This database contains urban outdoor air pollution levels, represented by estimated annual mean concentration of fine particulate matter (PM10 and PM2.5: particles smaller than 10 or 2.5 microns), for almost 1100 cities in 91 countries. The database covers the period 2003 to 2010, with the majority of values for 2008 and 2009. Data come from official national/subnational reports and websites; regional networks like the Asian Clean Air Initiative and Europe's Airbase; and selected publications.