During the next few weeks, the 1998 springtime Antarctic ozone hole will reach its maximum size. This ozone hole has opened up over Antarctica every year since 1978.

The World Meteorological Organisation said measurements from bases in the Antarctic and Argentina suggest the hole is 15% greater than in 1997.

Ozone is a form of molecular oxygen. It is an irritating pale blue gas, explosive and toxic. But in the atmosphere it forms a layer between six and 30 miles (10 - 50 km) high that absorbs harmful ultra-violet light from the Sun.

Man-made chemicals such as Cfc's are believed to have risen in the atmosphere and started chemical reactions that destroy ozone.

Releasing an ozone measuring balloon

The Antarctic ozone hole's current size is approximately 10 million square miles (26 million square kilometres.) It is approximately three times the area of Australia and is the largest ozone hole ever.

"Unfortunately, it's going to be several decades before we see the end of the Antarctic ozone hole," said Dr Susan Solomon of the United States National Oceanic and Atmospheric Administration.

Dr Solomon discovered that chemical reactions destroy ozone on surfaces such as the ice crystals within high-altitude 'polar stratospheric clouds' as well as on other atmospheric particles high in the atmosphere.

"We're clearly seeing a major impact on ozone depletion caused by the eruption in 1991 of Mt Pinatubo in the Philippines," says Dr Solomon.

"Just as polar stratospheric clouds, volcanic particles make chlorine from Cfc's more effective at ozone destruction,' she adds.

Chemical reactions on the volcanic sulphate particles blasted into the stratosphere by the eruption accelerated ozone depletion over the southern hemisphere by about 3%.

Scientists believe that ozone depletion in the atmosphere will continue until levels of ozone-destroying Cfc's drop to pre-1970 concentrations, around the middle of the next century.

Already, atmospheric scientists have detected reductions in atmospheric concentrations of a number of ozone depleting chemicals, a positive sign that the 1985 Montreal Protocol to reduce Cfc emissions is working.

The Montreal Protocol is the international agreement designed to protect the ozone layer by phasing out production of Cfc's and halons.