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 ingredients 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, all of which leads to younger-looking skin.
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: Facial Plastic Surgery Clinics of North America, February 2013, pages 55–60; 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; and Skin Pharmacology and Applied Skin Physiology, May-June 1999, pages 111–119). 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 daily eliminates this risk.
Note: AHAs are of little benefit when added to rinse-off products, as their contact with skin is too brief for them to function as exfoliants or absorb into skin (“Negligible penetration of incidental amounts of alpha-hydroxy acid from rinse-off personal care products in human skin using an in vitro static diffusion cell model”, Toxicology In Vitro, December 2011).