Cosmetics: all you need to know about sodium hydroxide.

The essential points to remember about sodium hydroxide.

Sodium hydroxide (NaOH) is a strong alkaline base, commonly known as caustic soda, used in numerous fields, including cosmetics.
In its pure form, caustic soda is corrosive and can cause skin burns, ocular irritation or hair damage.
In finished cosmetic products, Sodium Hydroxide is used in very small amounts, primarily to adjust the pH, facilitate the saponification of soaps or smooth hair.
After formulation, no free sodium hydroxide typically remains, rendering the ingredient safe under regulated conditions of use.
Concerns about the hazards of sodium hydroxide in cosmetics primarily arise from confusing the pure raw material itself with its inclusion in formulations at very low concentrations.

4 minutes to understand your skin. Our dermatological diagnostic guides you toward the ideal skincare for your specific needs. Simple, quick, personalized.

Sodium hydroxide: what is it?

Sodium hydroxide, known by its INCI name Sodium Hydroxide and having the chemical formula NaOH, is an alkaline mineral compound widely used in industry, chemistry and certain cosmetic processes. It is available in various forms, notably as a white solid — sometimes called caustic soda powder — or dissolved in water as caustic soda solution. Its composition is based on the association of sodium ions (Na⁺) and hydroxide ions (OH⁻), which gives it a very high pH and strong basicity. This alkalinity accounts for its reactive nature, as well as its technical utility in numerous fields, ranging from papermaking to water treatment.

Structure chimique de l'hydroxyde de sodium. — Chemical structure of sodium hydroxide.
Source: SEYEDINA S. H. & al. Investigation of NaOH properties, production and sale mark in the world. Journal of Multidisciplinary Engineering Science and Technology (2019).

In the field of skincare, cosmetic-grade sodium hydroxide is used at strictly controlled concentrations and under well-defined conditions. Caustic soda is particularly utilised during saponification, a chemical process transforming vegetable oils into solid soap. This is why sodium hydroxide is frequently found in soaps. However, this is not its only use, and sodium hydroxide features in the formulation of many other types of cosmetic products.

Chemical and physical characteristics	Values
Chemical formula	NaOH
Names	Sodium Hydroxide, caustic soda
Aspect	White powder
Molar mass	39.997 g/mol
Dosage	Generally used at approximately 0.1%
Solubility	Soluble in water

Physicochemical properties of sodium hydroxide.

What is sodium hydroxide used for?

The use of sodium hydroxide in cosmetics relies primarily on its alkalinising properties. More precisely, it is referred to as a pH regulator.

At very low doses, 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, typically between 4.5 and 5.5, to avoid irritation and preserve the balance of the microbiome and the integrity of the skin barrier. Many aqueous products, whether creams, lotions or deodorants, therefore require precise acidity adjustment: under these strictly controlled conditions, sodium hydroxide does not act on the skin but plays an essential technical role in the formulation of cosmetic products.

Relation entre le pH d'une solution et sa concentration en NaOH. — Relationship between the pH of a solution and its sodium hydroxide concentration.
Source: SAADON R. & al. Chemical route to synthesis hierarchical ZnO thick films for sensor application. Energy Procedia (2014).

As previously mentioned, 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 glycerine. Once the reaction is complete, there is normally no free soda left in the finished product, which explains why correctly formulated soaps do not pose the hazards associated with caustic soda itself.

Réaction de saponification. — Saponification reaction.
Source: RAHMAN W. & al. Prospect of castor oil biodiesel in Bangladesh: Process development and optimization study. International Journal of Green Energy (2017).

Finally, certain haircare or professional treatments employ caustic soda on hair to permanently modify its structure, particularly in alkaline straightening processes. In this specific context, the strongly basic action of sodium hydroxide cleaves specific bonds within the hair fibre to alter its shape. This explains why sodium hydroxide can be either beneficial or detrimental to hair. It depends entirely on concentration, exposure time and the conditions of use. If not properly controlled, such application of sodium hydroxide can be aggressive, hence the need to visit a professional salon to perform this type of straightening.

Mécanisme d'action des hydroxydes sur la structure de la fibre capillaire. — Mechanism of action of hydroxides on the structure of the hair fibre.
Source: MELO D. & al. Straight to the point: What do we know so far on hair straightening? Skin Appendage Disorders (2021).

Are there any dangers associated with caustic soda in cosmetics?

When it is mentioned, sodium hydroxide often raises concerns due to its raw form, better 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, marked irritation of the respiratory tract, as well as damage to hair fibres upon prolonged contact. However, these hazards of caustic soda pertain to the pure compound, not to finished cosmetic products, in which its use is strictly regulated.

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

After formulation, there is generally no residual free caustic soda in the final product, which explains why cosmetic-grade sodium hydroxide is regarded as 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 the skin under these controlled use conditions.