Phosphoric Acid

Phosphoric acid is primarily used as a pH adjuster/buffer (often ~0.1–1% in leave-on products, sometimes higher in rinse-off), and as an inorganic acid it can sting and irritate compromised skin depending on final formula pH. Clinical experience and patch-test data across acids show irritation risk rises as pH drops, with burning/stinging more likely in eczema, post-procedure, or barrier-impaired patients even at low concentrations. Because its safety is highly pH-dependent and it can contribute to cumulative acid load in routines, I rate it as a notable irritant requiring careful formulation and cautious use in very sensitive skin. Safety Notes: In consumer skincare, phosphoric acid is most often used as a pH adjuster/buffer in cleansers, toners, and lotions at very low levels (~0.01–0.3%), where it contributes negligible exfoliation and primarily controls finished-product pH. The highest OTC levels observed are in strong at-home chemical exfoliants/peels and some wart/callus-type cosmetic acids marketed to consumers, where phosphoric acid can be present at several percent up to ~10% in leave-on or short-contact formats; above this, products tend to fall into professional-use or non-cosmetic categories and are less common due to irritation/corrosivity constraints and pH limits.