India, Nov. 13 -- Delhi's air turned sharply more toxic on Thursday, with the concentration of PM2.5 across the city more than doubling in 24 hours to 244 micrograms per cubic metre by 6pm, four times the national standard, as a thick, smoky haze that usually thins by afternoon stayed trapped over the Capital in warm, damp and almost motionless air.

Two things closed in on the city at once, an HT analysis of official pollution and weather data showed. The dome of air in which Delhi's pollution mixes and thins shrank to about 400 metres high on Thursday afternoon, from 1,400 metres two days earlier; and the north-westerly that carries smoke from the farm fires of Punjab and Haryana, slack since Diwali, was blowing above the city again at ...