Initially, the earth’s surface was covered by water vapour, methane, carbon dioxide and ammonia. The ultraviolet rays of the sun decomposed water vapour to form hydrogen and oxygen. Hydrogen being a lighter gas escaped, while ultraviolet rays reacted with oxygen to form ozone.
About 90 per cent of ozone is found in the lower part of the stratosphere, forming the ozone layer. Depending on the functions of ozone, it can be categorised as ‘bad’ or ‘good’. In the troposphere, ozone molecules harm plants and animals and hence called ‘bad’ ozone. But, in the stratosphere, ozone molecules block the ultraviolet rays of the sun from reaching the earth, hence called ‘good’ ozone.
Various human activities have resulted in depletion of the ozone layer, the main cause being chlorofluorocarbons or CFCs which are mainly used as refrigerants in air conditioners, refrigerators and aerosol products. When CFCs are discharged into the atmosphere, they move up to reach the stratosphere. In the stratosphere, CFCs react with ultraviolet rays releasing chlorine atoms, which act as catalysts and degrade ozone, releasing molecular oxygen.
The depletion of ozone in the stratosphere has resulted in ozone holes, more in regions over the South Pole, that is, the Antarctic region. Similarly, there is an ozone hole over the Arctic region but over a smaller area. It is through the ozone hole that the ultraviolet rays reach the earth’s surface. These rays are harmful to organisms as they break the chemical bonds in DNA and protein molecules. Moreover, prolonged exposure to ultraviolet rays cause skin ageing, damage to skin cells, skin cancers, inflammation of the cornea and so on.
Realising the harmful effects of CFCs on the ozone layer, 31 nations signed the Montreal Protocol in 1987 to limit the emission of CFCs and other ozone-depleting substances or ODS. At present, 196 nations have signed the Protocol.