Bleach and colour damage

Key takeaways

• Bleach damage is oxidative and mostly irreversible. Peroxide-persulfate bleaching turns cystine into cysteic acid through the whole fibre and leaches cuticle and cortex protein in proportion to how hard you bleach. Grosvenor 2018; Kuzuhara 2006
• Bleaching hits the cuticle hard. Bleached cuticles fragment into small pieces and the cuticle protein S100A3 is irreversibly oxidised. Permed hair cuticles instead peel off in larger pieces. Kizawa 2005
• Damage compounds with each treatment. In an atomic-force study, surface roughness change accelerated with repeat bleaching (up to +65% after a further 10 minutes) and stiffness fell by up to 40% after each treatment. Humphry 2022
• Bleaching and dyeing damage every curl type. Both wore the cuticle and caused high protein loss regardless of curliness. Bloch 2019
• "Ammonia-free" does not mean gentle. In a Procter & Gamble study, monoethanolamine colourants caused up to 85% more damage than ammonia. Bailey 2014 See ammonia and MEA.
• Permanent dye carries a separate risk, allergy. In reviews, sensitisation to p-phenylenediamine (PPD) in dermatitis patients had a median of about 4-6%. Thyssen 2008 See p-phenylenediamine.
• Bond builders reduce but do not undo the damage, and their evidence is lab-only (see bond builders).

The science

Permanent colour and bleach use alkaline hydrogen peroxide, with persulfate boosters in lighteners. Peroxide destroys melanin to lighten the hair and oxidises the sulfur in keratin. Raman mapping shows cysteic acid rising across the whole cross-section, more so in dark hair, where metals bound to melanin speed up peroxide breakdown. Kuzuhara 2006 Cysteic acid cannot form disulfide bonds again. Bleach also strips the fatty surface layer and leaves a negatively charged surface.

Demi- and semi-permanent colours deposit dye without bleach and generally use less peroxide, but we found no study comparing their damage directly.

What it means for your hair

• Dark hair needs the most lift and takes the most damage, because there is more melanin to destroy.
• Fine hair has less mass to lose and tends to break sooner (reasoning).
• High-porosity or previously bleached hair compounds damage. Avoid overlapping new bleach onto already-lightened lengths.
• Curly and coily hair is damaged just as much; breakage shows up as lost curl definition.
• On a sensitive scalp, patch-test dyes. Allergy is separate from fibre damage.

Myths and facts

• Myth: a bond builder makes bleaching safe. It does not.
• Myth: "organic" or "ammonia-free" dye is gentle. Not necessarily. Bailey 2014
• Fact: each extra lift pass costs strength. Humphry 2022

Bottom line

Lift as little as you need, avoid double-processing, use the lowest developer strength that works, space sessions out and touch up only the regrowth. Be gentle with washing and heat afterwards (see heat, UV and chlorine). Do a patch test before dyeing and see a clinician for a severe reaction.