Farmers outside India’s capital have started to burn the post-harvest straw from their fields, heralding the onset of north India’s toxic-air season. Stopping the burning of such straw, or stubble, and other biomass could improve air quality in Delhi by 90%, according to a 2016 apportionment study by the Indian Institute of Technology, Kanpur.

Stubble burning was banned in 2015 by the National Green Tribunal (NGT), India’s apex court on matters concerning the environment. While the NGT asked Punjab, Haryana, Delhi, Rajasthan & UP to enforce the ban on ...