Most people get 80 percent of their sun exposure in the first 20 years of life, reports Dr. Sidney Hurwitz, a Yale University pediatrics and dermatology professor. Since solar rays cause at least 90 percent of all skin cancers, dermatologists and pediatricians have mounted a campaign to urge parents to limit their children's exposure to the sun.

"Children should be taught good sun protection habits in the same way that that they are taught not to swallow poisons, not to run in front of an oncoming car and not to play with matches," the Skin Cancer Foundation advocates in a new brochure for parents.

"It's just like teaching kids not to smoke," agrees Dr. Stephen Katz, chief of dermatology at the National Cancer Institute.

Skin cancer is the most common of all cancers. About 500,000 people are diagnosed with the disease each year in the United States, and rates are rising. This trend has dermatologists "very concerned," says Katz, who believes the number of cases reported may be "grossly underestimated."

One reason for the increase: the popularity of sun-worshiping. Another is the greater amount of leisure time for outdoor activities. There is also a trend toward wearing skimpier clothing, allowing more skin to be exposed.

"It is imperative that parents and physicans recognize this and start protecting children from the sun," Hurwitz advised pediatricians at the American Academy of Pediatrics annual spring meeting held earlier this month in Orlando, Fla. "Often childen play outdoors and don't realize that they are getting a lot of sun exposure." Yet studies show that getting just one blistering sunburn doubles the risk of developing malignant melanoma, one of the deadliest forms of skin cancer, he said.

"We're trying very hard to deal with the preventive techniques," says Dr. Lawrence Solomon, chief of dermatology at University of Illinois School of Medicine, "rather than dealing with the ravages of cancer."

Although a bronze glow may look healthy, tanning is really a sign of injury to the skin. To minimize sun damage, skin cells produce a pigment called melanin that darkens the skin. Sun exposure also promotes aging.

But perhaps the most striking reason to take adequate protection from the sun is its effect on the immune system. Recent studies in animals by Katz and his colleagues at NCI suggest that the ultraviolet rays in sunlight may interfere with production of cytokines in the skin. Cytokines are important for prompting certain protective immune reactions in the skin. "We know that UV makes that happen in animals," Katz says. "There is evidence that it happens in humans as well."

The message, experts say, is not to suggest that people hide in the shade, but rather that they be sensible about sun exposure. Katz recommends avoiding sunburn, limiting sun exposure and using sun protection when it's necessary -- or desirable -- to get a few of the sun's rays.

Among other recommendations:

Keep infants and young children out of the sun as much as possible during the first year of life.

Use creams or white lotions on young children rather than clear lotions, which can contain alcohol and are less soothing. Always test the child for an allergic reaction to the sun screen by placing a small amount on the inside of the child's wrist. If it turns red, try a different brand, perhaps one that does not contain PABA -- to which some people are allergic.

Never bask in the sun.

Always use sun protection, either a sun screen or sun block.

Apply sun lotions at least 30 minutes before going into the sun to allow for good penetration in the skin.

Reapply sun protection frequently, about every 90 minutes.

Limit exposure when the sun is strongest, the hours from 10 a.m. to 3 p.m. Resources

"For Every Child Under the Sun -- A Guide to Sensible Sun Protection" is a free booklet endorsed by the American Academy of Pediatrics and published by the Skin Cancer Foundation. To receive a copy send a stamped, self-addressed business-size envelope to the foundation at Box 561, New York, N.Y. 10156.

A free skin cancer detection clinic will be sponsored by the D.C. Dermatological Society and the American Academy of Dermatology on May 31 at the National Institutes of Health in Bethesda. Appointments must be made in advance. For information call 656-7892.