Denatured alcohol is a drying type of synthetic alcohol that irritates the skin and, when used in concentrations of 10% or more, weakens its barrier via depletion or improper reorganization of vital skin lipids. Typically, if denatured alcohol is listed as the fifth ingredient or higher, there is a significant risk of skin sensitization. This type of alcohol is also known as ethyl alcohol, SD alcohol and alcohol denat.
Denatured alcohol can increase the penetration of other ingredients into the skin; however, its effect in this regard irritates the skin and can trigger cell death (apoptosis). As a result, an alcohol-based skin care formula that contains other proven irritants such as menthol and/or fragrance oils can be doubly bad for the skin.
Ethyl alcohol is also the base from which alcoholic spirits such as gin and vodka are made. To prevent people from drinking cosmetic alcohol, a bitter-tasting denaturant is added that does not affect how alcohol works in skin care, makeup and hair care.
Denatured alcohol is used in over-the-counter products, including sunscreens, at levels up to 75%. Amounts lower than this are used based on formulation needs around aesthetics and absorbency. https://www.paulaschoice.com/ingredient-dictionary/ingredient-denatured-alcohol.html?q=Denatured%20Alcohol&fdid=ingredients