Lactic acid is found in nature in milk, it is also found in the skin. The role of lactic acid is to improve the integrity of the acid mantle of the skin and it acts as part of the skin’s natural moisturising function. It not only acts as an astringent, but also helps to moderate the pH of the skin to keep it slightly the acidic (the ideal value). Lactic acid is used in skin softening preparations and has been used at higher levels for the treatment of thickened skin conditions (e.g. calluses, warts, etc). The SCCNFP adopted a position paper (SCCNFP/ 0370/ 00) (1) on the safety of AHA based on studies on short term phototoxicity (sensitivity of human skin to UV-induced damage: sunburn cells and pyrimidine dimers production) and skin irritation. The SCCNFP considered that there was a need for more information in order to provide a full scientific assessment of the safety of AHA. However, on the precautionary principle, the SCCNFP suggested that: lactic acid up to a maximum level of 2.5 % and a pH ≥ 5.0. [CAS: 50-21-5; EINECS: 200-018-0]. Function: Buffering/ humectant/ skin conditioning. The actual or estimated LD50 value: 3,543 mg/kg body weight. AICS status (NICNAS Australia): AICS Compliant. Oral LD50 value (rat): 3,543 mg/kg. Dermal LD50 value (rabbit): 2,000 mg/kg. Lactic Acid.  CIR: Concentration or other limitation on use for safe with qualifications conclusion: </=10%, at final formulation pH>/=3.5, when formulated to avoid increasing sun sensitivity or when directions for use include the daily use of sun protection;</=30%, at final formulation pH>/=3.0, in products designed for brief, discontin-uous use followed by thorough rinsing from the skin, when applied by trained professionals, and when application is accompanied by directions for the daily use of sun protection. 

Source: Dweck, Anthony. Handbook of Natural Ingredients (Dweck Books 4) . Dweck Data.