Ozone is an tri-atomic form of oxygen-it has three oxygen atoms instead of the normal two. It is formed naturally in the upper levels of the Earth's atmosphere by high-energy ultraviolet radiation from the Sun. The radiation breaks down oxygen molecules, releasing free atoms, some of which bond with other oxygen molecules to form ozone. About 90 per cent of all ozone in the atmosphere are formed in this way, between 15 and 55 kilometers above the Earth's surface- the part of the atmosphere called the stratosphere. Hence, this is known as the 'ozone layer'. Even in the ozone layer, ozone is present in very small quantities; its maximum concentration, at a height of about 20-25 kilometers, is only ten parts per million.

Ozone is an unstable molecule. High-energy radiation from the Sun not only creates it, but also breaks it down again, recreating molecular oxygen and free oxygen atoms. The concentration of ozone in the atmosphere depends on a dynamic balance between how fast it is created and how fast it is destroyed
 

Figure 1 - Ozone Distribution in the Atmosphere

The ozone layer absorbs most of the harmful ultraviolet-B radiation from the sun. It also completely screens out lethal UV-C radiation. The ozone shield is thus essential to life as we know it. Depleting the ozone layer allows more UV-B to reach the earth. More UV-B means more melanoma and non-melanoma skin cancers, more eye cataracts, weakened immune systems, reduced plant yields, damage to ocean eco-systems and reduced fishing yields, adverse effects on animals, and more damage to plastics.

Scientific concern started in 1970 when Prof. Paul Crutzen pointed out the possibility that nitrogen oxides from fertilizers and supersonic aircraft might deplete the ozone layer. In 1974, Professors F. Sherwood Rowland and Mario J. Molina recognized that when CFCs finally break apart in the atmosphere and release chlorine atoms they cause ozone depletion. Bromine atoms released by halons have the same effect. The three scientists received the Nobel Prize for Chemistry in 1995 for their pioneering work.

The ozone layer over the Antarctic has steadily weakened since measurements started in the early 1980s. The problem is worst over this part of the globe due to the extremely cold atmosphere and the presence of polar stratospheric clouds. The land area under the ozone‑depleted atmosphere increased steadily to more than 20 million sq km in the early 1990s and has varied between 20 and 29 million sq. km since then.
 

Figure 2 - Measurements of Ozone and Reactive Chlorine from a Flight Into the Antarctic Ozone Hole, 1987

In 2000, the area of the ozone hole reached a record 29 million sq. kilometers on 12 September 2000. Although it was the largest and the deepest ozone hole on record for the month of September, it dissipated early in October, the earliest since 1991. The lowest value recorded at the South Pole was 86 DU on 12 October 1993. This year, the area of the ozone hole has been about 25 million sq. km. While no hole has appeared elsewhere, the Arctic spring has seen the ozone layer over the North Pole thin by up to 30%, while the depletion over Europe and other high latitudes varies between 5% and 30%.