NEW DELHI — A thick layer of toxic smog cloaked India’s capital on Friday as smoke from firecrackers used to celebrate Diwali, the Hindu festival of lights, pushed air pollution to hazardous levels. New Delhi’s air quality index plunged into the “severe” category, according to SAFAR, India’s main environmental monitoring agency. In many areas, levels of deadly particulate matter reached seven times the World Health Organization’s safety limit. Authorities in the capital have banned the use and sale of traditional firecrackers since 2017, asking people to opt for environmentally friendly ones or light shows instead, but the rule is often flouted. New Delhi, home to more than 33 million people, is regularly ranked one of the most polluted cities in the world. The air pollution crisis deepens particularly in the winter when the burning of crop residue in neighboring states coincides with cooler temperatures that trap deadly smoke. That smoke travels …