BHAs are beta hydroxy acids; the only BHA commonly used in cosmetics is salicylic acid. BHAs are oil-soluble, unlike AHAs, and can induce exfoliation in oily areas of the face. BHAs largely exfoliate inside the pore, and are hence comedolytic (resolve and prevent comedones/clogged pores) and may be recommended for the treatment of acne. Salicylic acid is also keratolytic, like AHAs, and may also address post-inflammatory hyperpigmentation over time. They are also most efficient at a pH of 3. They may have some antibacterial action. Opinions vary upon whether BHAs are safe for rosacean skin, although BHAs may be less irritating than AHAs.
See also Salicyloyl phytosphingosine and betaine salicylate.
Please note: If you’re allergic to aspirin, don’t use salicylic acid.