Hydrogen Peroxide

What it is

Hydrogen peroxide is the oxidising partner in almost every chemical hair service. In colour it performs two jobs at once: it lightens the natural hair pigment and at the same time starts the reaction from which the new colour pigments form inside the hair. In reshaping it serves as the fixing step, closing the previously opened bonds in their new arrangement.

Concentration is decisive. In the salon it is given in volume percent and determines how much lightening occurs — and to the same degree how much the fibre is stressed. The two cannot be separated: lightening happens through the very process that also attacks the keratin. Hydrogen peroxide decomposes into water and oxygen and leaves no residue in the hair.

Why

• Effect and burden come from the same source. There is no lightening without stressing the fibre — it is the same oxidation process. Control comes through concentration and processing time, not through whether the substance is “gentle”.
• Concentration is the single most important variable here, and it does not appear in the ingredient list. Between a low developer strength for a toner and a high one for several levels of lift there is a world of difference — in the result as much as in the stress on hair and scalp.
• Why the classification reads “increased attention”: because of the irritant potential for skin, eyes and airways at the concentrations used, and because correct application matters so much. It does not establish that a properly performed treatment causes harm — it records that execution decides the outcome here.
• Hazard and risk have to be kept apart. Concentrated hydrogen peroxide is corrosive; that describes the raw material. In the ready-to-use mixture it is diluted and acts for a limited time. Both statements hold at once and describe different situations.
• The substance leaves nothing behind in the hair. It decomposes into water and oxygen. What remains after the treatment is the change to the fibre — not the substance itself. That sets it fundamentally apart from conditioning agents, which work by remaining.

When the assessment differs

• Broken, irritated or inflamed scalp skin: not a suitable starting point. The manufacturer's instructions take precedence.
• Pre-damaged, repeatedly treated or already bleached hair: the risk of over-processing is markedly higher here; a strand test and the manufacturer's instructions govern.
• Eye contact: the most important acute hazard during application. Protective equipment and the manufacturer's specifications govern.
• Occupational contact: daily handling compared with occasional use. Gloves and ventilation are the topic here.
• Higher concentrations in bleaching: the classification assumes the concentrations usual in colour and reshaping; with strong lightening the stress is greater.

Open questions

• Concentration of the ready-to-use mixture: not available. With this substance it is the decisive variable and in principle cannot be read from the ingredient list.
• Regulatory status: maximum-level and labelling rules exist for hydrogen peroxide in cosmetic products, partly separated by professional and general use. They have not been checked here and are deliberately not quoted second-hand.
• Data on fibre stress as a function of concentration and processing time: not evaluated here. The statement above describes the mechanism, not a measurement series.
• Assessment of bleaching: not the subject of this entry. Further substances are involved there and would have to be considered separately.

Related substances

Applies to: Oxidising agents

• Potassium Persulfate — Oxidising agent, bleaching agent

Further substances from the same group for which a long-form assessment already exists.