Ozone is a gas and a highly reactive form of the element oxygen. It is a strong oxidising agent and readily reacts with organic compounds. It also kills bacteria and is used to sterilise and purify drinking water. It has a characteristic smell which one can detect when it is formed in electric discharges such as occur in sparking electric motors, during arc welding and in some high-voltage insect zappers and air sanitisers.

It is also formed by the action of ultra-violet (UV) light on oxygen. This occurs in some photocopying machines and sun lamps, and on a global scale due to the action of sunlight on oxygen in the stratosphere.

Health effects
Ozone gas is toxic and can quickly cause headaches, even at low concentrations. Most people can smell it at a concentration of 0.02 ppm, which is about one-sixth of the concentration at which the health of sensitive people can be affected if they are exposed to it for one hour. At high concentrations it can lead to nausea and vomiting and can cause damage to the lung function and the central nervous system. It can trigger asthma attacks in sensitive people and reduce lung function in others. It should be avoided, particularly in work situations, by providing good ventilation. Photocopiers and small electric motors like sewing machines are often sources of ozone.

Environmental effects
Oxygen strongly absorbs the far ultraviolet (UV, very short wavelength) part of sunlight and reacts to form ozone. This is the source of the ozone layer in the stratosphere, which extends from about eight kilometres above the earth at the poles and 17 kilometres at the equator, to about 50 kilometres. This layer itself absorbs most of the near-UV part of sunlight so that only a very small fraction of it reaches the Earth's surface. Thus the ozone layer protects living things from the harmful effects of UV radiation. The stratospheric ozone layer is now threatened by some synthetic chemicals. The CFCs and, to a lesser extent, the HCFCs are so stable and long-lived under ordinary conditions that they can diffuse up into the stratosphere. Under the action of intense UV radiation, they react with ozone in a chain reaction in which many thousand ozone molecules can be destroyed by a single CFC molecule. The extent of this reaction depends on many, as yet imperfectly understood, factors, but it is particularly severe in the southern hemisphere spring (September, October) over Antarctica, when up to 60 per cent of the total ozone is depleted. The extent of the destruction varies from year to year, but in 1989 for instance, up to 85 per cent of the ozone layer over the South Pacific and Indian Oceans was destroyed in October. The depletion reached as far as the South Island of New Zealand and Tasmania. Over most parts of the world the average depletion of stratospheric ozone is between five and six per cent throughout the year. High-flying supersonic aircraft also contribute to ozone depletion by producing nitrogen oxides not far below the ozone layer. Some of this diffuses up and reacts with the ozone. Ozone is also formed in the troposphere (see Glossary) by the reaction of pollutants (particularly nitrogen oxides) with oxygen under the action of sunlight. When the level of pollution is high, and there is no wind to disperse it, the ozone concentration can become so high that it injures plants and becomes a danger to health. Most major cities encounter such conditions in summer when there is intense sunshine. Ozone formation is not always accompanied by the formation of visible smog. (See also Car exhaust gases.)