Pollution an economic problem, says World Resources Institute
8 July 2014 Last updated at 02:41 BST
For the first time in history, over half the world's population now lives in cities - and that number is set to grow. However, urban centres have also become the biggest sources of pollution.
Unsafe air quality levels are worst in Asia, with India and Pakistan's major cities topping the list.
But pollution in cities is not just a health problem. According to a Washington-based think tank, it's a growing economic one too.
Andrew Steer, the head of the World Resources Institute, explains his thinking to the BBC's Leisha Chi.