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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, sulfur 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 (sulfur 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.
Ozone
Ozone (O3) is the tri-atomic form of molecular oxygen (O2). It is a toxic bluish, unstable gas, with pungent odor, found naturally in the atmosphere, particularly in the stratosphere 19 to 30 km above the Earth's surface where it forms the ozone layer. At these altitudes, ozone filters out the incoming ultraviolet (UV) radiation. Near ground level, however, it can impair lung function and cause irritation to the respiratory tract. Asthmatics are known to adopt these symptoms more easily. Irreversible damage to the respiratory tract and lung tissue can occur if ozone is present in sufficiently high quantities.
Most ground-level ozone is formed indirectly by the action of sunlight on volatile organic compounds in the presence of nitrogen dioxide, and as such is a secondary pollutant. There are no direct man-made emissions of ozone into the atmosphere. About 10 to 15% of ground-level ozone is transported from the stratosphere. As ozone concentrations are particularly dependent on sunlight, episodes are always likely to develop following sustained periods of warmth and calm weather. Once formed, ozone is scavenged by nitric oxide (NO), usually present in urban areas, as a result of traffic fumes, but less so in the countryside. Consequently, ozone usually occurs in higher concentrations during summer than winter, and in rural rather than urban areas. Background levels of ozone across Europe are usually less than 15 ppb but can be as high as 60 ppb. During photochemical smog episodes, levels can rise to over 100 ppb.
Nitrogen Dioxide
Nitric oxide (NO) is an odorless, colorless gas which is produced during high temperature burning of fuel in, for example, cars and other road vehicles, heaters and cookers. Once it is mixed with air it quickly combines with oxygen, forming nitrogen dioxide (NO2). Most nitrogen dioxide in the atmosphere is formed from the oxidation of nitric oxide (NO) in this way, although some is release directly from source. It is also present in tobacco smoke. It is a reddish brown, nonflammable, gas with a detectable smell. In significant concentrations it is highly toxic, causing serious lung damage with a delayed effect. Other health effects of exposure to nitrogen dioxide include shortness of breath and chest pains. Nitrogen dioxide is a strong oxidizing agent that reacts in the air to form corrosive nitric acid, as well as toxic organic nitrates. It also plays a major role in the atmospheric reactions that produce ground-level ozone or smog.
Since nitrogen dioxide is a traffic-related pollutant, emissions are generally highest in urban rather than rural areas. Annual mean concentrations of nitrogen dioxide in urban areas are generally in the range 10-45 ppb, and lower in rural areas. Levels vary significantly throughout the day, with peaks generally occurring twice daily as a consequence of rush hour traffic. Concentrations can be as high as 200 ppb.
Sulfur Dioxide
Sulfur dioxide (SO2) is a colorless, nonflammable gas with a penetrating odor that irritates the eyes and air passages. It reacts on the surface of a variety of airborne solid particles, is soluble in water and can be oxidized within airborne water droplets. The most common sources of sulfur dioxide include fossil fuel combustion, smelting, manufacture of sulfuric acid, conversion of wood pulp to paper, incineration of refuse and production of elemental sulfur. Coal burning is the single largest man-made source of sulfur dioxide accounting for about 50% of annual global emissions, with oil burning accounting for a further 25-30%. The most common natural source of sulfur dioxide is volcanoes.