Which are the world's two most polluted cities – and why?

Two cities - one in Iran and another in Nigeria - can claim title because WHO measures pollution in two different ways

The new WHO database of worldwide air pollution measures it in two different ways, and as a result two cities - one in Iran and another in Nigeria - can lay claim to the unenviable title of world's most polluted city.

It all comes down to which minute particles, or particulate matter (PM), in the air are being measured. These particles are between 2.5 and 10 microns in diameter, roughly 30 times smaller than the width of a human hair.