Acronym for alpha hydroxy acid. AHAs are derived from various plant sources or from milk. However, 99% of the AHAs used in cosmetics are synthetically derived. In low concentrations (less than 3%) AHAs work as water-binding agents. At concentrations over 4% and in a base with an acid pH of 3 to 4, these can exfoliate skin cells by breaking down the substance in skin that holds skin cells together. The most effective and well-researched AHAs are glycolic acid and lactic acid. Malic acid, citric acid, and tartaric acid may also be effective but are considered less stable and less skin-friendly; there is little research showing them to have benefit for skin. AHAs may irritate mucous membranes and cause irritation. However, AHAs have been widely used for therapy of photodamaged skin, and also have been reported to normalize hyperkeratinization (over-thickened skin) and to increase viable epidermal thickness and dermal glycosaminoglycans content. A vast amount of research has substantially described how the aging process affects the skin and has demonstrated that many of the unwanted changes can be improved by topical application of AHAs, including glycolic and lactic acid (Sources: Plastic and Reconstructive Surgery, April 2005, pages 1156-1162; Cutis, August 2001, pages 135–142; Journal of the European Academy of Dermatology and Venereology, July 2000, pages 280–284; American Journal of Clinical Dermatology, March-April 2000, pages 81–88; Skin Pharmacology and Applied Skin Physiology, May-June 1999, pages 111–119; Dermatologic Surgery, August 1997, pages 689–694 and May 2001 pages 1–5; Journal of Cell Physiology, October 1999, pages 14–23; and British Journal of Dermatology, December 1996, pages 867–875).
Because AHAs exfoliate sun damaged from the surface of skin, and this layer imparts some minimal sun protection for skin, there is a risk of increased sun sensitivity when using an AHA. (Source: Photodermatology, Photoimmunology, and Photomedicine, February 2003, pages 21-27) However, wearing a sunscreen eliminates this risk.