- Ozone, a pale-blue gas with a pungent odour, is a form of oxygen which was first discovered in laboratory experiments in the mid-1800s. It is created in the atmosphere when ultraviolet light breaks up oxygen molecules.

- The ozone layer is vital to life on earth because it filters dangerous solar radiation. Holes in the layer have been blamed for increased risk of skin cancer and cataracts in humans, and harm to crops and marine life.

THE OZONE LAYER

- Some 90 percent of the ozone is found in a layer of the atmosphere called the stratosphere, a region that begins about 10-16 km (6-10 miles) above the earth's surface and extends to an altitude of 50 km (31 miles). The ozone layer sits about 15-30 km (9-19 miles) above the earth.

- While ozone is useful in the stratosphere it becomes a pollutant greenhouse gas at altitudes below 10 km (6 miles) in the region known as the troposphere.

- Ozone is present in the atmosphere in very small quantities: out of every 10 million air molecules, about 2 million are normal oxygen but only 3 are ozone. The concentrations of ozone are lower at the equator than at the poles.

OZONE DEPLETION

- Chlorofluorocarbons (CFCs) containing chlorine and bromine have been blamed for thinning the layer because they attack the ozone molecules, causing them to break apart. Many CFCs, once commonly used in refrigeration, air conditioning and industrial cleaning, were banned by the 1985 Vienna Convention and its Montreal Protocol clinched in 1987.

ANTARCTIC OZONE HOLE

- Large reductions in the ozone layer take place each winter over the polar regions, especially the Antarctic, as low temperatures allow the formation of stratospheric clouds that assist chemical reactions breaking down ozone.

- The presence of a "hole" in the ozone layer over the Antarctic was recognised in 1985. Scientists say the hole spanned a record 29 million sq km (11 million sq miles) in September 2003, exposing the southern tip of South America.

THE PROGNOSIS

- The World Meteorological Organisation and the UN Environment Programme (UNEP) said in August 2006 that the protective layer would be back to pre-1980 levels by 2049 over huge areas of Europe, North America, Asia, southern Australasia, Latin America and Africa. This was five years later than forecast in the last major scientific report in 2002.

- Over Antarctica, where so-called "ozone holes" have grown over the past 30 years, recovery was likely to be delayed until 2065, 15 years later than earlier hoped.