As people live longer, they spend more time in the sun. But for half a million Americans each year, the result can be cancer -- and possibly fatal. MALIGNANT MELANOMA Cases:

26,000 a year.

Deaths:

5,800 a year.

Symptoms:

Unusual skin conditions, such as scaliness, oozing or bleeding. A mole that changes shape, color or size or that becomes ulcerated or bleeds easily from a small injury. A new bump or nodule. A pigmented spot that spreads or becomes tender, itchy or painful.

Risk factors:

Excessive sun exposure or severe sunburn in childhood. Fair complexion. Occupational exposure to coal tar, creosote, pitch, arsenic compounds or radium.

Prevention:

Avoid the sun between 10 a.m. and 2 p.m. Wear protective clothing. Use a sunscreen, especially one containing PABA, or para- aminobenzoic acid. Do a self-examination once a month.

Treatment:

Surgery, used in 90 percent of cases; radiation therapy; electrodessication -- tissue destruction by heat; or cryosurgery -- freezing affected tissue.

Survival rates:

Overall 5-year survival rate for malignant melanoma in white* patients is 80 percent; for localized malignant melanoma, 89 percent; for malignant melanoma detected after it has spread elsewhere in the body, 46 percent.

SQUAMOUS-CELL AND BASAL-CELL CARCINOMAS Cases:

Of more than 500,000 cases of skin cancer each year, the vast majority are basal or squamous-cell cancers, which rarely metastasize, or spread, but can cause damage to skin tissue.

Deaths:

Of an estimated 7,800 skin cancer deaths a year, 2,000 are attributed to either basal- or squamous-cell cancers.

Symptoms:

New skin growths, particularly emergence of a pale, waxlike, pearly nodule or a red, scaly, sharply outlined patch. Changes in appearance or sensation of small sections of skin.

Survival rates: Nearly 100 percent, with early detection and treatment -- usually minor surgery to freeze or scrape off the cancer cells. *Among blacks, because of heavy protective skin pigmentation, skin cancer is rare. Source: American Cancer Society, "Cancer Facts & Figures, 1987."