Nine urban areas in Western Europe (Amsterdam, Athens, Barcelona, Cologne, Helsinki, London, Lyon, Milan, Paris, and Rotterdam)—which differ in terms of sociodemographic characteristics, climate, geographic conditions, and air pollution (in terms of the nature of pollutants and ambient levels)—participated in the study. These nine metropolitan areas have a combined average population of 22 million. In each of them, a retrospective ecological time-series study was conducted over a study period—which varied by metropolitan area—between 1977 and 1992. This type of study involves examining the day-to-day association between pollution indicators reflecting population exposure and indicators of mortality and hospital morbidity reflecting health status. In all these urban areas, excess risks in terms of mortality and cardiorespiratory morbidity were identified for pollution levels below the thresholds defined in European directives. These results have been taken into account by the expert groups currently working on the revision of the European directives. (adapted from the text).