For the past two weeks, air pollution in the Indian capital of Delhi has been off the scale. A toxic mix of dust, soot from farmers burning paddy field stubble, car fumes and construction all combined into a murky grey film over the city. The pollution was so bad visibility was reduced and the government moved to close schools and advise people to stay in doors.

This week, host James Haines Young talks to Dr Vikas Maurya (Specialist Pulmonology at Fortis Hospital in Delhi) and Professor Guojun (Economics Department of Hong Kong University of Science and Technology) about why so many cities are struggling to breath. James also talks to Tanya Dutta, a reporter with The National in Delhi.