NEW DELHI: New Delhi residents woke up on Friday under a blanket of smog darkening the city, the most dangerous air pollution of the year after Diwali revellers defied - as usual - a fireworks ban during India's annual Hindu festival of lights.

New Delhi has the worst air quality of all world capitals, but even by its sorry standards Friday's reading - the morning after the end of Diwali - was extra bad, the price for celebrating India's biggest festival in the noisiest and smokiest way.

The Air Quality Index (AQI) surged to 463 on a scale of 500 - the maximum recorded in 2021, indicating "severe" conditions that affect even healthy people let alone those with existing respiratory diseases.

The AQI measures the concentration of poisonous particulate matter PM2.5 in a cubic metre of air. In Delhi, a city of nearly 20 million people, the PM2.5 reading on Friday averaged 706 micrograms, whereas the World Health Organization deems anything above an annual average of 5 micrograms as unsafe.

Airborne PM2.5 can cause cardiovascular and respiratory diseases such as lung cancer. And, in India, toxic air kills more than a million people annually.

On the morning after the close of Diwali, thick smog turned daylight into dusk in and around Delhi, with car and building lights only barely penetrating the murk, and the ubiquitous detritus of firecrackers coating the ground.

"The firecracker ban didn't seem to be successful in Delhi, which led to hazardous pollution levels adding on top of existing perennial sources," said Sunil Dahiya, analyst at the Centre for Research on Energy and Clean Air (CREA).