Encyclopedia · chemical treatment
Hydrogen peroxide
The oxidiser in every bleach and permanent dye; it lifts colour by destroying melanin but also oxidises the hair's own proteins, so damage rises with strength and repeat use.
What it is
Hydrogen peroxide is mixed as "developer" (sold as 10, 20, 30 or 40 volume, which are roughly 3, 6, 9 and 12 percent) with an alkaline dye or bleach base. It is a cosmetic ingredient, and the EU, UK and US limit its content in hair products.
How it works
In alkaline bleach, peroxide plus persulfate degrades melanin even at mild settings, oxidises cystine to cysteic acid and leaches cuticle and cortex protein in proportion to severity. Grosvenor 2018 Raman mapping shows the disulfide signal falling and cysteic acid rising through the fibre cross-section, faster in dark hair. Kuzuhara 2006 Cysteic acid cannot be turned back into a disulfide (see bond builders).
What the studies show
- Bleaching oxidised sulfur amino acids and removed cuticle and cortex protein in proportion to severity. Grosvenor 2018
- The disulfide band fell and the cysteic acid band rose through the cross-section of bleached fibres. Kuzuhara 2006
- The alkalizer matters too: monoethanolamine colourants caused up to 85% more damage than ammonia in the extreme case (Procter & Gamble study). Bailey 2014
How it's used in studies
Salon bleach typically mixes developer 1:1 or 1:2 with lightener, with strength chosen by the lift needed. These are practice norms, not study doses.
Side effects and safety
Scalp burns and irritation, and breakage or a "chemical haircut" with over-processing. Never overlap on previously lightened hair. Skin allergy to peroxide itself is rare, as dye and persulfate allergens are the bigger problem. Eye contact is serious.
Bottom line
It is unavoidable for lightening. Damage scales with strength, time and number of passes, so lift only as far as needed, use the lowest developer that works and avoid overlap. Bond builders are harm reduction only. We are fairly sure of the damage mechanism and less sure about any particular mitigation. See bleach and colour damage.
Studies 3
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 ↗Kuzuhara A. Analysis of structural changes in bleached keratin fibers (black and white human hair) using Raman spectroscopy. Biopolymers. 2006
Tested: bleaching, Raman mapping across the fibre cross-section
Result: The disulfide signal fell and the cysteic acid signal rose across the whole cross-section after bleaching, more so in dark hair.
Read the study ↗Bailey AD et al. Comparison of damage to human hair fibers caused by MEA- and ammonia-based hair colorants. J Cosmet Sci. 2014
Tested: monoethanolamine versus ammonia colourants
Result: Colourants with monoethanolamine caused up to 85% more damage than ammonia-based ones.
Read the study ↗Related
- Persulfates (ammonium, potassium, sodium persulfate)The booster salts in powder lighteners; they speed up bleaching and are a documented cause of occupational asthma and contact allergy in hairdressers.
- Ammonia and monoethanolamine (MEA)The alkaline agents that swell the cuticle so dye and bleach can enter; "ammonia-free" MEA dyes are not gentler.
- p-Phenylenediamine (PPD)The main dark permanent dye ingredient and one of the commonest hair-dye allergens; it colours well but can cause severe contact dermatitis in sensitised people.
- Bis-aminopropyl diglycol dimaleateThe active in Olaplex; it strengthens bleached hair in the lab, but the claim that it re-links broken disulfide bonds has never been independently confirmed.
- Bleach and colour damageBleaching and permanent dyeing chemically damage the hair fibre in every hair type, and most of that damage cannot be undone.