A thick blanket of toxic smog covered parts of New Delhi on Monday after millions of revelers let off firecrackers to celebrate Diwali, the Hindu festival of lights.

The System of Air Quality and Weather Forecasting and Research (SAFAR) said that pollution in the city of 16 million had reached the highest official category of "severe" in several districts, and warned residents to stay indoors and avoid physical activity.

Concentrations of the tiny lung-clogging particulate matter known as PM2.5 were recorded above 900 micrograms per cubic meter in some parts of the city early Monday. That's well above the World Health Organization's recommended mean of 25 mcg per cubic meter over a 24-hour period.

"You could see and sense how the visibility had come down, and there was a choking haze all around," said Anumita Roy Chowdhury, executive director of the Centre for Science and Environment, a Delhi-based research organization. "Where is the plan for short- and medium-term action, and for emergency action?" she added. "We need one very urgently."

(DW)