Ozone is formed when an electrical discharge passes through oxygen, or by the action of ultraviolet light on oxygen. It can also be destroyed by UV light, which can break it back down into an oxygen molecule, O2, plus a single "free radical" atom of oxygen. At ground level, it is a pollutant, corrosive to many materials, damaging to lungs and plants. It is found in the "smog" that is formed by the action of light on vehicle exhaust smoke.
Up in the stratosphere, ozone is more appreciated. It forms a thin layer that protects us from the carcinogenic UV part of the Sun's spectrum.
Without this protection, there would be little life on Earth. So, ozone is formed by UV, destroyed by UV, and in the process it protects us from UV.
What this means is that there is an "ozone balance" - a state in which ozone is being created and destroyed at equal rates - which keeps the ozone layer in being. The balance is naturally fragile and fluctuating, and anything that upsets it and increases the rate of ozone destruction is potentially life-threatening - hence the worry, since the 1980s, about the effect of the release into the atmosphere of chlorofluorocarbons (CFCs), such as those used in aerosols, refrigerators and air conditioners. These interfere with the ozone balance by promoting complex chemical reactions that speed up the breakdown of ozone. The problem is aggravated by the fact that CFCs were used for many years in the belief that they were inert, with no environmental penalties. Their very stability, however, means that even after they have been phased out, they will remain in the atmosphere for a long time.
Their slow migration to the stratosphere brings them into contact with the ozone layer. There is hope that ozone depletion - largely seen in the form of "holes" in the ozone layer over both Poles - has slowed and the layer is showing signs of recovery.
Another worry is the increase in "greenhouse" gases. The greenhouse effect is the name given to the increase in temperature at the Earth's surface caused by the way that certain gases in the atmosphere - notably carbon dioxide and water vapour - allow heat radiated from the Sun to pass freely to the Earth, and then absorb it when it is radiated back, heating up and thus keeping the atmosphere warm where it is closer to the Earth.
The greenhouse effect itself is natural - without it, the Earth would be in the grip of permanent ice age with an average temperature of - 15oC, 30oC below its current level.
But for 150 years, industrialisation has meant that much more carbon dioxide is being pumped into the atmosphere, and most scientists believe that this is increasing the greenhouse effect, leading to a range of radical effects such as the shrinking of the ice caps, rising sea levels and increasingly frequent extreme weather.