Commercial tanning salons that promise a safer indoor tan without a burn may pose a long-term threat of skin damage and cancer, a Food and Drug Administration study suggests.

Disputing what he called "unproven claims" of safety, FDA official C. David Lytle said yesterday that new laboratory findings provide the first "strong indication" that radiation emitted by tanning devices marketed in recent years may increase risk of skin cancer.

Many indoor tanning booths and beds use ultraviolet A, or UVA, a form of radiation just beyond visible light. Its wavelengths are shorter than visible light but longer than those of ultraviolet B, or UVB, the radiation emitted by older sunlamps. UVB is far more likely to produce severe reddening of the skin.

Natural sunlight contains both forms, with UVA predominating. While the smaller UVB portion has long been considered the major culprit in damaging skin and eyes, dangers of UVA by itself have been less well documented.

Because newer UVA tanning devices are less likely to cause quick sunburn in most people, some advertisers have touted them as "safer than the sun," Lytle said.

However, a team headed by Dr. Victoria Hitchins at the FDA's Center for Devices and Radiological Health has found that cells taken from mice and exposed to levels of UVA radiation comparable to those from tanning devices showed an increased rate of mutation. Lytle said this test is considered predictive of the presence of potential carcinogens.

The results are to be published in a scientific journal and may give federal regulators new ammunition to combat safety claims about the newest UVA devices.

"We're very concerned" about the claims, said Robert Handren of FDA's compliance staff, citing one estimate of at least 15,000 tanning salons nationwide. In addition, there are new products for home use.

Lytle, acting director of the division of life sciences, said longer wavelengths in UVA radiation are also known to penetrate the skin more deeply than UVB radiation, producing a "deeper tan" and perhaps other long-term damage.

He warned that UVA tanning devices can produce serious burns in "high-risk" persons who have sensitive skin, are "photosensitive" because of drugs or diet, have cold sores or have had certain kinds of eye surgery. He urged those who use tanning salons to protect their eyes.

Medical experts have long warned that excessive tanning, naturally or artificially, can lead to premature aging of skin and a common but usually treatable form of skin cancer. "But we know people want" tans, Lytle said. "It's a personal choice."