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. Manmade emissions are largely the result of the combustion of fossil fuels such as coal, oil, petrol or diesel as well as dust generated through activities such as demolition and construction. 

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 mainly human activity that produces 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 rapaidly blown away, or removed from the air by rain and particles are subsequently prevented from being resuspended. During still hot and dry weather pollution is able to build up to harmful concentrations, leading to what are known as 'pollution episodes'. Concentrations often increase in winter when low wind speeds do not assist in the mixing and dispersion and this can lead to a build up of pollution in London.