Washington, Oct 21 (IANS): Scientists worldwide are looking for evidence that the ozone layer is beginning to heal, but this year's data from Antarctica does not hint at a turnaround.

National Oceanic and Atmospheric Administration (NOAA) and NASA use balloon-borne instruments, ground instruments and satellites to monitor the annual South Pole ozone hole, global levels of ozone and the manmade chemicals that contribute to ozone depletion.

The ozone layer helps protect the earth from harmful ultraviolet radiation, but environmental factors punch a hole through it every year, for several weeks.

The Antarctic ozone hole, which yawns wide every spring, reached its annual peak on Sep 12, stretching 10.05 million square miles, the ninth largest on record.

"The upper part of the atmosphere over the South Pole was colder than average this season and that cold air is one of the key ingredients for ozone destruction," said James Butler, director of NOAA's Global Monitoring Division in Boulder, Colorado, according to a NOAA statement.

Other key ingredients are ozone-depleting chemicals that remain in the atmosphere and ice crystals on which ozone-depleting chemical reactions take place.

Levels of most ozone-depleting chemicals are slowly declining due to international action, but many have long lifetimes, remaining in the atmosphere for decades.