For years, Nancy Eddy of Chevy Chase prided herself on having the best tan of anyone she knew. "I used nothing on my skin when I sat outdoors," said Eddy, publications director for the Center for Strategic and International Studies. The attraction for Eddy, 57, who describes herself as slightly overweight, was that "a tan was good for taking off 10 pounds, and I tan beautifully."

But four years ago, she noticed a few rough red spots on her face that never seemed to heal. Her dermatologist diagnosed a pre-cancerous condition, known as actinic keratoses, and Eddy joined the growing ranks of Americans who have found that, good as the sun may feel, unprotected exposure is hazardous to their health.

This year alone, an estimated 600,000 Americans will be diagnosed with skin cancer, one of the fastest-growing afflictions in the United States. Skin cancers strike more Caucasians than all other cancers combined.

Most are basal cell carcinomas -- the kind former President Ronald Reagan had on his face, which, along with the neck, is where most cases of skin cancer appear. Basal cell carcinomas are slow-growing cancers that respond well to surgery and are rarely fatal. An additional 15 percent of cases are squamous cell carcinomas, which grow a little more quickly but with prompt treatment are usually not life-threatening. The least common, but deadliest, are malignant melanomas, which account for about 5 percent of cases.

The rapid rise in cases of melanoma alarms doctors and public health officials, who predict that at the current rate, at least 1 in 100 people could develop the disease by the end of this century.

About 6,300 people will die of melanoma this year, nearly as many as the number of women who die of cervical cancer, according to Charles Smart, chief of the Early Detection Branch at the National Cancer Institute.

One of the major factors in the increase in melanoma cases, some scientists theorize, is the depletion of the protective upper ozone layer, which is being eaten away by pollutants. Others discount this hypothesis and blame the popularity of sunbathing.

Skin cancer is only one of the problems associated with too much sun. The others are premature aging -- chiefly in the form of wrinkles and leathery skin -- and possible negative effects on the immune system, which enables the body to fight infection.

For most people, the solution is not to avoid the sun altogether but to engage in safe sunning. "We don't want people to become hermits and stay out of the sun altogether," said Margaret Kripke, chairman of immunology at the M.D. Anderson Cancer Center in Houston. "On the other hand, most people get enough sunlight in their day-to-day activities."

"The key," according to Nicholas Lowe, a dermatologist at the University of California at Los Angeles, "is to use sun protection." Sunscreen Confusion

But which sun protection?

"Sunscreens are confusing," acknowledged Warwick Morison, associate professor of dermatology at Johns Hopkins Medical Institutions. "There are many different SPF's {sun protection factors} and many different claims."

SPF, the sun protection factor designation that adorns most sunscreen labels today, is a measure established by the Food and Drug Administration; it indicates the level of protection provided. The higher the SPF number, the greater the protection.

Most sunscreen users fail to realize that SPF numbers refer only to protection from one type of ultraviolet ray -- UVB. Sunlight contains two types of rays: the longer wavelength -- UVA -- penetrates into the deepest layer of the skin, while the medium ray -- UVB -- is largely absorbed by the upper layers of the skin. Each causes its own type of damage.

Because of its long wavelength, UVA light passes through skin and glass, such as car windows, with ease. It dries the skin and increases the cancer-causing effects of UVB damage.

Often called the "safe" ultraviolet ray by tanning parlors, UVA light actually "penetrates the skin more efficiently than UVB," the FDA's William E. Gilbertson, director of the agency's over-the-counter drug evaluation program, said in a letter to the Schering-Plough Corp., a manufacturer of sunscreens. About 40 to 50 percent of UVA rays pass through the epidermis, the upper layer of the skin, compared with just 10 to 30 percent of UVB rays. "UVA radiation is becoming much more important than previously realized," Gilbertson said.

As yet, there are no standards for UVA light exposure. "In {1978} when the FDA panel first reviewed sunscreen ingredients, UVA light wasn't as well appreciated, so that panel didn't go into any great detail on it," Gilberston said.

Confusion about the truth of claims made by some sunscreen manufacturers of broad-spectrum protection is one reason the FDA recently announced it planned to implement standards for UVA exposure and warned some manufacturers to modify the language of their advertisements.

"Such labeling may lead consumers to believe that they are obtaining a greater degree of protection than they actually are," Gilbertson said.

