Arumugam Chinnaswamy set up his makeshift booth selling firecrackers in a Chennai neighbourhood a week ahead of Diwali, the Hindu festival of lights, with great expectations of doing a brisk trade.

On Diwali day, it improved marginally to 218 (poor), before firecrackers in the evening ... “We see this every year, that Delhi’s background emissions are already high at this time of the ...