Where is the world's most polluted city?

Outdoor air pollution kills 3.3 million people, mostly in cities, every year. That's more than HIV, malaria and influenza combined - yet the sparse coverage of official data means many cities are not even monitored

Every day, hundreds of millions of people step outside into an environment that has become unsafe for human survival. Outdoor air pollution kills 3.3 million people every year, mostly in cities; more than HIV, malaria and influenza combined. But the search for this insidious mass killer reveals something astonishing. As the governments of more than 190 nations gather in Paris to discuss a possible new global agreement on climate change, not only don't we know where it kills the most, in many places we aren't even looking.

As November's cold winds sweep down from Mongolia, the coal burning season begins in northern China. At the time of publication, Chinese maps on the World Air Quality Index website were awash with red, purple and maroon flags, indicating dangerous levels of fine particulate matter (PM2.5) - the world's primary air pollutant.