Acronym for alpha hydroxy acid. AHAs are derived naturally from various plant sources and from milk, but 99% of the AHAs used in cosmetics are synthetically derived. In low concentrations (less than 3%), AHAs work as water-binding agents. At concentrations greater than 4% and in a base with an acid pH of 3 to 4, these ingredients can exfoliate by breaking down the substance that holds dead skin 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 sensitize mucous membranes. However, AHAs have been widely used for improving signs of aging, dry skin and an uneven skin tone, all of which lead to younger-looking skin.
There is a vast amount of research that substantially describes how the aging process affects skin and that demonstrates that many of the unwanted changes can be improved by topical application of AHAs, including glycolic and lactic acids. AHA use can result in increased sensitivity to the sun, though 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.
References for this information:
Journal of the German Society of Dermatology, July 2012, issue 7, pages 488-491
Clinical, Cosmetic and Investigative Dermatology, November 2010, issue 3, pages 135-142