Lye Middel Irritant No known benefits Lye at a glance Highly alkaline ingredient Used in small amounts in cosmetics (such as in exfoliants and cleansers) to adjust pH Also used in hair and nail products to maintain pH Called sodium hydroxide or potassium hydroxide Lye description Lye is a term for a highly alkaline chemical compound known scientifically as sodium hydroxide. Also called caustic soda, sodium hydroxide is an ingredient used in small amounts in cosmetics to adjust the pH of a cosmetic formula. Lye is also in reference to potassium hydroxide when used in liquid formulas such as soaps. Its used as a pH adjuster is seen particularly in products such as cleansers and in leave-on exfoliants, where the product’s pH needs to be held within a very specific range for maximum efficacy (between 3-4 is ideal for acid-based exfoliants, while cleansers are generally pH 4.5-6). Pure lye’s pH level is 13, but in cosmetics, it is only used up to pH 11. Lye is seen in many applications in beauty products, such as nail cuticle solvents and hair straighteners, where it’s used in greater amounts (between 2% and 5% concentrations by weight) than in leave-on skincare products or makeup (for example, some lipsticks contain 0.25% sodium hydroxide). It’s also used as a cleansing and denaturing agent. It’s a significant skin sensitizer in concentrations of 10% or greater. Much lower amounts are used in solution to pH control leave-on, acid-based exfoliants. Related ingredients: Potassium Hydroxide Sodium Hydroxide