Thus, the remaining current challenge in formulating with AHA is balancing the efficacy, mildness and stability at formulation pH levels of 3.5-4.0. This specific pH requirement is not limited to AHA systems. It also applies to all acidic and protondonor actives, such as kojic acid, ascorbic acid, nicotinic acid retinoic acid and folic acid, all of which are more stable and effective in a low pH environment