The global environment is changing rapidly, partly in response to economic globalization. These global changes are clearly evident at the local level, even in the quality of air that people breath. In some high income countries air quality has been improving, due to a combination of de-industrialization, improved technologies and environmental regulation. However, advances in the science of epidemiology suggest that even air that would until recently have been considered ‘clean’ may contain pollutants that are hazardous to people’s health. Moreover, in many low and middle income countries, economic growth is still associated with declining air quality