New Delhi’s air quality was running at levels considered hazardous for three consecutive days earlier this week The thick smog that blankets northern India with the approach of winter has a particularly grim ally this year: Covid-19. Air pollution from the annual burning of rice stubble across the states of Haryana, Uttar Pradesh and Punjab is reversing the gains that gave the north of the country its cleanest air in two decades under lockdown earlier this year. New Delhi’s air quality was running at levels considered hazardous for three consecutive days earlier this week, and the usual peak in pollution around the Diwali festival won’t even hit until Saturday. In Alipur in the north of the capital, an air quality index that conventionally tops out at 500 posted an off-the-charts reading of 851 Monday. That’s particularly troubling given the evidence that Delhi air pollution is a major factor raising the risk of dying from coronavirus. The decline in pollution and roa...