Hair bleach is a two-part system: a powder (or cream) containing persulfate salts and alkaline agents, mixed with a liquid developer containing hydrogen peroxide. Together, these ingredients create a chemical reaction strong enough to dissolve the pigment inside your hair shaft. Here’s what each component does and why it matters.
The Bleach Powder
The powder side of hair bleach contains two main categories of ingredients: persulfate salts and alkaline (basic) agents. Persulfate salts are the heavy lifters. Most formulas use one or a combination of ammonium persulfate, sodium persulfate, and potassium persulfate. These compounds boost the oxidizing power of hydrogen peroxide, making the bleach far more effective at breaking down pigment than peroxide alone could manage.
The alkaline agents serve a different purpose. Ingredients like sodium carbonate, magnesium silicate, sodium metasilicate, and ammonium chloride raise the pH of the mixture to create the highly basic environment bleach needs to work. Your hair shaft naturally sits at a pH around 3.7, and your scalp hovers around 5.5. Activated hair bleach pushes well above that, into strongly alkaline territory. This high pH forces open the outer layer of each hair strand, the cuticle, so the bleaching chemicals can reach the pigment stored deeper inside.
The Developer
The liquid you mix into bleach powder is called a developer, and its active ingredient is hydrogen peroxide. Developers come in different “volumes” that correspond to specific peroxide concentrations:
- 10 volume: 3% hydrogen peroxide
- 20 volume: 6% hydrogen peroxide
- 30 volume: 9% hydrogen peroxide
- 40 volume: 12% hydrogen peroxide
Higher volumes lift more pigment but also cause more damage. Most bleaching systems use peroxide concentrations in the 3 to 6% range for on-scalp application. At higher concentrations, the risk of chemical burns to the scalp increases significantly. A 40-volume developer is typically reserved for off-scalp techniques like foil highlights, where the mixture doesn’t sit directly against the skin.
How These Ingredients Work Together
When powder and developer are combined, the persulfate salts and hydrogen peroxide react to generate highly reactive oxygen molecules, including hydroxyl radicals and perhydroxyl ions. These are the molecules that actually destroy your hair’s pigment.
The process works in two stages. First, the stronger radicals pre-oxidize pigment molecules, breaking them into simpler chemical structures. Then the second type of reactive molecule, the perhydroxyl ion (which forms when peroxide loses a hydrogen atom in the alkaline mix), cracks open the rings of those simplified pigment structures. When both types of reactive molecules are present together, the rate and extent of pigment breakdown increases dramatically compared to either one working alone. This is why bleach powder mixed with developer is so much more powerful than hydrogen peroxide by itself.
Trace metals naturally present in your hair, particularly iron and copper, also play a role. They catalyze additional breakdown of hydrogen peroxide into reactive oxygen species, adding to the overall bleaching effect. This is one reason bleaching results can vary from person to person: differences in mineral content affect how the chemistry plays out.
What Bleach Does to Hair Structure
Pigment destruction is only half the story. The same chemical reactions that dissolve color also attack the protein structure of your hair. Hair gets its strength from keratin, a protein held together by bonds between sulfur-containing amino acids called disulfide bonds. These crosslinks act like rungs on a ladder, giving each strand its structural stability, elasticity, and mechanical strength.
Bleaching breaks some of those disulfide bonds. The oxidation process partially destroys them in both the cuticle (the protective outer shingle-like layer) and the cortex (the inner core where pigment lives). Damaged cuticle cells develop holes in their structure, which is why bleached hair feels rougher and tangles more easily. In the cortex, broken crosslinks mean the hair is physically weaker and more prone to snapping.
This damage is cumulative and permanent. Each round of bleaching breaks additional bonds that the hair cannot repair on its own. Conditioning products can temporarily smooth the surface and improve how the hair feels, but no topical product rebuilds disulfide bonds. The only way to fully “fix” over-bleached hair is to grow it out and cut the damaged sections off.
Conditioning and Buffering Additives
Most modern bleach formulas include additional ingredients meant to reduce damage or improve the application experience. These vary by brand but commonly include thickeners that give the mixed bleach a workable consistency so it stays in place on the hair. Some formulas add conditioning agents like hydrolyzed keratin proteins, oils, or silicones that coat the strand during processing. These don’t prevent the underlying chemical damage, but they can reduce friction and moisture loss during and after the bleaching session.
You’ll also find buffering agents in some formulas. These help control how quickly the pH rises and stabilize the reaction so it proceeds more predictably. Some professional-grade bleaches include chelating agents that bind to the metal ions in hair, aiming to moderate the Fenton reactions that can cause uncontrolled bursts of oxidation.
Why Timing Matters
Once mixed, hair bleach is a ticking clock. The chemical reaction between persulfates and hydrogen peroxide begins immediately and eventually exhausts itself as the reactive ingredients are consumed. Leaving bleach on past the point where it stops actively lifting color doesn’t lighten hair further. It just continues degrading protein structure with diminishing returns on pigment removal. This is why most bleach instructions specify a maximum processing time, and why going beyond that window primarily increases damage rather than lightness.
The speed of the reaction depends on the developer volume, the ratio of powder to developer, the temperature (heat accelerates it), and the starting condition of the hair. Previously bleached or chemically treated hair has fewer intact disulfide bonds to begin with, so it’s more vulnerable to structural breakdown during reprocessing.