

It absorbs colour unevenly. Porosity varies along the strand depending on how many times each section has been lightened, so toner takes differently at the roots and ends.

What "repair" can honestly mean

Three categories, with very different claims.

Bond builders. The chemistry that started with bis-aminopropyl diglycol dimaleate, now joined by several others, targets the disulfide bonds directly — reconnecting some of what oxidation broke. This is a genuine intervention on the structure rather than a cosmetic one, and it is why salons apply it during the bleach rather than after: the best moment to save a bond is before it fully oxidises into cysteic acid. It does not restore everything, and it does not touch the lipid layer.

Protein treatments. Hydrolysed proteins fill gaps in the damaged cortex and stiffen the fibre. Temporary, washes out, genuinely useful in rotation. Overused, it makes brittle hair. The balance is the whole skill.

Conditioning and film-forming. Cationic conditioners, silicones and oils restore slip and reduce further mechanical damage. This is cosmetic, in the good sense — the hair behaves better, breaks less, and looks like hair. Nothing is being rebuilt.

There is also research into replacing 18-MEA with lipid analogues, and some products claim it. Treat any "restores the lipid layer" claim as unproven for now.

The routine that actually follows

Never brush it wet, and detangle only with conditioner in. This is the single highest-value rule for bleached hair, because wet strength is where the loss is. The order of operations matters.

Alternate protein and moisture rather than picking one. Test with a shed strand every couple of weeks and let the result decide the next treatment.

Treat heat as expensive. A fibre with broken cross-links and no lipid layer has far less margin. The temperature bands drop accordingly.

Use a chelating shampoo occasionally if your water is hard. Damaged, negatively charged hair attracts mineral deposits far more than virgin hair does — it is the group most affected.

Space out the lifts, and never overlap onto previously bleached hair. Root touch-ups exist for this reason.

Accept that the ends are finite. Bleached ends have a lifespan. Planning to cut them gradually is a better strategy than trying to keep them forever, and it is the only thing in this whole guide that actually removes damage.