What it does
The EU's CosIng database lists two functions for sodium hydroxide: buffering and denaturant.1 Buffering means altering or keeping a product's pH at the desired level; a denaturant makes a product unpalatable, mostly one containing ethyl alcohol.2
The US Cosmetic Ingredient Review (CIR) describes the inorganic hydroxides as working mainly as pH adjusters, raising the pH of an otherwise acidic formulation. They are also used at very high pH in hair straighteners and depilatories, and sodium hydroxide has been reacted with fats to make soap for millennia. In US industry data from 2015 it had 5,147 reported uses, about half of them in leave-on skin care.3
Why little of it usually stays free
Sodium hydroxide is a strong base, and the CIR panel recognised that the hydroxides can irritate skin and eyes when used to make highly alkaline products such as depilatories and hair straighteners. In other products, it noted, most of their alkalinity will be neutralised by other ingredients in the formulation.3
So the panel judged them not by the amount added but by the free hydroxide ions that remain, which depend on the acidity of the other ingredients and are expected to be low in cosmetics, except in some depilatories and hair straighteners.3 An EU ingredient list, by contrast, is ordered by weight at the time each ingredient is added (Article 19(1)(g))4, so the name appears according to what went in, not what is left.
The EU limits
| Use | Limit in the ready-for-use product | Required warnings |
|---|---|---|
| Nail cuticle solvent | 5% | Contains alkali; Avoid contact with eyes; Can cause blindness; Keep out of reach of children |
| Hair straightener, general use | 2% | Contains alkali; Avoid contact with eyes; Can cause blindness; Keep out of reach of children |
| Hair straightener, professional use | 4.5% | For professional use only; Avoid contact with eyes; Can cause blindness |
| pH adjuster for depilatories | pH below 12.7 | Keep out of reach of children; Avoid contact with eyes |
| Other uses as a pH adjuster | pH below 11 | None |
The amounts are expressed as the weight of sodium hydroxide, and where sodium, potassium or lithium hydroxide are combined, their sum should not exceed the limits.4 The same entry covers Potassium Hydroxide.
The CIR panel concluded that the inorganic hydroxides are safe in hair straighteners and depilatories under conditions of recommended use, with users minimising skin contact, and safe in all other present practices of use and concentration when formulated to be nonirritating.3
Separately, the pure substance has an EU harmonised classification as Skin Corr. 1A, with the hazard statement H314 (CLP Annex VI, index 011-002-00-6).5 That classification concerns the concentrated chemical: the CLP Regulation does not apply to cosmetic products in their finished state (Article 1(5)).5
How it appears on a label
As Sodium Hydroxide.1 Its position reflects the amount weighed in, and below 1% EU lists may run in any order4, so neither tells you how alkaline the product is; ingredient list order explains the rules. On a depilatory, hair straightener or cuticle remover, the warnings in the table above must be on the container and packaging (Article 19(1)(d)).4 To see every pH adjuster on a list, paste it into the ingredient checker.