PERSONAL HEALTH : A Bottle of Prevention . . .

A thinning ozone layer and the lax sunscreen habits of teen-agers will boost skin cancer rates far into the next century, experts report.

In a recent survey of 220 teen-agers--the majority of whom are white and upper-middle class--81% reported being avid sun worshipers but only 9% said they always used sunscreen. One-third of the teens said they never did.

Teens at higher risk for skin cancer--fair-skinned youths and those with a family history of skin cancer--were no more likely to use sunscreen, according to the report in the January issue of Pediatrics.

Even parents appear unmoved by warnings. A report in the May 15 Journal of the American Medical Assn. shows that 80% of parents surveyed thought suntans made their children look healthier. Only half of the 82 parents questioned used sunscreen on their children.

According to experts, 80% of lifetime exposure to the sun occurs before age 21. Moreover, exposure before age 20 is more strongly associated with the most deadly skin cancer, malignant melanoma, than exposure after age 30. Even one severe sunburn in childhood can cause skin cancer later.

The thinning ozone layer will add to the risks. The U.S. Environmental Protection Agency reported last year that ozone depletion will cause an additional 4,000 skin cancer deaths annually, including more cases of malignant melanoma. In the 1980s, cases of malignant melanoma rose 500%.

Experts say using a sunscreen with a sun protection factor (SPF) of 15 in childhood can reduce skin cancer incidence by 78%.