Since 1950 the population of the world has more than doubled, and the proportion of people living in cities has increased by a factor of four. In the year 2000 nearly half of the world's population will live in urban areas. Air pollution has always been one of the major nuisances of urban living, but in recent decades the sources of pollution have changed in importance in most of the industrialised world. Earlier they were dominated by individual heating systems, industry and local power plants; now they are mainly related to traffic. Concurrent with this development, the composition of the...