Ozone is a colourless gas with a pungent smell. It is an air pollutant of much concern in Europe, because it can affect human health and damage the environment.

Ozone at ground level – in the air we breathe – is not to be confused with the ozone layer in the upper atmosphere, which shields the Earth from the sun's ultraviolet rays. Up there ozone is 'good' – at ground level it is 'bad'.

Ozone is formed when certain gases, for example from car engines and power stations, react under the action of sunlight.

Since the formation of ozone requires sunlight, the highest levels of ozone pollution occur during periods of hot and sunny weather.

Human activites have lead to much higher ground-level ozone concentrations in Europe today than in the pre-industrial era.