In the past air pollution reflected activities that were unique to a locality. During the twentieth century it became increasingly similar everywhere to the extent that the air pollution problems faced by cities became global in their presence and character. The formation, control and perception of air pollutants seemed increasingly universal. Despite this, the polluted urban atmosphere emerged as more complex than that of the past. It no longer necessarily derived from single and obvious sources such as a factory chimney, and was less visible, often without any obvious source. Photochemical smog, for example, contains the reaction products of pollutants from a range of sources. Pollutant emissions are dependent on our choice of transport, agriculture and industry. Individual ownership of automobiles has widened responsibility for air pollution, while a convergence in lifestyle, attitudes to work and gender have led to pollution exposures that are increasingly similar. Regulation has adopted international health standards and common legislative frameworks. Solving modern air pollution problems involves detailed understanding, analysis and modelling, removing it from satisfyingly direct approaches of the past. Thus communicating the issues to politicians and the public can be difficult, weakening the sense of local involvement and favouring solutions imposed by distant bureaucracies.