Alpha hydroxy acids are a group of natural acids found in foods. Lactic acids (found in sour milk) are applied to the skin (used topically) for moisturizing and removing dead skin cells, for treating acne and improving the appearance of acne scars, for improving the appearance of photo-aged skin, and firming and smoothing skin.  Alpha hydroxy acids are also used topically to treat extremely dry skin (xerosis), an inherited disease marked by dry, scaly skin (ichthyosis), and a condition that causes darkening of the skin (melasma).