A fatal type of skin cancer is affecting more people at a younger age than ever before, and the trend may get worse if chemical pollution continues to leach natural ozone from the atmosphere, an expert says.

Dr. Darrell S. Rigel, an assistant clinical professor of dermatology at New York University Medical School, said Tuesday that melanoma, a type of skin cancer that can be fatal if not treated, is now diagnosed at the rate of one of 128 Americans, an increase of about 1,500 percent compared to 1935.Even more disturbing, the ages of patients developing melanoma is dropping dramatically, Rigel told the American Academy of Dermatology annual meeting.

"Five years ago it was unusual to see someone under the age of 40 with skin cancer," said Rigel. "Now we're commonly seeing people in their 20s with skin cancer."

About 25 percent of the estimated 27,300 Americans diagnosed as having melanoma this year will be aged 39 or under, he said. Only a few years ago, melanoma was considered a disease of the aged, Rigel said.

"Not only has the overall rate grown, but the age group where it has grown the most rapidly is in the youngest group," said Rigel. "The rate of increase is highest among the youngest people, not only here, but in other countries."

Most medical experts believe that melanoma, a skin cancer that will spread throughout the body if not excised by surgery early, develops after 10 to 20 years of heavy or damaging exposure to ultraviolet rays of the sun. But Rigel said the increasing rate of the disease among the young suggests that the time it takes to develop the disease may be decreasing.

A loss of ozone in atmosphere "might be the explanation of why people are getting it younger and younger - maybe. That has been brought up as a hypothesis," said Rigel. "For every decrease of 1 percent in the ozone layer, there is a 2 percent increase in the incidence of skin cancer."

Ozone is a natural gas present in the atmosphere that partially blocks ultraviolet solar rays from reaching the Earth's surface. Studies in recent years have shown that some types of chemical pollutants, principally the fluorocarbons that are used for refrigerants and solvents, destroy the atmospheric ozone, thus allowing more ultraviolet rays to reach Earth.

Counterbalancing the ozone-loss effect, said Rigel, is a growing consciousness about the dangers of excessive exposure to the sun. He said more people are using solar screens, a lotion that is applied to the skin to filter out ultraviolet sun rays.

"People are so much more sun conscious than they were just five years ago," he said in an interview. "The use of sun screen has risen very dramatically."

Many parents have become aware that sun damage on the skins of children under the age of 10 can result in cancer among those youngsters before they are 30, said Rigel. As a result, he said, even children now are using sun screen.

"We won't begin to see the effects of that sun screen usage for another 10 to 20 years from now," he said. "The hope is that by the turn of the century, the sun screen use and more common sense about avoiding the sun will kick in" and cause a decline in skin cancers.

If not, said Rigel, he predicts that by the year 2000, melanoma may be striking one in every 90 Americans.

Melanoma first appears as a black, brown or mixed colored spot of irregular shape on the skin's surface. Left untreated, the cancer penetrates to the interior of the skin and releases cells that can move through the bloodstream and establish colonies in the bone, brain, lung or even heart. Once this happens, the disease cannot be cured, said Rigel.

But if the cancer is detected early, when it is still about the size of a pencil eraser, it can easily be cut from the skin and cure is almost certain, he said.

Rigel said the number of melanoma cases in the United States has almost doubled during the last eight years, and the number of deaths has tripled, to about 5,800, since 1960. Yet, the survival rate for melanoma patients has risen from 60 percent in the 1960s, to about 83 percent now.