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There is a particular concern over air pollution in urban areas arising from a wide range of sources - particularly traffic and local factories. Emissions from homes and offices also play a part, mainly through their use of products containing organic solvents such as glues, paints and inks. A major focus of the many criteria and standards set for air quality is the protection of human health. In urban areas where the levels of most air pollutants are highest, standards for the protection of human health are of the greatest significance. The principal urban air pollutants are nitrogen dioxide, which can give rise to respiratory problems and which in urban areas arises almost exclusively from traffic; sulphur dioxide, which arises particularly from combustion plants such as power stations, and from diesel vehicles; fine particles (PM10), which has health implications and also arises primarily from diesel vehicle emissions; volatile organic compounds (VOCs), some of which are potentially injurious to health, and which arise from traffic generally and from the use of products containing organic solvents; and carbon monoxide, which at high concentrations can be toxic and which arises almost exclusively from traffic.
There have been different trends in urban air quality for the last 25 years, or as long as records have been kept. Continuous measurements of nitrogen dioxide have been made at a number of sites in England and Wales since 1977. For central London no clear trend is discernible in either the mean annual level or the 98th percentile (level exceeded during only 2% of the hours in the year). Sulphur dioxide is measured at several hundred sites in the Department of the Environment's Basic Urban Network in England and Wales and shows a distinct downward trend in levels, both of the mean and the maxima.