Another problem is that sunscreens offer only partial protection from the most harmful effects of ultraviolet light. They "don't protect completely against the sun and should not be used as a total sense of security," said Stephen I. Katz, chief of dermatology at the National Cancer Institute.

Even when sunbathers avoid a burn or mute a tan by using sunscreen, "it doesn't mean that they are not getting bad effects {such as premature aging} from the sun," Katz said.

Many people fail to apply enough sunscreen -- roughly one ounce, or about a quarter of an average bottle -- required to protect the bathing-suit-clad body from sun exposure. "It has been shown that if you hand someone a sunscreen and tell them to put it on, the average person only puts on half as much as used in tests {by the FDA}," said Morison.

Most experts, including the American Academy of Dermatology, recommend SPF 15 sunscreens for all but the fairest or darkest complexions. Those who want to tan "will still tan through SPF 15," said Arthur Sober, associate chief of dermatology at Massachusetts General Hospital.

People who burn "will still get the same sunburn," added M.D. Anderson's Kripke. "It just takes you 15 times longer to get it than you normally would."

There are also changes in store for SPF. "We have no published standards for anything over SPF 15," Gilbertson said, although there are sunscreens with SPF values as high as 45. "We've looked at standards for SPF's above 15, and we will be addressing that."

Sun lotions with the highest SPF numbers "offer more protection," Gilbertson said, but are probably largely unnecessary, except for use in the Caribbean. "We'll probably come up with a cutoff of SPF 25 to 30," he said.

Only one product -- Photoplex -- is FDA approved for broad-spectrum coverage. But the drawback to Photoplex is that it offers less than SPF 15 protection against UVB light.

Few people have side effects from the approximately 100 sunscreens currently on the market. Some people are allergic to para amino benzoic acid -- PABA -- an ingredient once found in most sunscreens. PABA is not used in most lotions because it causes allergies and because it stains clothing and other surfaces. The most common reaction to PABA is a rash that clears up when use is discontinued.

No adverse effects have been documented with long-term sunscreen use. "There seems to be little concern about that," said Lowe of UCLA. If there are any potential risks, he says, "they are more than outweighed by the benefits."

"If we could get people to use sunscreens as routinely as they brush their teeth, we could undoubtedly have a fall-off in skin cancer," said Lowe. Sun Damage

Several weeks ago, a 37-year-old Arizona woman essentially died of a sunburn and heatstroke after she fell asleep while sunbathing beside an apartment house swimming pool in Tucson. The woman, who officials said spent about five hours lying in 107 degree heat, had been drinking heavily before she sunbathed.

"Basically, she drank a lot and cooked to death," said a fire official who transported her unconscious body to a hospital, where she died a short time later. She had suffered second-degree burns on her body, and although hospital officials packed her in ice, she never regained consciousness.

Few cases of sunburn are so severe, but all involve destruction of part of the epidermis -- the outer layer of the skin about as thick as a hair -- situated just below the stratus corneum -- the layer of dead cells that keep viruses and bacteria out and water in.

The most severe sunburns, much like third-degree burns, destroy the entire epidermis causing inflammation, bad blisters and a lot of pain. Milder sunburns produce redness and subsequent peeling.

The best treatment for a sunburn, doctors say, is to avoid one altogether, since there's basically not much you can do, according to NCI's Katz.

Like genetic destiny, the sun's ultraviolet rays affect how soon a face will be a candidate for the plastic surgeon's scalpel and how fast skin will start to lose its elasticity and sag.

Ultraviolet light produces a series of irreversible changes, from altering the shape of collagen and elastin -- key structural proteins in the skin -- to changing the DNA of cells.

The sun also triggers the production of melanin, the colored pigment manufactured by cells known as melanocytes, which produces a tan. Despite wide variation in skin color, everyone has roughly the same amount of melanocytes in their skin. The difference is in the ability of the cells to produce melanin to protect against ultraviolet radiation.

The lighter the skin, the less melanin produced and the less likely it is that a person will tan. Blacks rarely develop skin cancer of any kind, because they are protected by larger quantities of melanin.

Tanning, of course, causes less damage to the skin than a sunburn, but it too, exacts a price. "Tanning is visible evidence of injury to the skin," said David Bickers, a dermatologist at University Hospital in Cleveland, who chaired a National Institutes of Health panel last year on the effects of sunlight and ultraviolet radiation.

