Topic
Hair damage: what it is and where it comes from
Damage is cumulative and cannot be biologically healed, and chemical treatments, heat, sun, water and grooming each add to it.
Key takeaways
- Hair is dead keratin, so damage is cumulative and cannot be biologically healed. Products can coat, lubricate or temporarily reinforce, but they do not regrow tissue. Only new growth is undamaged.
- Four sources do most of the harm: chemical treatments (bleach, colour, perms, relaxers), heat, sun and water (chlorine), and mechanical grooming. Chemical treatments are the most aggressive in studies: bleaching and dyeing caused cuticle wear and high protein loss in every hair type tested. Bloch 2019
- Oxidation is irreversible. Bleach turns cystine into cysteic acid. Reduction by perm lotion can be partly reversed by the neutraliser. Grosvenor 2018; Kizawa 2005
- Wet hair is the weak point. Heated tools on wet hair cause bubble hair, and surfactant washing leaches protein and raises internal porosity. Li 2023; Kaushik 2022
- Repair claims are mostly lab results on cut hair. The best-supported protective steps are doing less damage, pre-wash oil and conditioning (see oils, bond builders, silicones).
- Breakage is often cumulative fatigue from repeated grooming, not one event.
The science
A hair fibre has an outer cuticle of overlapping scales, a cortex of keratin held together by disulfide and weaker bonds, and a lipid-rich cement between cells. Damage shows up in different ways depending on the layer:
- Surface and cuticle: scale lifting, chipping and loss of the fatty surface lipid, seen as roughness, tangles and dullness.
- Cortex: loss of protein and cystine, vacuoles and bubbles, seen as weakness, breakage and split ends.
- Pigment: melanin destroyed by oxidation or UV, seen as fading or golden, brassy tones.
| Source | Page |
|---|---|
| Bleach and colour | Bleach and colour damage |
| Perms, relaxers, smoothing | Relaxers, perms and smoothing |
| Heat tools | Heat damage and heat protectants |
| Sun | UV and hair |
| Pool water | Swimming and chlorine |
| Repair chemistries | Bond builders |
| Split ends | Split ends and trimming |
| Drying | Hair drying |
| Frizz | Frizz |
What it means for your hair
Porosity, thickness and curl pattern change how damage shows and which steps help (see hair porosity). Tightly coiled fibres have twists that concentrate stress, which is a reasoned explanation for their fragility rather than something shown here.
Myths and facts
- Myth: conditioner repairs split ends. It cannot; trimming does.
- Myth: damage is only from bleach. Heat, UV and grooming add up.
- Fact: damage worsens toward the tips because they are the oldest hair.
Bottom line
Count your treatments, because each chemical service or heat pass adds to the total. Handle wet hair gently and never use heated tools on it. Condition at every wash and consider a pre-wash oil. Trim regularly and shield hair from sun and pool water.
Studies 5
Bloch LD et al
Result: Chemical and physical treatments such as bleach, UV and flat ironing damaged both Caucasian and Afro-ethnic hair fibers.
Read the study ↗Grosvenor AJ et al. The physical and chemical disruption of human hair after bleaching: studies by transmission electron microscopy and redox proteomics. Int J Cosmet Sci. 2018
Tested: peroxide-persulfate bleaching at several levels
Result: Bleaching oxidised cystine and other sulfur amino acids and leached cuticle and cortex protein in proportion to bleaching severity.
Read the study ↗Kizawa K et al. Dissimilar effect of perming and bleaching treatments on cuticles: S100A3 model. J Cosmet Sci. 2005
Result: Bleached cuticles fragmented into small pieces with irreversible oxidation of the protein S100A3, while permed cuticles delaminated in larger pieces.
Read the study ↗Li YM et al. A review of bubble hair deformity. 2023
Result: Most patients had used a heated tool on wet hair; treatment is stopping the heat.
Read the study ↗Kaushik V et al. Benefit of coconut-based hair oil via hair porosity quantification. Int J Cosmet Sci. 2022
Result: Repeated surfactant washing raised pore surface area in an industry study; pre-treatment with coconut-based oil limited it.
Read the study ↗Related
- Bleach and colour damageBleaching and permanent dyeing chemically damage the hair fibre in every hair type, and most of that damage cannot be undone.
- Relaxers, perms and smoothing treatmentsChemical straighteners and perms all damage hair, and some carry scalp, respiratory, kidney or cancer-association concerns worth knowing before you book.
- Heat damage and heat protectantsHeat on wet hair is the clearest harm, and there is little independent evidence that retail heat-protectant sprays work.
- UV and sun damage to hairSunlight fades colour and weakens hair through free-radical damage, with light, bleached and coloured hair most affected.
- Swimming, chlorine and green hairHeavy swimming can strip and discolour hair, and green tints usually come from copper rather than chlorine itself.
- Bond builders (Olaplex, K18, maleic acid, "acidic bonding")Bond builder is a marketing label for four unrelated chemistries; all have lab results on cut hair and none has a trial on real heads.
- Oils and hairOils coat hair and some enter it, which can cut damage from washing, but they do not repair split ends or add moisture.
- Silicones in hair careSilicones coat hair to cut friction and add shine without repairing it; buildup is real but manageable, and the scare claims are unsupported.
- Split ends and trimmingSplit ends cannot be repaired, only smoothed over or cut off, and trimming preserves length by stopping splits travelling up the strand.
- Hair dryingAir drying is not clearly safer than moderate-heat blow-drying; high heat, close range and heat on wet hair are the real problems.
- FrizzFrizz comes from a rough, damaged surface and moisture uptake; products manage it, while less damage prevents it.
- Hair porosityPorosity describes how easily water moves in and out of a hair fiber; in practice it is mostly a marker of damage, and the popular float test does not measure it.