Earth's protective ozone layerthinned considerably over parts of Europe for three days in January,highlighting a growing problem of mini ozone holes over the continent thatcould threaten peoples' health, scientists said Wednesday.

The thinning, observed bysatellite, was triggered by unusual bursts of warm, tropical air that movedinto Central Europe and changed the chemistry of the upper atmosphere, whereozone resides and blocks most of the Sun's harmful ultraviolet rays. The rootcause of the depletion is thought to involve industrial pollutants that breakthe ozone molecules apart.

Studies suggest higher exposureto UV rays can lead to cancer and premature aging of the skin.

The mini ozone holes occur inaddition to larger holes seen each year above Earth's polar regions. Thesmaller events have been noticed only in recent years and are occurring morefrequently and, strangely, only over Europe.

"From 28-30 January, weobserved a pronounced streamer event, where streamers of tropical air pushingup from the equatorial regions spread over southern Spain, France andGermany," said Thilo Erbertseder of the German Aerospace Center DLR."Ozone levels in tropical air are much lower than those over more northernregions."

The event increased exposure toUV rays by 20 to 30 percent over the affected regions on clear days, accordingto a press statement issued by the European Space Agency.

"The increase in frequencyof mini ozone holes over Europe is resulting in an increase in harmfulbiologically active UV radiation," Erbertseder said.

The Global Ozone MonitoringExperiment (GOME) instrument aboard the ESA's ERS-2 satellite monitors theozone levels. Mini ozone holes were spotted in November of 1999and again in November 2001.

Ozone occurs naturally in thestratosphere and fluctuates naturally based on temperature and sunlight. Its levelsvary as much as 5-8 percent seasonally. But above the poles it can vary up to25 percent during the year, a remarkable change that is almost certainlyhuman-induced.

Most researchers agree thatchlorofluorocarbons, used as refrigerants and in aerosol containers, areresponsible for the large holes. Over Antarctica, the seasonal hole reached recordsize in 2000.