Sun damage may be far more than just skin deep. The most recent evidence suggests that exposure to ultraviolet light may actually tamper with the immune system, the body's line of defense against infection and cancer.

Animal studies have long documented the link between UV exposure and immune suppression, but more recent research in humans suggests that the same process may operate there as well.

At the University of Miami, Wayne Streilein, chairman of microbiology and immunology, exposed human skin to low doses of ultraviolet light, below the level needed to cause a sunburn. Then he tried to immunize people through this exposed skin and found that a significant proportion of healthy individuals were unresponsive, suggesting that part of the immune system had been turned off.

"This is exactly what we see in animal models," said Kripke, who has studied the phenomenon. "It's important because it validates a lot of what we have collected in animal studies."

Exactly how ultraviolet light alters the immune system is not yet understood, but recent findings from a variety of animal and human studies suggest that sunlight can switch off key points in the immune system. Scientists believe this is only a temporary effect.

Australian researcher Peter Hersey reports that healthy people exposed to UVA light from tanning booths showed a drop in the function of natural killer cells that wander the body in search of virus-infected cells.

Other work suggests that UV light may activate suppressor cells, which are programed to decrease the immune response to foreign antigens, such as that produced by poison ivy, which enter the body through the skin. These suppressor cells can act locally in the skin or, with repeated exposure, circulate through the body. As long as the cells exist, they prevent the immune system from reacting to the foreign agent. This reaction may last for several weeks or several months.

Studies at the University of Michigan document immunological changes in biopsied human skin that has been exposed to ultraviolet light. "These cells appear to have suppressive immune responses," Kripke said.

International research, including studies of Australian aborigines, increasingly suggests that long-term exposure to the sun promotes the development of cataracts, which are far more common in tropical climates.

The National Health and Nutrition Examination Survey has found that those with greater sunlight exposure are more likely to suffer from cataracts. So did a 1988 study of Chesapeake Bay watermen, conducted by Hopkins researchers.

Convincing people to take precautions against the sun may take some doing. A 1989 poll by the American Academy of Dermatology showed that 100 percent of adults surveyed "could name at least one or more negative aspects of the sun, from sunburn to wrinkles, aging skin and skin cancer." Yet 50 percent said they intentionally work on a tan -- a figure that has not changed in three years.

Even a brush with skin cancer doesn't always motivate people to change their habits. As Northwestern University dermatologist June Robinson reported recently in the Archives of General Dermatology, about 2 out of every 5 people who have a skin tumor surgically removed still refuse to wear sunscreens.

That's something Nancy Eddy understands. "It's still awfully tempting to sunbathe," said Eddy, who has had several precancerous lesions removed from her face. "I've stopped going to the beach, and I use SPF 15 on my face always."

Her dermatologist has recommended that she use SPF 15 on her legs as well. That's where she draws the line.

"I use SPF 8 there," she said. "I can't have white legs."

SAFE SUN TIPS

"There are a lot of ways that people can reduce their sun exposure without having a significant impact on their life style," said Margaret Kripke, chairman of immunology at the M.D. Anderson Cancer Center in Houston.

Among the recommendations from the experts:

Avoid peak sun hours whenever possible. Worst times: 10 a.m. to 2 p.m. or 11 a.m. to 3 p.m. (daylight savings time), when the sun's rays are most intense. A good rule of thumb: Stay out of the sun whenever your shadow is shorter than you are.

Use sunscreens. Apply liberally: use about one ounce of sunscreen per application about 20 minutes before going into the sun. Re-apply after swimming. Use an SPF 15 or higher if fair-skinned or for prolonged exposure.

Wear protective clothing. Wide-brimmed hats, long-sleeved shirts and pants of tightly woven fabric, since it is possible to burn through light clothing. To check fabric: hold clothes up to the light. If most of the light is filtered out, the clothes will protect against the sun.

Be careful of reflective surfaces. Sand, snow, water and even concrete can reflect up to 85 percent of the sun's rays. Be especially careful of sun exposure on cloudy or hazy days, when 80 percent of rays can penetrate the clouds.

