This column is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. Carla Helfferich is a science writer at the institute.

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Suntan season in the north is shaded slightly this year by nervous medical researchers who want us to stay out of the sun. Entirely, please. If we must expose ourselves at all to frightful sunshine, they say it had better be while we are wrapped in long pants and sleeves, and standing under a parasol or at least a wide-brimmed hat.

The experts' horror of ultraviolet light isn't really new. For years they've been warning sunlovers like me that sunshine ages skin--young tanned hides quickly become old leathery ones, so to speak. And sunburn is as damaging as any other kind of burn. But now the medical warnings have a new urgency.

The British journal New Scientist, issue of 16 May 1992, contained an article reviewing what's known about sun damage. The article's title indicates the main problem with too much sunshine: "The resistible rise of skin cancer."

Skin cancer of all kinds is more common than ever before, and the most dangerous kind--melanoma--is blooming apace wherever pale skins have a chance to meet strong sunshine. Australia is the model of such a place, and it is fast becoming the world's skin cancer capital.

The correlation works even where sunshine is not so strong. Melanoma occurrence increased in Scotland by 82 percent between 1979 and 1989, but the trend nicely parallels the growing popularity among Scots of seeking sunny vacation spots. Apparently, reaching the Mediterranean countries and points south is now easy and comparatively inexpensive for people who reside in cloud-shielded northern European regions.

The statistical correlation between sun exposure and skin disease is very clear for the less dangerous cancers. They are most likely to develop in pale-skinned people who work outdoors, and they usually appear on patches of skin most likely to be exposed--hands, face, neck, arms. Melanoma development is more complicated. Its usual victim is a relatively affluent office worker. Its most likely site is on a man's torso, or a woman's legs.

Researchers are beginning to get a handle on the reasons for the patterns. Some of the new understanding stems from studies in genetics. For example, in a rare human genetic disorder called xeroderma pigmentosum, the gene encoding the enzyme that repairs DNA damaged by ultraviolet light does not function properly. People with a defect in that gene are a thousand times more likely to develop melanoma than are people with functional versions of that gene.

The real breakthroughs in understanding why sunbathing is dangerous have come thanks to work in immunology. It's now well understood that exposure to ultraviolet light suppresses the immune system in many animals, including mice and humans. The exact way in which the light does its nefarious work isn't known, but it is recognized to have two effects. One is confined to the irradiated skin, while the other upsets the whole system.

Thus, for example, the UV light could damage the DNA of a skin cell, turning it cancerous--the first effect. Then, the UV radiation-damaged immune system would be unable to attack the newly cancerous cell--the second effect. Together, the two can produce serious problems.

Ultraviolet light suppresses the immune system no matter how well tanned a person is, and it afflicts blacks as well as whites. Sunscreens don't protect against the immune suppression effect. A malfunctioning immune system permits other diseases beside cancer, but no one has yet quantified what those other ailments are in well-sunned victims.

It seems odd that sunshine should turn off the immune system, but evolutionary theoreticians have a possible explanation. Sunburn surely has been a common ailment over the ages, and sunburned skin cells look alien to a healthy immune system. A sunburned body would attack itself at the cellular level, perhaps dangerously; a little immune suppression thus could actually be a survival advantage.

Knowing now that sun damage is more than skin deep, I'll modify my summer habits--a little. Besides, a big hat can be used to swat mosquitoes.

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