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The protective ozone layer over North America has rebounded from its extremely low level of two winters ago, but that doesn't mean it's time to relax.

High-altitude "ozone over the U.S. during the winter of 1993-94 recovered from the record low values of the previous winter," a team of scientists reports in Geophysical Research Letters. Ozone levels that were as much as 15 percent below normal in 1992-1993 have risen to slightly above normal.The layer of ozone high in the atmosphere helps block dangerous ultraviolet radiation from the sun. Too much of this radiation can lead to skin cancer, premature aging of the skin and eye damage.

Samuel J. Oltmans, one of the researchers, stressed that the recovery of the ozone layer doesn't mean it's safe to spend time in the sun without protection.

"There's still the long-term decline (in ozone) that's been going on for the past dozen years or so," he said.

"The recovery really is a recovery to the levels represented by the long-term decline, it's not a recovery to levels that we saw in the mid-1970s," he said in a telephone interview.

Researchers at the University of Athens, Greece, reported in the same issue of the journal that their studies indicate a per-decade decline in ozone levels of 2.5 percent in summer and 7 percent in winter over populated areas.

Chemicals from the eruption of Mount Pinatubo in the Philippines helped cause the unusual ozone decline in the winter of 1992-93, but that material has largely settled out of the air, scientists report.

The new study was based on measurements of ozone in the atmosphere above Nashville, Tenn.; Fresno, Calif.; Wallops Island, Va., and Boulder, Colo. Oltmans said satellite measurements of ozone had similar findings.

Damage to the ozone is associated with the chemical chlorine in the air. Environmentalists have fought in recent years to reduce this by limiting use of chlorine-containing gases in cosmetics and other manufacturing processes.

The ozone damage was sharply increased by the chemicals blasted into the air by Pinatubo in 1991, said Oltmans, of the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory in Boulder. Also talking part in the study were researchers from the University of Colorado and the University of Wyoming.