Blonde hair has been a global beauty trend for decades – and behind the effect lies fascinating chemistry. When bleaching or lightening hair, you intervene in the hair’s natural pigment system: melanin molecules are oxidatively destroyed, the hair fiber is opened and structurally altered. This article explains how the chemical process of hair bleaching works, which ingredients are involved, what damage can occur – and how the hair best regenerates afterwards.
What Is Melanin – and Why Does It Make Hair Dark?
Melanin is the key pigment responsible for human hair color. It is produced in specialized cells called melanocytes in the hair follicles and gives each hair its individual shade.
Biologically, two basic types are distinguished:
- Eumelanin: Dark brown to black pigment – responsible for brown and black hair
- Pheomelanin: Yellow to reddish-brown pigment – responsible for blonde and red hair
The individual hair color results from the specific ratio of both melanin types. Dark hair contains significantly more eumelanin, while light hair has a higher proportion of pheomelanin. Eumelanin is chemically more stable and more resistant to oxidizing agents than pheomelanin – which is why dark hair is harder to bleach.
Melanin is embedded in the keratin structure of the hair and is present there as solid, insoluble pigment granules. To permanently lighten hair color, these melanin molecules must be chemically destroyed – a process that is only possible through powerful oxidizing agents.
The Structure of Hair: Layers and Architecture
Before bleaching agents can take effect, they must first penetrate into the interior of the hair fiber. The structure of the hair determines how and how quickly this occurs:
- Cuticle (scale layer): The outermost protective layer, consisting of overlapping, keratin-containing scale cells. In healthy hair, it lies tightly and protects the interior.
- Cortex (hair cortex): The middle layer making up the bulk of the hair fiber. This is where the keratin filaments and the embedded melanin granules are located.
- Medulla (hair marrow): The innermost core, contributing to mechanical stability.
Healthy, untreated hair has a natural pH of approximately 3.67. This mildly acidic value keeps the cuticle scales closed and the hair fiber sealed. Alkaline substances, however, lift the cuticle scales and open the hair fiber – the first necessary step of any bleaching process.
Ingredients in a Hair Bleach: What’s Really Inside?
Modern bleaching products typically consist of two or three components mixed immediately before application:haut
1. Alkalizing Component – Ammonia or Ethanolamine
The alkalizing agent – most commonly ammonia (NH₃) – is the key to allowing the oxidizing agent to enter the hair fiber. Ammonia raises the pH of the bleaching mass to approximately 9–10. This alkaline pH achieves two things:hair-shop+1
- The cuticle layer swells and opens
- The permeability of the hair fiber for peroxide increases substantially
Without this opening mechanism, hydrogen peroxide would never reach the melanin pigments in the cortex. Alternatively, in modern, less-odorous formulations, ethanolamine is used, which also creates an alkaline pH but is far less volatile.
2. Oxidizing Agent – Hydrogen Peroxide (H₂O₂)
Hydrogen peroxide is the actual active substance in bleaching. It oxidizes the melanin molecules and destroys their chromophoric bonds – those chemical structures responsible for light absorption and thus for color.
EU cosmetic regulations permit concentrations of up to 12% H₂O₂ for hair treatment:
| Peroxide Concentration | Degree of Lightening | Application |
|---|---|---|
| 1.9% (6 Vol.) | approx. 1 tone | Gentle refresh |
| 3% (10 Vol.) | approx. 1–2 tones | Mild toning |
| 6% (20 Vol.) | approx. 2–3 tones | Standard bleaching |
| 9% (30 Vol.) | approx. 3–4 tones | Intensive lightening |
| 12% (40 Vol.) | up to 5 tones | Maximum bleaching |
In an alkaline environment, hydrogen peroxide decomposes into water, molecular oxygen and highly reactive hydroperoxide anions (HOO⁻). These highly reactive species are the actual effective oxidizing agents.
3. Boosters – Persulfates (Potassium, Sodium, Ammonium Persulfate)
To achieve lightening of more than 5 tones, bleaching powders are supplemented with so-called boosters or bleach accelerators:
- Potassium peroxodisulfate (K₂S₂O₈)
- Sodium peroxodisulfate (Na₂S₂O₈)
- Ammonium peroxodisulfate ((NH₄)₂S₂O₈)
These persulfates act as additional oxidizing agents that help overcome the kinetic sluggishness of hydrogen peroxide. With such combinations of H₂O₂ and persulfates, lightenings of up to 8 tones are achievable.
Further Auxiliary Components
- Surfactants: Ensure even distribution of the bleaching mass over the hair
- Thickeners (e.g. carboxymethylcellulose): Provide the desired consistency (cream, powder, gel)
- Protective and conditioning agents (e.g. lanolin, lecithin, stearin): Intended to protect the hair fiber during the aggressive bleaching process
- Stabilizers and buffers: Control pH value and reaction rate
The Reaction Mechanism: How Melanin Is Degraded
The chemical bleaching process proceeds in several stages:
Step 1 – Opening of the cuticle:
Ammonia or another alkalizing agent raises the pH to approximately 10. The cuticle layer of the hair opens, allowing hydrogen peroxide to penetrate into the cortex.
Step 2 – Formation of reactive oxygen species:
In the alkaline environment, hydrogen peroxide decomposes into water, molecular oxygen and highly reactive hydroperoxide anions (HOO⁻). These intermediates are the truly effective oxidizing agents.
