Acetic Acid

Acetic acid is a low–molecular weight acid that is clearly irritating at effective (low pH) use levels; even dilute solutions can sting and cause erythema on compromised skin, with higher concentrations being caustic. In cosmetic/skin-contact products it is typically used as a pH adjuster at low percentages, but because its irritancy is strongly pH- and concentration-dependent and sensitive/eczema-prone users can react sharply to acidic exposure, it warrants a very high irritancy score for patient safety. Safety Notes: In consumer skincare, acetic acid is most often used at very low levels (~0.01–0.3%) as a pH adjuster/acidulant in both leave-on and rinse-off products, and these trace levels are widely seen on INCI lists. Higher, consumer-available concentrations appear mainly in rinse-off chemical exfoliants/peels and callus/foot preparations where “acetic acid” is the declared active acid (commonly ~2–8%), while true household vinegar (~5% acetic acid) is not typically sold as a regulated cosmetic but informs the upper bound of consumer exposure in cosmetic-adjacent use; leave-on products at these higher levels are uncommon due to irritation potential and pH constraints.