Sodium Carbonate

Sodium carbonate (washing soda) is a strongly alkaline pH adjuster/buffer; even at low leave-on levels it can raise product pH enough to disrupt the stratum corneum, increasing stinging and barrier damage in eczema-prone or compromised skin. In rinse-off products it’s often used at higher effective alkalinity, where clinical experience and patch/occupational data show notable irritant potential, especially with repeated exposure. Given its caustic alkalinity and high likelihood of barrier disruption in sensitive users, it warrants a high irritancy score. Safety Notes: In consumer skincare/cosmetic products, sodium carbonate is most often used at very low levels (~0.01–0.5%) as a pH adjuster/alkalizing buffer in rinse-off cleansers, soaps, and some bath products, with occasional use around ~1% in strongly alkaline cleansing systems. The highest consumer-available concentrations are found in highly alkaline cleansing powders/pastes and specialty exfoliating/cleansing products positioned as “deep-clean” or “soap-based,” where sodium carbonate can function as a primary builder/alkalinity source and reach roughly 10–20% in anhydrous or low-water formulas. It is generally unsuitable for leave-on skincare at meaningful levels due to high alkalinity/irritation risk, so high-end concentrations are overwhelmingly rinse-off.