Sodium Hydroxide
Also known as Caustic soda, Lye, Soda lye, Sodium hydrate, White caustic
“CIR Expert Panel says: safe as used in cosmetics.”
Sodium Hydroxide (caustic soda, lye; CAS 1310-73-2; NaOH) is a strong inorganic base used in cosmetic formulations at trace concentrations as a pH adjuster (typically 0.01-0.5%) to neutralize acidic components, adjust formulation pH, and saponify fatty acids in emulsion preparation. At higher concentrations it is used in hair straightening and depilatory products to break disulfide and keratin bonds. The Cosmetic Ingredient Review Expert Panel assessed Sodium Hydroxide together with other inorganic hydroxides (potassium hydroxide, calcium hydroxide, magnesium hydroxide, lithium hydroxide) and concluded these ingredients are safe in hair straighteners and depilatories under conditions of recommended use — with the qualification that users should minimize skin contact — and safe for all other present practices of use and concentration when formulated to be non-irritating (Final Report September 2015; amended re-review published as Burnett, Bergfeld, Belsito et al., Int J Toxicol 2021). At the 5,147 cosmetic formulations reported in the 2015 VCRP data, Sodium Hydroxide is by far the most-used ingredient in the inorganic hydroxides assessment. About half of all uses are in leave-on skin care formulations where it serves purely as a trace pH adjuster — not as an active caustic agent. The QRT Finding 'SQ' and the specific hair-straightener/depilatory language reflect the dual-use nature of sodium hydroxide: it is safe as a trace pH adjuster in leave-on products, but in high-concentration professional applications (relaxers, depilatories) the caustic nature requires the recommended-use restrictions. Adverse events reported in the literature are primarily associated with hair-straightening formulations where sodium hydroxide is present at high concentrations; these events are not clearly attributable to sodium hydroxide alone rather than to co-formulated ingredients (e.g., thioglycolates) in such products.
Primary strong-base pH adjuster in cosmetic formulations — neutralizes acidic components and produces the alkaline environment needed for carbomer gel formation and fatty-acid saponification
Enables carbomer-based gel formulations: carbomer is not water-soluble until neutralized with a base; sodium hydroxide is the most common neutralizer
Essential for emulsion preparation via fatty-acid saponification: reacting sodium hydroxide with stearic acid or palmitic acid produces in-situ sodium stearate/palmitate surfactants that stabilize oil-in-water emulsions
CIR Expert Panel concluded safe as used in cosmetics (Final Report September 2015; Burnett et al. amended re-review 2021) with the specific hair-straightener/depilatory qualification; leave-on skin care at trace pH-adjuster concentrations carries no special restriction beyond 'formulated to be non-irritating'
Caustic/corrosive at high concentrations — in hair straighteners and depilatories (typically 1-3% NaOH for professional relaxers), direct skin contact can cause chemical burns; CIR qualifies these uses as safe only 'under conditions of recommended use' with the explicit caveat that users should minimize skin contact
Adverse events reported in the literature are primarily associated with hair-straightening formulations and are often not clearly attributable to sodium hydroxide alone — co-formulated thioglycolates and other reactive ingredients may contribute
In leave-on skin care formulations, sodium hydroxide is used at trace concentrations (0.01-0.5%) purely for pH adjustment; at these levels it is fully neutralized in the finished product and does not pose a caustic risk
CIR Quick Reference Table (12/2017, revised 07/2018) - Sodium Hydroxide row: Finding 'SQ' (Safe Qualified) with conditions 'safe in hair …
“Sodium Hydroxide | SQ | safe in hair straighteners and depilatories under conditions of recommended use: users should minimize skin contact; all other uses safe when formulated to be non-irritating | Final report 09/2015 available from CIR”— QRT-122017revised072018.pdf, p. 120