The seasonal ozone hole over Antarctica has widened sharply this year, making it the biggest hole since 2000 and the third largest on record, according to measurements reported by the European Space Agency (ESA).

Data sent back ESA's Earth monitoring satellite Envisat showed the hole had swollen to an area of 10 million square kilometers in mid-August, approximately the same size as Europe.

The hole is likely to expand further before reaching its maximum in September, ESA said in a press release.

"This year's hole is large for this time of year, based on results from the last decade. Only the ozone holes of 1996 and 2000 had a larger area at this point in their development." it said.

Ozone, a molecule of oxygen, is a stratospheric shield for life on the Earth, for it filters out dangerous ultraviolet rays from the Sun that damage vegetation and can cause skin cancer and cataracts.

But the protective layer has been damaged by chemicals, such as chlorine and chlorofluorocarbons (CFCs).

CFCs are an aerosol gas and refrigerant whose use was belatedly controlled by an international treaty, the Montreal Protocol.

The size of the hole -- in effect a thinning of the ozone layer -- fluctuates according to the season and prevailing weather.

High-altitude cloud formation, carrying traces of chlorine, is a big factor. A single molecule of chlorine can break down thousands of molecules of ozone.

At ground level, ozone is formed by a reaction between road traffic exhausts and sunlight, becoming a potentially dangerous irritant for people with respiratory problems.

(China Daily September 1, 2005)