In almost all developing countries, meanwhile, urban air pollution is worsening. Rapidly growing cities, more traffic on roads, use of dirtier fuels, reliance on outdated industrial processes, growing energy consumption, and the lack of industrial zoning and environmental regulations are all contributing to reduced urban air quality and deteriorating public health. In response to this, the World Health Organization (WHO) and the United Nations Environment Programme (UNEP) have been collaborating since 1974 on a project to monitor urban air quality. Known as GEMS/Air, the project has assessed the levels and health effects of five of the most common and ubiquitous air pollutants, drawing data from 50 countries. This publication summarizes the assessment's results.