Step 3 – Oxidative destruction of eumelanin:
The darker eumelanin contains an extensive aromatic π-electron system whose conjugated double bonds are responsible for light absorption in the visible spectrum. The reactive oxygen species attack these chromophoric structures, cleave the indole and dihydroxyindole rings of eumelanin and destroy the light absorption. The result: the dark pigments are lightened.
Step 4 – Oxidation of pheomelanin:
Pheomelanin contains sulfur and has a different structure. Although it is easier to oxidize than eumelanin, it tends to leave yellowish intermediate tones during the bleaching process – the well-known “yellow tinge“ in dark blonde or brown hair after incomplete bleaching.
Step 5 – Dissolution of melanin granules:
When the melanin molecules are fully oxidized, the granules lose their coloring effect and appear colorless. The hair appears light yellow to white.
Natural Bleaching: Why Hair Turns Gray With Age
Hydrogen peroxide plays a role not only in the bottle but also within the body when it comes to hair color. Researchers at the University of Mainz demonstrated that small amounts of H₂O₂ are naturally produced in hair – as a byproduct of cellular metabolism. The enzyme catalase normally breaks down this endogenously produced peroxide.
However, with increasing age, catalase activity in the hair follicles decreases significantly. The rising H₂O₂ concentration attacks the enzyme tyrosinase and oxidizes the amino acid methionine in its active site. Tyrosinase is, however, the key enzyme for melanin biosynthesis – if inactivated by oxidation, no melanin can be produced. The result: the hair turns gray or white. Cosmetic bleaching accelerates this mechanism in a targeted and controlled manner.p
What Damage Does Bleaching Cause
The oxidative attack on melanin is unfortunately not selective: hydrogen peroxide and persulfates also attack the protein structure of the hair.
Damage to the Keratin Structure
Hair consists of approximately 95% keratin, a protein with a high cysteine content. Cysteine residues are crosslinked by disulfide bonds (S–S bridges), giving hair its strength and elasticity. Oxidizing agents can attack these disulfide bonds:
- Disulfide bonds are oxidized to sulfonic acid groups → permanent structural weakening
- Peptide chains can be cleaved by further oxidation
- The sulfur bridges holding keratin filaments together are irreversibly damaged
Damage to the Cuticle
Due to the alkaline environment and longer exposure times, the scale cells of the cuticle are lifted and cannot fully close again. The consequence: the hair fiber loses its protective layer, moisture escapes more easily, the hair becomes porous, dry and dull.
Typical Symptoms of Bleached Hair
- Increased porosity: Water and other substances are absorbed and released too quickly
- Moisture loss: Bleached hair dries out faster
- Loss of elasticity: Hair breaks more easily and is less stretchable
- Surface roughness: Dull appearance, lack of shine due to raised cuticle scales
- Brittleness and split ends: Particularly pronounced at the tips
Hair Care After Bleaching: Repair at the Molecular Level
pH Value Management
After every bleaching treatment, the hair fiber pH is significantly elevated. Acidic rinses and products with pH 4–5 re-close the cuticle, seal the hair fiber and immediately improve shine and texture.
Bond Protection Technologies
Since the introduction of Olaplex, a new category of hair care products has emerged based on bond building. The active ingredient bis-aminopropyl diglycol dimaleate forms covalent bridges between broken disulfide bonds, thereby restoring the structural integrity of the keratin proteins. Similar technologies are now found in further bond protection products such as pH Plex, K18 or Smartbond.
Keratin Treatments
External keratin applications – as intensive treatments, shampoos or in-salon services – fill gaps in the porous hair fiber, strengthen the keratin layers and improve the manageability of bleached hair.
Further Care Recommendations
- Intensive conditioning: Deep-acting masks with ceramides, lipids and proteins at least once a week
- Heat protection: Bleached hair reacts more sensitively to thermal stress from blow-drying or straightening
- UV protection: Sunlight can further lighten already bleached hair and damage the keratin structure
- Toning: Violet or blue pigments in so-called silver shampoos neutralize unwanted yellow and orange tones after bleaching
Safety Aspects and Regulatory Framework
The EU Cosmetics Regulation (Regulation (EC) No. 1223/2009) governs the permitted concentrations of bleaching agents for home use and professional application. Hydrogen peroxide is approved up to 12% for hair bleaching products; persulfates are also subject to clear restrictions. Persulfates are considered potential allergens – ammonium peroxodisulfate in particular can trigger contact allergies and, in rare cases, occupational asthma in hairdressers.
Important safety notes:
- Patch test recommended before every bleaching treatment
- Processing time must be observed precisely – overexposure leads to irreversible keratin damage
- Professional application strongly recommended for more than 3–4 tones of lightening
- Do not apply to the scalp – intensive bleaching agents can cause chemical burns
Conclusion: Bleaching Is Applied Oxidation Chemistry
Bleaching hair is far more than a cosmetic procedure – it is a precisely controlled oxidative process that fundamentally alters the chemistry of the hair’s natural color system. Hydrogen peroxide destroys melanin molecules in an alkaline environment, persulfates accelerate the process, and ammonia opens the pathways into the hair fiber. The result is visible – but not without cost: the keratin structure is attacked, the cuticle weakened, the fiber made porous.
Modern bond-building technologies and targeted care routines can significantly minimize this damage – provided that bleaching and aftercare are chemically understood and consistently applied. Knowledge of the fundamental chemistry of bleaching is therefore not a luxury, but the foundation for sustainably healthy-looking light hair.
