Low-concentration alpha hydroxy acids (AHAs) and salicylic acid (called cosmetic acids) are chemicals promoted to improve signs of photoaging such as rough skin surface texture, fine lines, and uneven pigmentation. Home use of cosmetic acids originally became popular because they provide quick results on areas of rough skin by peeling off or exfoliating the upper layers of the epidermis, exposing smoother skin underneath.

In addition, numerous medical studies have also shown that use of low-concentration cosmetic acids results in improvements to the structure of the epidermis and the upper dermis and modest improvements in skin texture and fine lines. They may also stimulate the growth of collagen.

See for example "Epidermal and Dermal Effects of Topical Lactic Acid," Journal of the American Academy of Dermatology, 35(3 Pt 1), 388-391, 1996 Sep; and "Topical 8% Glycolic acid and 8% L-lactic Acid Creams for the Treatment of Photodamaged Skin: A Double-Blind Vehicle-Controlled Clinical Trial," Archives of Dermatology, 132(6), 631-636, 1996 Jun; (abstracts available on Medline). Net reviews of cosmetic acids are generally positive, though some people with sensitive skin report significant irritation.

Note that these acids are more than cosmetic in their effect, but the phrase "cosmetic acid" has become common. You may also hear the invented term "cosmeceutical" applied to acids, retinol, and copper peptide - that is, products which have some effect on skin biology, but don't require a prescription.

Products With AHA's

There are a variety of cosmetic acid products available for home use. Acid concentrations are typically listed as 2% to 20%, though concentrations are calculated differently among different manufacturers. (The important specification is actually pH or acidity, but manufacturers rarely list this.)

Most AHA products contain either glycolic acid, malic acid, or lactic acid. Salicylic acid is sometimes labeled a "beta hydroxy acid," indicating a different chemical structure than AHAs. It is also used in some acne medications. Unlike AHAs, salicylic acid is toxic at high-concentration and more caustic, so most salicylic acid products for home use contain concentrations below 7%.

Too much salicylic acid applied to the skin can cause salicylism, which is characterized by headache, dizziness, and ringing in the ears. (By the way, salicylic acid is not aspirin, that's acetlysalicylic acid, a slightly different chemical.)

Cosmetic acids are available as creams, lotions, and gels on the shelf at specialty cosmetics stores and most drugstores for less than $15. Some dermatologists, plastic surgeons, and cosmetologists sell the higher concentration products (above 10% for AHAs) in their offices, but prices are usually higher than in stores.

Note that cosmetic acids are not moisturizers, but some vendors classify them with moisturizers because they have a similar appearance effect on the skin. One popular, reasonably-priced AHA brand is Alpha Hydrox, in the red boxes. The Alpha Hydrox Oil-Free Formula is a 50 ml tube of water-based, 10% glycolic acid gel that can be diluted with water if necessary; it costs under $10 and is available at many drug stores and on the Net. Many drug and discount stores also sell an inexpensive store-brand 8% AHA lotion or cream.

Cosmetic acids can be used regularly if you have abnormally rough skin due to overgrowth of the top epidermal layers. If you have chronically rough and dry skin, you should also try various moisturizers including special urea-based moisturizers like Carmol 20, available from pharmacists. (A higher strength, Carmol 40 is also available by prescription.)

However, despite what some advertising says, people with normal skin should limit their use of cosmetic acid products. (This also applies to strong abrasive scrubs.) The upper epidermis provides an important environmental barrier and protection from UV radiation, so burning it off everyday with acid for the rest of your life doesn't make sense. FDA studies have confirmed that AHAs make the skin more sensitive to solar UV radiation damage (though the effect is reversible when use is discontinued).

Also, some doctors have raised questions about the safety of long-term cosmetic acid use. The continuous cell turnover that they produce could potentially accelerate some intrinsic skin aging processes. You should also know that there are Net reports of AHA use preceding the development of rosacea, which is sometimes triggered by skin irritation. If you have a family history of rosacea, you should probably not use cosmetic acids.