Acetaldehyde, also known as ethanal, is sometimes used to create specific scents or notes in cosmetic and skincare products. It’s sometimes used as a preservative in skincare products to extend shelf life, but concentration is regulated because of potential skin irritation. It is also used as an active ingredient in products like exfoliators (e.x. chemical peels, callus removers, etc), but these products would generally require specific labeling and warnings.