Today's skin-care products do more than just clean and moisturize: They promise smoother, clearer, more radiant skin with fewer fine lines and wrinkles. But there's a downside to some of the ingredients.

A seemingly endless array of creams, lotions and cleansers use alpha hydroxy acids to improve skin texture and make it firmer by thickening the collagen beneath the surface. But these acids also can make the skin more sensitive to the ravages of the sun. It becomes more apt to burn, which not only contributes to aging but also can lead to skin cancer.

The product packaging, however, doesn't specify how the acids work or that they can make the skin irritated and sun-sensitive.

Now federal regulators are recommending that consumers be warned about the risks of alpha hydroxy acids. The Food and Drug Administration is collecting public comments until the end of January and will review them before making a final decision on the wording.

With manufacturers "putting more and more active ingredients in products, such as AHAs, they should be warning the public that there may be side effects such as sun sensitivity," said Dr. Elisabeth Shim, a skin cancer specialist in Santa Monica and Pasadena. Such sensitivity can be a hidden risk, she said, because it's not as apparent as the redness and irritation the products also can cause.

Alpha hydroxy acids, which come from fruit and milk sugars, exfoliate the skin, which means they strip dead cells from the skin's top layers. But in thinning that outermost shield against the elements, they can increase the possibility of sunburn and sensitivity for up to a week after they've been applied.

The dangers can be avoided with careful use of sunscreens, but that message apparently isn't getting through.

The FDA has proposed that labels state: "Sunburn Alert: This product contains an alpha hydroxy acid (AHA) which may increase your skin's sensitivity to sunburn. Use a sunscreen and limit sun exposure while using this product and for a week afterward." However, because the recommendation doesn't have the force of law, the manufacturers must decide whether to warn customers.

Dr. Richard Glogau, a dermatologist at UC San Francisco, said patients who regularly use these products often complain that their skin is irritated and getting redder, even if they get only intermittent exposure to sunlight. He advises that they use sunscreens with a high SPF, such as 30 or 40. "Most of the people I see get the stern skin cancer lecture and go out the door with a much better sunscreen than they were using when they came in."

Most cosmetic products containing alpha hydroxy acids have concentrations of 6% to 10% of the acid. Stronger versions are used by cosmetologists, facialists and dermatologists for various facial peels. The category includes glycolic acid, lactic acid, citric acid and malic acid.

But Shim said alpha hydroxys shouldn't be singled out; other active ingredients cause similar irritation and sensitivity. They include some vitamin A-derived chemicals, such as retinol (which helps fade age spots and plump up tiny lines) and salicylic acid (which is used to exfoliate). Both can be found alongside the AHAs on store shelves.