Sodium hydroxide, also known as lye, is a highly alkaline (also known as "base") ingredient used in small amounts in cosmetics to establish and maintain a product's pH in an acidic or basic formula. This is especially seen in products such as cleansers and in leave-on exfoliants, where the pH of the product must be kept within a very specific range for maximum effect (between 3-4 is ideal for acid-based exfoliants, while cleansers are generally pH 4.5-6) . The pH value of pure sodium hydroxide is 13, but in cosmetics it is only used up to pH 11.
Sodium hydroxide is seen in many applications in beauty products, such as nail polish removers and straighteners, where it is used in larger amounts (between 2% and 5% concentrations by weight) than in skin care products or makeup (eg leave-on), some lipsticks contain 0.25% sodium hydroxide). It is also used as a cleansing and denaturing agent. It is a significant skin sensitizer at concentrations of 10% or more. https://www.paulaschoice.com/ingredient-dictionary/ingredient-sodium-hydroxide.html?q=Sodium%20Hydroxide&fdid=ingredients