Alpha hydroxy acid, or AHA, and beta hydroxy acid, BHA, are naturally derived, closely related chemical exfoliants. Both acids have a rejuvenating effect on skin and are used in a range of topical cosmetic preparations, from over-the-counter blemish creams to professional quality facial peels. But despite their similar qualities, AHA and BHA are not identical; the use of one over the other depends on skin type and the purpose of treatment.
Alpha hydroxy acids are naturally derived from fruits, milk and sugars. Well-known AHAs include lactic acid from milk; glycolic acid from sugar cane; malic acid from pears and apples; and citric acid from oranges and lemons. The BHA most commonly used in cosmetics is salicylic acid, derived from willow bark.
Both AHA and BHA work to break apart the uppermost layer of skin. This facilitates the removal of dead and damaged surface cells and encourages the growth of the newer, healthier skin beneath.
The acids are commonly found in skin care products formulated to "peel away" the fine lines, wrinkles and uneven pigmentation associated with aging skin. BHA and AHA are also used to treat acne; the chemicals work to unplug clogged hair follicles and counteract the overproduction of sebum.
The main difference between alpha and beta hydroxy acids stems from the fact that AHAs are water-soluble and BHAs are lipid--oil--soluble. This means that AHAs are more effective at exfoliating damaged, dry skin while BHAs are better able to penetrate oily, blemished skin and pores.
The amount of exfoliation that occurs depends on the product's strength and other ingredients. Furthermore, it can take multiple applications of any AHA or BHA treatment before the desired results are achieved.
According to studies conducted at the Ohio State University and Massachusetts General Hospital, regular use of AHA products can result in a smoother, more even toned complexion. The research on how BHA affects the skin's appearance is less conclusive, say the authors of "The Idiot's Guide to Beautiful Skin." Additionally, though BHAs are often marketed as a milder alternative to AHAs, studies have not yet confirmed this claim.
As superficial exfoliants, AHA and BHA only affect the top, dead layer of skin. Theoretically, this means that even the strongest hydroxy acid concentrations will not produce adverse reactions. However, side effects can vary according to the product and the user's skin type and sensitivity.
Common side effects of AHA and BHA use include temporary stinging and mild redness. Skin may initially become dry and flaky as the cells begin to shed, but this should clear up within a few weeks. Alpha and beta hydroxy acids can cause skin sensitivity and raise the risk of sunburn. Should any side effect become severe, discontinue use of the product immediately.