In both developed and rapidly industrialising countries, the major historic air pollution problem has typically been high levels of smoke and sulphur dioixide arising from the combustion of sulphur-containing fossil fuels such as coal for domestic and industrial purpose.
The major threat to clean air is now posed by traffic emissions. Petrol and diesel-engined motor vehicles emit a wide variety of pollutants, principally carbon monoxide (CO), oxides of nitrogen (NOx), volatile organic compounds (VOCs) and particulates (PM10), which have an increasing impact on urban air quality.