Ultraviolet A solar rays, a form of sunlight not blocked by many sun lotions now on the market, may be a major cause of premature wrinkles and sagging in sun-damaged skin, according to research published recently.

John Simon of Duke University, author of a study in the Proceedings of the National Academy of Sciences, said UVA radiation turns a natural molecule found on the skin into a destructive form of oxygen that accelerates skin aging.Researchers have long known that sunlight can cause the skin to wrinkle and turn leathery. But experts said the Simon study was the first to show why this happens - and to link it to UVA rays.

The study underscores the importance of using a lotion that blocks both types of ultraviolet radiation - UVA and UVB. Right now, most sunscreens focus on UVB, which causes most sunburns.

Simon said UVA is totally blocked by zinc oxide, that white goo lifeguards and other beach lovers often smear on their noses.

Missy Gough of the American Academy of Dermatology said her organization recommends that people use broad-spectrum sunscreens that have a sun protective factor, or SPF number, of at least 15 and provide some protection against both UVA and UVB.

Consumers should check the label to make sure lotions contain ingredients that protect against UVA, she said. These include benzophenone, oxybenzone, sulisobenzone, titanium dioxide, zinc oxide and avobenzone, a sunscreen chemical recently approved by the Food and Drug Administration and sold under the brand name Parsol 1789.

"This study shows that we have to pay more attention (to UVA) and to find blockers that are more effective in that region" of solar radiation, said Janna Wehrle, a research director at the National Institute of General Medical Sci-ences.

Wehrle, whose organization is one of the National Institutes of Health, called the discovery a major advance in understanding how the sun ages the skin.

Simon's study shows that UVA sunlight is absorbed by urocanic acid, a natural molecule made by the outermost skin cells. The sunlight chemically changes urocanic acid and causes it to create within the cells a type of oxygen-free radical, a highly active molecule that can be damaging to cells.

This oxygen radical, said Simon, "degrades collagen and elastin, which are the major molecules that make up the skin. This accelerates photoaging."

By degrading collagen and elastin, he said, "you decrease the elasticity of the skin. It makes you look older than you might be."

The oxygen free radicals from urocanic acid also may play a role in skin cancer, but that link has not been proven.

"People knew there was something doing damage, but they couldn't find any molecule that caused it," Wehrle said. "By finding this indirect mechanism, with the oxygen free radical, they have shown how light can cause the skin damage."

Although the study did not link the mechanism to cancer, Wehrle said the research "is a warning flag" that will need further in-ves-ti-gation.

Both Simon and Wehrle recommended long sleeves and hats for people going into the sun.

"People would do well to stay out of the sun in the center of the day," said Wehrle. "People should be covered more of the time they are outside. Hats are a good idea. Moderation is the key."