Air pollution levels skyrocketed in New Delhi and left India’s capital shrouded in toxic smog as millions of Indians set off firecrackers the evening of Nov. 7 for Diwali, the Hindu festival of lights.

The air quality index, or AQI, hit 980 at 4:30 a.m. Nov. 8, according to website AirVisual, which monitors air pollution around the world. The levels surged as people violated an order of the Supreme Court and burst firecrackers before and after a two-hour window set for the purpose. Readings below 50 are considered safe, while anything above 300 is considered hazardous.

Children light firecrackers during Diwali Festival celebrations in New Delhi on Nov. 7.
Photographer: Dominique Faget/AFP via Getty Images

The air quality...