Don't assume that a tan protects against the sun. A tan is a sign of damage to the skin. Keep using sunscreen, especially on children. It's still possible to get burned.

Keep infants and children protected. Babies younger than 6 months need to avoid direct sun exposure on their very tender skin. For older children, using a SPF 15 sunscreen until age 18 can reduce the risk of skin cancer by about 78 percent, according to the American Academy of Dermatology.

Medications and sun often don't mix. Some drugs, including tetracycline and ibuprofen, can react to sunlight, producing an itchy rash, redness or swelling or make skin more vulnerable to burning. Check with your doctor before going into the sun or using a tanning parlor.

Avoid tanning parlors and sun lamps. Despite claims of safety by operators, ultraviolet light emitted from these sources can cause sunburn, premature aging and increase the risk of skin cancer, a National Cancer Institute workshop reported last year.

Wear sunglasses. Don't assume the darkest glasses offer the most protection. For general outdoor use, look for glasses that screen at least 95 percent of UVB, 60 percent of UVA and 60 to 92 percent of visible light.

To ease a sunburn, take aspirin to help reduce swelling and pain; cool compresses or ice may help. Lotions won't prevent peeling but may help soothe and lubricate the skin for a few days afterward. If a burned area forms blisters or oozes, consult a doctor. Deadly Melanomas May Respond to New Vaccine Treatment

Malignant melanoma, the deadliest of skin cancers, is caused by the uncontrolled growth of the pigment-producing cells, the melanocytes.

Like other forms of skin cancer, it is the result of long-term exposure that began as much as 30 years earlier.

Melanoma is very rare among blacks and Hispanics. When it does occur in people with dark skin, it often appears on the palms or soles of the feet, areas that have the least pigmentation.

Those at highest risk of developing melanoma include redheads or blondes; those who experienced excessive sun exposure before age 15 or had at least one blistering sunburn during childhood or adolescence, and people in professions with long-term exposure, such as farmers, lifeguards, sailors or construction workers.

There appears to be a genetic link as well: People whose parents, children or siblings have had malignant melanoma are eight to 10 times more likely to develop the disease themselves.

With early detection, malignant melanoma is highly curable. Because it progresses rapidly -- and can quickly spread or metastasize throughout the body -- it is frequently fatal.

About 30 percent of cases arise from or near moles, one reason why the American Cancer Society urges people to regularly check moles. Those that are large, irregular or unevenly colored are potential melanoma sites.

Surgery is the usual treatment for melanoma. Chemotherapy is often used to treat cases that have spread to other parts of the body.

One of the newest therapies is an experimental vaccine that is now being tested by Jean-Claude Bystryn and his colleagues at New York University School of Medicine.

Unlike vaccines used to combat communicable diseases such as measles, polio or influenza, the melanoma vaccine is used to prevent recurrences of cancer, rather than the illness itself.

Made from extracts of human melanoma cells, the vaccine contains proteins, known as antigens, which Bystryn hopes can stimulate the body's immune system to identify and respond more vigorously to the melanoma cells.

More than 100 patients have now been treated with the vaccine in clinical trials. So far, there have been "no complications in any patients," Bystryn said. "It has clearly been safer than chemotherapy."

The vaccine appears to be biologically active and has stimulated an immune response to the melanoma in more than half the patients, according to a report presented recently at the annual meeting of the American Association for Cancer Research.

"Over all, our patients are doing better than the controls," Bystryn said, noting that patients treated with the vaccine have survived an average of 65 months without a recurrence of cancer, compared to 12 months for the controls.

"Progression of melanoma seems to be delayed in those people in whom the vaccine induces a good response," he said.

But, Bystryn noted, larger randomized trials of the vaccine are needed to determine its effectiveness.

More information on melanoma is available from:

American Cancer Society, 1825 Connecticut Ave. NW, Suite 315, Washington, D.C. 20009. Send a stamped, self-addressed business-size envelope for a free copy of "Melanoma/Skin Cancer: You Can Recognize the Signs" and "What You Should Know About Melanoma." Phone 483-2600 in the District; from Maryland or Virginia, call 1-800-ACS-2345.

Skin Cancer Foundation, Box 561, Dept. AB, New York, N.Y. 10156. To receive a free copy of "The ABCDs of Moles & Melanomas," send a stamped, self-addressed business-size envelope.