BACKGROUND: Ambient air pollution, including particulate matter (such as PM10 and PM2·5) and nitrogen dioxide (NO2), has been linked to increases in mortality. Whether populations' vulnerability to these pollutants has changed over time is unclear, and studies on this topic do not include multicountry analysis. We evaluated whether changes in exposure to air pollutants were associated with changes in mortality effect estimates over time. METHODS: We extracted cause-specific mortality and air pollution data collected between 1995 and 2016 from the Multi-Country Multi-City (MCC) Collaborative Research Network database. We applied a two-stage approach to analyse the short-term effects of NO2, PM10, and PM2·5 on cause-specific mortality using city-specific time series regression analyses and multilevel random-effects meta-analysis. We assessed changes over time using a longitudinal meta-regression with time as a linear fixed term and explored potential sources of heterogeneity and two-pollutant models. FINDINGS: Over 21·6 million cardiovascular and 7·7 million respiratory deaths in 380 cities across 24 countries over the study period were included in the analysis. All three air pollutants showed decreasing concentrations over time. The pooled results suggested no significant temporal change in the effect estimates per unit exposure of PM10, PM2·5, or NO2 and mortality. However, the risk of cardiovascular mortality increased from 0·37% (95% CI -0·05 to 0·80) in 1998 to 0·85% (0·55 to 1·16) in 2012 with a 10 μg/m3 increase in PM2·5. Two-pollutant models generally showed similar results to single-pollutant models for PM fractions and indicated temporal differences for NO2. INTERPRETATION: Although air pollution levels decreased during the study period, the effect sizes per unit increase in air pollution concentration have not changed. This observation might be due to the composition, toxicity, and sources of air pollution, as well as other factors, such as socioeconomic determinants or changes in population distribution and susceptibility. FUNDING: None.

Center for Climate Change Adaptation National Institute for Environmental Studies Tsukuba Japan

Center for Environmental and Respiratory Health Research University of Oulu Oulu Finland; Finnish Institute of Meteorology Helsinki Finland

Department of Environmental Health Harvard T H Chan School of Public Health Harvard University Boston MA USA

Department of Environmental Health Instituto Nacional de Saúde Dr Ricardo Jorge Porto Portugal; Instituto de Saúde Pública da Universidade do Porto Porto Portugal; Laboratory for Integrative and Translational Research in Population Health Porto Portugal

Department of Epidemiology Instituto Nacional de Saúde Dr Ricardo Jorge Lisbon Portugal

Department of Epidemiology Lazio Regional Health Service ASL ROMA 1 Rome Italy

Department of Geography Geoinformatics and Meteorology University of Pretoria Pretoria South Africa

Department of Global Health Policy Graduate School of Medicine The University of Tokyo Tokyo Japan

Department of Hygiene Epidemiology and Medical Statistics National and Kapodistrian University of Athens Greece

Department of Primary Care and Population Health University of Nicosia Medical School Nicosia Cyprus

Department of Public Health Environments and Society London School of Hygiene and Tropical Medicine London UK

Department of Statistics Computer Science and Applications G Parenti University of Florence Florence Italy

Environment and Health Modelling Lab Department of Public Health Environments and Society London School of Hygiene and Tropical Medicine London UK

Faculty of Environmental Sciences Czech University of Life Sciences Prague Czech Republic; Institute of Atmospheric Physics Czech Academy of Sciences Prague Czech Republic

Institute for Medical Information Processing Biometry and Epidemiology Faculty of Medicine LMU Munich Munich Germany

Institute of Environmental Assessment and Water Research Spanish Council for Scientific Research Barcelona Spain

Institute of Epidemiology Helmholtz Zentrum München German Research Center for Environmental Health Neuherberg Germany

Institute of Epidemiology Helmholtz Zentrum München German Research Center for Environmental Health Neuherberg Germany; Institute for Medical Information Processing Biometry and Epidemiology Faculty of Medicine LMU Munich Munich Germany

Institute of Epidemiology Helmholtz Zentrum München German Research Center for Environmental Health Neuherberg Germany; Institute for Medical Information Processing Biometry and Epidemiology Faculty of Medicine LMU Munich Munich Germany; Department of Environmental Health Harvard T H Chan School of Public Health Harvard University Boston MA USA; Munich Heart Alliance German Center for Cardiovascular Research Munich Germany

Institute of Social and Preventive Medicine University of Bern Bern Switzerland; Oeschger Center for Climate Change Research University of Bern Bern Switzerland

School of Epidemiology and Public Health Faculty of Medicine University of Ottawa Ottawa ON Canada; Environmental Health Science and Research Bureau Health Canada Ottawa ON Canada

School of Public Health and Preventive Medicine Monash University Melbourne VIC Australia

School of the Environment Yale University New Haven CT USA; Korea University Seoul South Korea

School of Tropical Medicine and Global Health Nagasaki University Nagasaki Japan

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