Air pollution is a term used to describe any unwanted chemicals or other materials that contaminate the air that we breathe resulting in the degradation of air quality. Air pollutants occur both outdoors or indoors , and can be natural or man-made. Outdoor air pollution, sometimes called ambient air pollution, occurs in both urban and rural areas, although a different mix of air pollutants may be found in the countryside to that found in a city. Typical urban air pollutants from man-made activities include nitrogen oxides , carbon monoxide , sulphur dioxide , hydrocarbons and particulate matter . All these pollutants are called primary pollutants because they are emitted directly into the atmosphere.
Ozone is a secondary pollutant, formed in the air as a result of chemical reactions .
Whilst ozone does build up within cities on hot summer days, higher levels are usually found in the countryside, because of the special nature of the reactions involving the formation of ozone. In addition, there are biological pollutants such as dust mites and mould .