Along the line of lounge chairs beside the pool, bared stomachs undulate softly with shallow breathing. Stillness is everything: A twitching foot does not tan. The stoic reveries of the sun worshipper are only disturbed by a neighbor's bolt from chair to water: Another sunbather has suddenly hit the broiling point.

In search of the healthy look, they pursue the tan. But since this glow may lead later to skin cancer and the wrinkles of premature aging, the art of burn-and-peel-and-tan is being discarded. Tanning-by-number is coming into vogue. The poolside conversation opener "What's your sunsign?" has been supplanted by "What's your sunscreen? You remind me of a Two I once knew."

Slathering on a sunscreen before going out in the sun makes good sense. "Basically, sunscreens block out ultraviolet light and prevent damage to the skin," says Ronald N. Shore, a Silver Spring dermatologist and assistant professor of dermatology at Johns Hopkins. "If a person is capable of tanning, he can achieve the tan with minimal or no sun damage by using a sunscreen."

In 1978 an FDA panel proposed labeling sunscreens with numbers up to 15: the greater the number, the greater the protection. Sunscreens with lower numbers permit a degree of tanning. Wearing a lotion rated "4" means you can stay in the sun without getting burned about four times longer than if you were unprotected. (You need to reapply it after swimming.)

The panel said that the people most likely to suffer skin damage or cancer from a lifetime of exposure to sunlight have light eyes and skin, are of Northern European descent and live in sunny places. (There are more skin cancers in Texans than Minnesotans.) They recommended that people stay out of the sun from 10 to 2. Several ingredients work as sunscreens, from para-aminobenzoic acid (PABA) to red veterinary petrolatum (RVP or red vet pet).

What's the best sunscreen for one person isn't always best for another, Shore says. For example, someone with a very light complexion, in their 50s or 60s, with signs of sun damage over the years, will want the best protection. That's a sunscreen with a sun protection factor of 15. The person won't tan, and this will limit sun damage.

Someone with a naturally darker complexion generally will tolerate the sun better, and wanting a deeper tan could use a sunscreen with a sun-protection factor of from 4 to 8.

A regular sunscreen may not be adequate for people taking certain medications -- some antibiotics and some tranquilizers -- which may render a person particularly susceptible to effects of the sun. In this case, check with your doctor. Another caveat: some ingredients in sunscreen preparations will block pores. Cocoa butter is one of them. "In a person who is acne-prone," says Shore, "use of cocoa butter can be detrimental."

"By using sunscreens, we are actually preventing skin cancers," he says, adding that if young people were to use sunscreens now, "they would also have far less wrinkling 20 or 30 years from now."

If you're still reluctant to take the plunge, you won't be alone: when Shore goes to the beach, which he does occasionally, he uses a sunscreen, too. TANNING BY NUMBER 8 OR MORE -- Labeled "Maximal" (8 to 14) or "Ultra" (15 or greater). For someone who always burns easily; never tans. 6 TO 7 -- "Extra." Always burns easily; tans minimally. 4 TO 5 -- "Moderate." Burns moderately; tans gradually (light brown). 2 TO 3 -- "Minimal." Burns minimally; always tans well (moderate brown). 2 -- "Minimal." Rarely burns; tans profusely (dark brown).