Air pollution is the release of particles and noxious gases into the atmosphere; these emissions can be natural or manmade and are considered to have an effect on human health. Natural emissions of particles come from the sea, the soil and from plants. Particle pollution from human activiity is largely the result of the combustion of fossil fuels such as coal, oil, petrol or diesel.

The main pollutants of concern are carbon monoxide, nitrogen dioxide, ground level ozone, particulates, sulphur dioxide, hydrocarbons and lead. Each has different sources, health effects and chemical behaviours, making the task of understanding and controlling air pollution as a whole very complex.

While it is we who produce the pollution, it is primarily the weather that dictates what will happen once it is released into the air. During wet or windy conditions pollution concentrations remain low, either blown away, or removed from the air by rain. During still hot weather pollution is able to build up to harmful amounts, leading to what are known as pollution episodes. Concentrations also increase in winter when low winds lead to a build up of traffic pollution in London.