Ozone is a nearly colourless (but faintly blue) gaseous form of oxygen, with a characteristic odour like that of weak chlorine. It is usually found in trace amounts in the atmosphere, but is primarily found at 30 000 to 150 000 feet. Its production results from photochemical process involving ultraviolet radiation. Because it absorbs harmful ultraviolet radiation at those heights, it is a beneficial gas. What is the difference between stratospheric and tropospheric Ozone? The stratosphere is the region of the Earth's atmosphere from 10 to 50 kilometres above the surface. Approximately 90 per cent of all ozone is produced naturally in this layer. This band of ozone-rich air found in the stratosphere is known as the ozone layer and shields the earth from damaging ultraviolet radiation. This layer of ozone is thus essential to life on earth. The troposphere extends to between 10 and 18 kilometres above the surface of the Earth and tropospheric ozone often called ground level ozone occurs at the bottom of this layer. The abnormally high concentrations in this layer are caused by human activities in which large amounts of fossil fuels are burnt. Ozone at this level is a major component of smog and thus a form of pollution. Thus Ozone has different effects depending on where it is located in the atmosphere. How is stratospheric Ozone formed? Ozone is created in the stratosphere when highly energetic solar rays strike molecules of oxygen (O2) and cause the two oxygen atoms to split apart. If a freed atom bumps into another O2, it joins up, forming ozone (O3). Ozone is also naturally broken down in the stratosphere by sunlight and by a chemical reaction with various compounds containing nitrogen, hydrogen and chlorine. These chemicals all occur naturally in the atmosphere in very small amounts. In an unpolluted atmosphere there is a dynamic balance between the amount of ozone being produced and the amount of ozone being destroyed. As a result, the total concentration of ozone in the stratosphere remains relatively constant. However, because of release of a number of atmospheric pollutants this delicate balance is being disrupted. This destruction of the ozone has lead to the international effort to limit ozone destructive pollutants and a worldwide effort to preserving the ozone layer.