[noun] AHAs, or alpha hydroxy acids, and BHAs, or beta hydroxy acids, work to chemically exfoliate the skin, meaning they are able to dissolve the bonds that keep dead skin cells clinging to surface of the epidermis, making it easier to remove them. Exfoliation is essential to cellular renewal.
AHAs are water-soluble. The most widely used AHA is glycolic acid, with a low molecular weight that means it can quickly penetrate the skin. They have what is called keratolytic action, as the molecule dissolves the keratin proteins that exist between the dead cells. BHAs act on the surface and are fat-soluble (meaning oil-soluble). They are recommended for normal to oily skin that may be prone to blemishes. The best-known BHA is salicylic acid, a very effective molecule that is also valued for its soothing and cleansing properties.