What is "Sodium Hydroxide" and what is it used for?

Sodium hydroxide, or by its INCI name Sodium Hydroxide, with the chemical formula NaOH, is an alkaline inorganic compound widely used in industry, chemistry, and certain cosmetic processes. It appears in various forms, notably as a white solid—sometimes called caustic soda powder—or dissolved in water as caustic soda solution. Its composition relies on the association of sodium (Na⁺) and hydroxide (OH⁻) ions, which gives it a very high pH and strong basicity. This alkalinity explains its reactive nature and underscores its technical value in many fields, from paper manufacturing to water treatment.

In the world of skincare, cosmetic-grade sodium hydroxide is used at strictly controlled concentrations and under well-defined conditions. Caustic soda is particularly used during saponification, a chemical process that transforms vegetable oils into solid soap. This is why sodium hydroxide is often found in soaps. However, that is not its only function, and sodium hydroxide is also included in the formulation of many other types of cosmetic products.

The use of sodium hydroxide in cosmetics is primarily based on its alkalizing properties. More precisely, it serves as a pH regulator.

In very low concentrations, Sodium Hydroxide is primarily used to adjust the pH of formulations and ensure their compatibility with the skin. Skincare products must indeed be formulated within a pH range close to the skin’s physiological pH, generally between 4.5 and 5.5, to avoid irritation and preserve the balance of the microbiome as well as the integrity of the skin barrier. Many aqueous products—whether creams, lotions, or deodorants—therefore require precise pH adjustment: under these strictly controlled conditions, sodium hydroxide does not act on the skin but serves a critical technical role in formulating cosmetic products.

As mentioned earlier, this compound is also central to saponification, the chemical reaction responsible for traditional bar soaps. During this transformation, caustic soda reacts with fats to form fatty acid salts — in other words, soap — and the glycerin. Once the reaction is complete, no free caustic soda normally remains in the finished product, which explains why properly formulated soaps do not present the hazards associated with caustic soda itself.

Finally, some hairdressing or professional applications use caustic soda on hair to permanently modify its structure, particularly in alkaline straightening processes. In this specific context, the highly basic action of sodium hydroxide breaks certain bonds in the hair fiber to change its shape. This explains why sodium hydroxide can be either beneficial or harmful to hair. It depends entirely on the concentration, processing time, and conditions of use. If not properly controlled, such use of sodium hydroxide can be harsh, which is why it’s necessary to go to a professional salon to perform this type of straightening.

When mentioned, sodium hydroxide often raises concerns because of its raw form, more commonly known as caustic soda. In its pure state—whether as caustic soda powder or caustic soda solution—this compound is indeed highly corrosive: it can cause severe chemical burns to the skin and eyes, pronounced respiratory irritation, and damage to hair fibers upon prolonged contact. However, these hazards of caustic soda apply to the pure compound, not to finished cosmetic products, where its use is strictly regulated.

In cosmetics, Sodium Hydroxide is used at very low concentrations, solely to adjust pH or to enable certain reactions such as saponification (≈ 0.1%).

After formulation, there is generally no free caustic soda remaining in the final product, which explains why cosmetic-grade sodium hydroxide is considered safe by health authorities when used in accordance with regulations. Available data do not indicate that sodium hydroxide is harmful to hair or dangerous to skin under these controlled-use conditions.

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