The database contains results of urban outdoor air pollution monitoring from almost 1100 cities in 91 countries.
The map above was created by the WHO to show particulate matter (PM) pollution around the world, measuring specifically PM10, or particulates with a diameter of 10 micrometers.
The database covers the period from 2003 to 2010, with the majority of values for the years 2008 and 2009. The primary sources of data include publicly available national/subnational reports and web sites, regional networks such as the Asian Clean Air Initiative and the European Airbase, and selected publications. The database aims to be representative for human exposure, and therefore primarily captures measurements from monitoring stations located in urban background, urban traffic, residential, commercial and mixed areas.
The world's average PM10 levels by region range from 21 to 142 ug/m3, with a world's average of 71 ug/m3. (source)