World Health Organization (WHO) has published data about the urban outdoor air pollution levels of almost 1,100 cities in 91 countries. Air quality is represented by annual mean concentration of fine particulate matters of PM10 (smaller than 10 um) and PM2.5 (smaller than 2.um).
The database covers the period from 2003 to 2010, with the majority of values for the years 2008 and 2009. The world's average PM10 levels by region range from 21 to 142 ug/m3, with a world's average of 71 ug/m3.
From the map we are able to see that cities in North America are doing very well. The worst polluted regions are mainly located at East and South Asia and Middle-East.