Air pollution is one of the most important environmental concerns. This is particularly the case in urban areas, where the majority of people live in developed countries and, increasingly so, in the developing regions of the world. It is now widely recognized that air pollution can affect our health as well as the environment. Particles and other pollutants adversely affect the quality of life of critical groups such as children and the elderly, and can lead to a significant reduction in life span (Pope et al. 2002; WHO 2003; Anderson, H.R. et al. 2004).

With rising population, pressure on urban environments is increasing. For example, there is the ever greater demand for travel and the need to increase energy production and consumption. Although other sources, such as industrial pollution, are still a problem in some parts of the world, the greatest threat to clean air is coming from increasing traffic pollution. The link between poor air quality and adverse health conditions is also becoming clearer. Our response to improve air quality in cities at national and local levels, however, is not homogeneous across the globe, with richer nations usually having more stringent and comprehensive pollution management strategies. For example, in the European Union comprehensive legislative frameworks exist to ensure that member states comply with limit values set in the air quality directives and daughter directives (see Directives 96/62/EC, 99/30/EC, 2000/69/EC, 2002/3/EC).