At present, roughly half of the world's population lives in urban centers. There are now more than 20 cities with a population of over 10 million inhabitants, compared to less than 5 about 50 years ago. This tendency toward urbanization is expected to continue, particularly in the developing world. A consequence of this growing trend is that millions of people are being exposed to harmful levels of urban air pollutants caused mainly by emissions from motor vehicles and from industrial and domestic activities involving the combustion of fossil fuels. The driving force for the design and...