In the past, the main cause of air pollution was smoke and sulphur dioxide from burning coal. Often this air pollution would be seen as smog in major cities and towns. Today our air is much cleaner – there are fewer coal fires and factory emissions are closely controlled. But motor vehicles also cause air pollution, and the rising numbers of vehicles and congestion on our roads is leading to poorer air quality, particularly in urban areas.

Air pollution levels vary from day to day and area to area. It is worse on busy roads next to high buildings, and at busy road junctions. Calm sunny weather and cold foggy weather can also increase pollution levels.