Top 10 world’s most polluted cities

Foggy morning skyline view of downtown Los Angeles with steaming Southbay refineries and industrial facilities in background. Photo taken from hilltop near Pasadena, California.

According to the 2011 World Health Organisation (WHO) report, pollution is measured as the microgramme (mcg) concentration per cubic metre of air of particulate matter smaller than 10 micrometres (PM10) – about a seventh of the width of a human hair.
The figures below are an average for the year. Seasonal spikes can be many times higher.
The WHO’s health guidelines’ maximum exposure of 20 mcg/m3 is measured as an annual average.

Ahvaz, Iran, 372 mcg/m3 (2009 data)
Ulan Bator, Mongolia, 279 mcg/m3 (2008 data)
Sanandaj, Iran, 254 mcg/m3 (2009 data)
Ludhiana, India, (2008 data)
and Quetta, Pakistan, (2004 data) tied at 251 mcg/m3
Kermanshah, Iran, 229 mcg/m3 (2009 data)
Peshawar, Pakistan, 219 mcg/m3 (2004 data)
Gaborone, Botswana, 216 mcg/m3 (2005 data)
Yasuj, Iran, 215 mcg/m3 (2009 data)
Kanpur, India, 209 mcg/m3 (2008 data)
Lahore, Pakistan, 200 mcg/m3 (2004 data).