Strictly too much of any substance in the wrong place or at the wrong time is a pollutant. More specifically, atmospheric pollution may be defined as the presence of substances in the atmosphere, resulting from man-made activities or from natural processes, causing adverse effects to man and the environment. 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. Common sources of these primary pollutants include power station and industrial plants (sulphur dioxide), and road transport (carbon monoxide, particulate matter and nitrogen oxides). 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.
Common indoor air pollutants include carbon monoxide and nitrogen dioxide from faulty gas heaters and cookers, carbon monoxide and benzence from cigarette smoke, and volatile organic compounds (VOCs) from synthetic furnishings, vinyl flooring and paints. In addition, there are biological pollutants such as dust mites and mould. Since most of us spend 80 to 90% of the time indoors, air quality could have a real bearing on our health.