Sodium Hydroxide

What it is

Sodium hydroxide is among the most powerful substances that occur in cosmetics at all — corrosive in concentrated form, with a pH close to 14. In this lexicon it appears in four products, and in all four in an entirely different role: as a pH adjuster in small amounts, far down the ingredient list.

That makes this record the most instructive case in the lexicon for why a substance name on its own says nothing. The same substance is the actual active in straightening products of the “lye relaxer” type and is responsible there for chemical burns to the scalp. No such product is in this range.

Why

• Why the classification reads “low risk”: in all four products the substance stands far down the list and serves to set the final pH. After that adjustment it is no longer present as a caustic solution but in dissolved form in the water of the formulation. Three of the four products are rinse-off care.
• What the substance does elsewhere belongs in the assessment. A peer-reviewed review from 2024 summarises the safety situation of chemical straighteners: for sodium hydroxide-based products, irritation and chemical burns of the scalp from the extreme alkalinity are documented, and scarring hair loss as a consequence. That describes a product category, not a substance — and that category is not represented here.
• The only straightening product in this range works differently, and that was checked specifically while writing this assessment. Schwarzkopf Strait Styling Glatt contains ammonium thioglycolate and hydrogen peroxide, that is, the reduction and oxidation process of a perm. Sodium hydroxide stands there too only as a pH adjuster. Anyone seeing the ingredient list and thinking of a lye relaxer is mistaken.
• A counterintuitive finding that belongs here nonetheless: a South African study measured the pH of 70 straightening products, including ones advertised for children. All of them were in a range corrosive to skin — including those marketed as “no lye”. And the frequently recommended mixing-in of a conditioner produced a measured pH of 13.8 and did not protect the scalp.
• Origin corrected: the short field said “inorganic”. That is a substance class, not a statement of origin. Sodium hydroxide is produced industrially by electrolysis of brine — the raw material is a mineral, the process is industrial. The short field now reads “synthetic” in all four languages.

When the assessment differs

• Irritated or broken scalp skin: an alkaline substance carries more weight on pre-damaged skin, even in small amounts.
• Occupational contact: whoever mixes and applies daily has the higher total exposure on the hands.
• Products that stay on the skin: for three of the four products the assessment assumes rinse-off use.
• Anyone using a sodium hydroxide-based straightener: this assessment expressly does not apply to that. It describes the substance in the role of a pH adjuster.
• Very dry or porous hair structure: alkaline conditions make the fibre swell; how strongly depends on the pH that is set, not on the substance name.

Open questions

• Concentration in the specific product: not available.
• The pH that is set in the finished formulation: not documented and not readable from the ingredient list. It, and not the substance name, decides tolerability.
• Transferability of the findings on straightening products to the products listed here: expressly not given. The findings appear in the text for context, not as an assessment of these products.
• Share of sodium hydroxide in the overall effect: cannot be quantified. In all four products further pH-active substances stand alongside it, among them citric acid.
• Regulatory status: not checked; statements on use and maximum levels are not quoted second-hand.

Sources for this assessment

• Needle C. u. a.: Safety of chemical hair relaxers — a review article. JAAD Reviews, 2024. Begutachtete Übersichtsarbeit; beschreibt Reizungen, Verätzungen der Kopfhaut und vernarbenden Haarausfall bei Glättungsmitteln auf Natrium- oder Kaliumhydroxidbasis. Abgerufen am 11.09.2026.
• The pH of lye and no-lye hair relaxers, including those advertised for children, is at levels that are corrosive to the skin. South African Medical Journal, 2019. Messung an 70 Produkten; auch die als „ohne Lauge“ ausgelobten lagen im ätzenden Bereich, und das Untermischen einer Pflege ergab pH 13,8. Offener Zugang, abgerufen am 11.09.2026.

Related substances

Applies to: Alkalising and pH-adjusting agents

• Aminomethyl Propanol — Formulation ingredient
• Ammonium Bicarbonate — pH buffer, alkalising
• Ammonium Hydroxide — Alkalising agent
• Citric Acid — pH regulator and chelating agent

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