BHA acids, or beta hydroxy acids, are exfoliants that help remove dead skin cells and unclog pores, reducing the appearance of acne, improving the overall skin texture, and reducing redness and inflammation. Salicylic acid is a common BHA acid. They are particularly effective for oily and acne-prone skin types. They work by breaking down the substances that hold dead skin cells together and by promoting cell turnover. It's important to use them in a low concentration, gradually increasing the frequency of use, and use sun protection as they can increase sun sensitivity.