Sunscreen can't give blanket protection.

Sunlovers' security blanket against cancer-causing rays -- sunscreen lotion -- appears to have some holes in it, researchers now report.

Such lotions help prevent mild skin cancers and sunburn. But exposure to the sun's ultraviolet (UV) rays may have another, less visible effect that sunscreens fall to stop - impairing the ability of immune cells to fight melanoma, report Margaret L. Kripke and her colleagues of the University of Texas M.D. Anderson Cancer Center in Houston. New cases of melanoma, the deadliest of skin cancers, have doubled since 1980. About one in 105 Americans will develop it; 20 percent of them will die from it, U.S. melanoma deaths totaled about 6,800 in 1993.

"Protection against sunburn does not necessarily imply protection against other possible UV radiation effects, such as enhanced melanoma growth," Kripke and her co-workers report in the Jan. 19 JOURNAL OF THE NATIONAL CANCER INSTITUTE.

In fact, by preventing the pain and realness of sunburn, sunscreen may enable people to stay longer in skin-scorching sun, putting them at higher risk for developing melanoma, Kripke says.

In their study, the researchers applied either a sunscreen or an oil-water mixture to mice's ears and tails. After 20 minutes, the team exposed some mice for 20 to 27 minutes to UV-B, the rays most sunscreens try to block. They repeated the exposure twice a week for three weeks, so the mice received about three times the UV-B needed to give them a sunburn. After the final session, the researchers injected all of the mice's ears with melanoma cells from genetically identical mice to see how well their immune system fought off the cancerous cells.

The group of mice put under the sunlamp had a higher incidence of melanoma tumors than the unexposed mice, Kripke and her co-workers write. The lotion protected them against sunburn, however.

Finding that protection against sunburn does not necessarily imply protection against melanoma is "really kind of surprising," Kripke says.

In other studies using lower light levels, sunscreen reduced the DNA damage that researchers believe leads to melanoma, she notes. She speculates that even a small amount of DNA damage may increase an animal's melanoma risk.

The study "reminds us that the pathophysiology of melanoma appears to differ markedly from that of non-melanoma skin cancer," note Howard K. Koh and Robert A. Lew of the Boston University Schools of Medicine and Public Health in an editorial accompanying the article.

"This is really a particularly interesting study" because it shows that the radiation stimulates the melanoma cells. to, grow, says John Epstein of the University of California, San Francisco. However, a stronger sunscreen than the one the researchers used might have offered more protection, he says.

Darrell S. Rigel of New York University Medical School also warns that these results may not apply to humans, since melanoma acts differently in mice.

Other animal studies have found that light not screened out by most sunscreen lotions, such as UV-A, may cause the cell changes that lead to melanoma (SN: 7/24/93, p.53). To ward off skin cancer, use lotions and dress properly, researchers conclude. "With a wide-brim hat you can prevent 70 percent of the rays" from reaching you, Epstein says.