Skin cancer is the most common kind of cancer, affecting some 500,000 Americans each year. Fortunately, it is also the most treatable kind--95% of those who develop skin cancer are cured by surgery, X-ray therapy, or drug therapy. In addition, medical researchers are fairly certain that they have identified the cause of most types of skin cancers, so that preventative measures can be taken.

In 90% of skin cancer cases, sunlight is regarded as a causative factor. Sunlight radiates through the fragile atmosphere of our planet, particularly in areas where the ozone layer has been depleted. Ultraviolet (UV) light--the component of sunlight that is potentially damaging to the skin--is emitted in different wavelengths that vary in their ability to penetrate the ozone layer. UVC rays, those with the shortest wavelength, do not penetrate the ozone layer. UVB rays, intermediate in wavelength, are largely blocked by ozone, but enough get through to cause about 80% of sunburn damage. UVA rays, with the longest wave-length, are not blocked by ozone. Until the 1980s, UVA rays were thought to be relatively safe because they take longer then UVB rays to produce a burn. Now, however, it is recognized that skin damage can occur even without a burn. UVA rays penetrate the skin deeply, breaking down connective tissue and causing the skin to sag and wrinkle. They also are believed to potentiate the effect of cancer-causing UVB rays, and they themselves may lead to the development of cancer.

People who stay out in the sun too long can get a burn just as severe as that caused by fire or boiling water. Both UVA and UVB rays stimulate the pigment producing melanocytes to produce more melanin, and it is the buildup of this melanin that produces your tan. The tan may look attractive, but it is by no means healthy--tanned skin is damaged skin. The consequences of repeated exposures may not appear for 15 to 20 years, but when they do, they range from wrinkles, blotches, and unattractive growths to various forms of skin cancer.

The most frequently encountered skin cancers are of three types:

Basal cell carcinoma, the most common skin cancer, accounts for about 70% of total cases. It usually occurs where the sun strikes hardest--on the face and arms. This type of cancer arises from cells in the stratum basale. It appears first on the surface of the skin as a small, shiny bump. As the bump enlarges, it often develops a central crater that erodes, crusts, and bleeds. Fortunately, there is little danger that it will spread (metastasize) to other areas of the body. These carcinomas are usually treated by excision (surgical removal).

Squamous cell carcinoma arises from cells immediately superficial to the stratum basale. Normally, these cells undergo very little division. But in squamous cell carcinoma they continue to divide as they produce keratin. The result is usually a firm, red keratinized tumor, confined to the epidermis. If untreated however, it may invade the dermis and metastasize. Treatment usually consists of excision and X-ray therapy.

Malignant melanoma, the most life-threatening form of skin cancer, arises from the melanocytes located in the stratum basale. Often, it begins as a small molelike growth, which enlarges, changes color, becomes ulcerated, and bleeds easily. Metastasis occurs quickly, and unless treated early--usually by widespread excision and X-ray therapy--this cancer is often fatal.

A variety of measures are recommended to protect yourself against all three varieties of skin cancer. First, use a sunblock to protect your skin. The currently available products at SPF 15 ratings will filter out about 90% of the damaging light. If you prefer not to slather yourself with lotion, wear protective clothing. Long-sleeved, dark fabrics with tight weaves are best. Baseball caps offer some protection, but wide-brimmed hats are better. In addition, wear sunglasses designed to block both UVA and UVB radiation; they not only protect the skin around your eyes, but may also help to prevent cataracts. And, finally, stay out of tanning booths. Despite merchants' claims that they have blocked out all damaging wavelengths, patrons are typically exposed to 2 to 20 times the amount of UVA that they get in natural sunlight.

It is also important to be viligant in monitoring your body. Do you have warts or moles? Check these for changes in size, shape, color, or differences in sensation. Check also for any new, suspicious growths. If a mole has ragged edges, is multicolored, is asymmetrical, or has a diameter greater than that of a pencil eraser, consult your physician promptly.

If you take preventive measures, your chances of developing skin cancer will be greatly reduced, and you will probably enter old age looking better than your less cautious contemporaries. For those of you who are diehard sun-worshippers, the American Cancer Society has a succinct warning: Fry now, pay later.