We obtained health and environmental data from the MCC database, which has been described previously.10 (link),12 (link) The current analysis was limited to locations that had available data on air pollution (652 urban areas in 24 countries or regions, with the data covering the period from 1986 through 2015) (Table S1 in the Supplementary Appendix, available with the full text of this article at NEJM.org). Data on mortality were obtained from local authorities within each country. Causes of death were classified according to codes in the International Classification of Diseases, 9th Revision (ICD-9) or 10th Revision (ICD-10), whichever was available. In each location, mortality was represented by daily counts of either death from nonexternal causes (ICD-9 codes 0 to 799 and ICD-10 codes A0 to R99) or, when such data were unavailable, daily counts of death from any cause. We also collected mortality data for two main causes of death: cardiovascular disease (ICD-10 codes I00 to I99) and respiratory disease (ICD-10 codes J00 to J99).13 We obtained daily data on PM10 in 598 cities and on PM2.5 in 499 cities. Data on both pollutants were available in 445 cities in 16 countries or regions. The geographic distributions of the cities that had data on PM10 and PM2.5, as well as the annual mean PM concentrations over the period studied for each city, are provided in Figure 1 and Figure 2, respectively (also see the interactive map, available at NEJM.org). Daily data on gaseous pollutants (ozone, nitrogen dioxide, sulfur dioxide, and carbon monoxide) were obtained where available. We also collected data on the daily mean temperature and daily mean relative humidity. To avoid potential consequences of including outlying values of exposure data, we used trimmed data, in which the highest 5% and lowest 5% of PM10 and PM2.5 measurements were excluded.14 (link)