Ozone is a naturally occurring gas that is found in two layers of the atmosphere. In the layer surrounding the Earth's surface "the troposphere" ground-level or "bad" ozone is an air pollutant that is a key ingredient of urban smog. The troposphere extends up to the stratosphere, which is where "good" ozone protects life on Earth by absorbing some of the sun's UV rays. Stratospheric ozone is most concentrated between 10 to 50 Km above the Earth's surface.

In the 1980s, scientists began accumulating evidence that the ozone layer was being depleted (discovery of the Antarctic "Ozone Hole" in 1985 and observations, since then, of ozone depletion in the middle and higher latitudes). Estimates have been published of. ~10% over the UK during winter and spring, and ~ 5% during summer and autumn (DETR 1996). Depletion of the ozone layer results in increased UV radiation reaching the Earth's surface, which in turn can lead to a greater chance of over-exposure to UV radiation and the related health effects of skin cancer, cataracts, and immune suppression.

The main cause of ozone depletion are emissions of Chlorofluorocarbons (CFCs), which were - until recently- widely used in aerosols, foams, refrigeration, air conditioners, solvents, fire extinguishers etc. Strong winds carry CFCs into the stratosphere in a process that can take as long as 2 to 5 years. When CFCs break down in the stratosphere, they release chlorine, which attacks ozone. Each chlorine atom acts as a catalyst, repeatedly combining with and breaking apart as many as 100,000 ozone molecules during its stratospheric lifetime. Other ozone-depleting substances include pesticides such as methyl bromide, halons used in fire extinguishers and methyl chloroform used in industrial processes.

Countries around the world have recognized the serious threats posed by ozone depletion and agreed under the auspices of the UN to phase out the production and use of ozone-depleting substances ("Montreal Protocol"). Production and consumption of CFCs and other ozone depleting substances have decreased dramatically due to international control measures. However, do to past emissions and the long atmospheric lifetime of CFCs, scientists predict that ozone depletion will continue to increase, with the peak expected between 2000 and 2010. Given the full implementation of the Montreal Protocol, natural atmospheric processes will repair the ozone layer around the middle of the 21^st century.

References:

DETR. The potential effects of ozone depletion in the UK. London, The Stationery Office; 1999 p. .

of Sciences RSA. Effects of ozone depletion. Ambio. 1995;24(Special Issue No. 3.).

UNEP. Environmental effects of ozone depletion: 1998 assessment. In: UNEP. UNEP. Nairobi; 1998. p. . Available from: http://www.gcrio.org/ozone/toc.html

Text adapted from: http://www.unep.org/ozone/