Here are the top 10 cities for which data is available, according to a 2011 World Health Organisation (WHO) report.
The figures are the average for the year. Seasonal spikes can be many times higher.
The WHO's health guidelines are maximum exposure of 20 mcg/m3, measured as an annual average.
1) Ahvaz, Iran 372 mcg/m3 (2009 data)
2) Ulan Bator, Mongolia 279 mcg/m3 (2008 data)
3) Sanandaj, Iran 254 mcg/m3 (2009 data)
4) Ludhiana, India (2008 data) and Quetta, Pakistan (2003/4 data)
tied at 251 mcg/m3
5) Kermanshah, Iran 229 mcg/m3 (2009 data)
6) Peshawar, Pakistan 219 mcg/m3 (2003/4 data)
7) Gaborone, Botswana 216 mcg/m3 (2005 data)
8) Yasuj, Iran 215 mcg/m3 (2009 data)
9) Kanpur, India 209 mcg/m3 (2008 data)
10) Lahore, Pakistan 200 mcg/m3 (2003/4 data)
Paris, whose levels hit a high of 180 mcg/m3 last week, has an annual mean level of 38 mcg/m3 according to 2008 data.
Beijing, which has also been in the news over smothering smog, is listed with an annual mean figure of 121 mcg/m3.
The Middle East and North Africa is the world's most polluted region with an annual mean level well over 130 mcg/m3, followed by Southeast Asia with a level near 100 mcg/m3.