Millions of people in Delhi, India, and neighboring states are struggling to cope with eye-watering smog that has settled on the region—creating some of the worst air quality in years. Government authorities have declared a public-health emergency, closing schools, halting construction, and restricting cars to an “odd-even” system, based on their license plates, to try to halve the number of vehicles on the roads. The toxic stew filling the air comes from a combination of vehicular and industrial emissions and smoke from the seasonal burning of rice-paddy stubble on farms in nearby states.