Concern over the quality of air we breathe, part icularly i n urban areas, and its effect on human health, has been increasing. A recent study confirms that there are 'serious grounds for concern about London's air quality ' , w i t h numerous transgressions of European guidelines, and a series of high pol lut ion episodes throughout the year (South East Institute of Public Health, 1995). Ambient concentrations of pollutants increased at most L o n d o n monitor ing sites between 1993 and 1994. The government has placed great faith i n catalytic converters, w h i c h have been fitted to a l l new cars f rom 1993, to reduce urban air pollution. To date, at least i n London, the evidence suggests that this measure alone w i l l not be sufficient to reduce air pollution levels.