Delhi's 20 million residents were effectively breathing smoke on Thursday as the air quality index (AQI) breached the “severe” and “hazardous” categories in nearly all monitoring stations of the Indian capital, raising calls to close schools.

The AQI exceeded 450 at many places early in the day, according to data from the Central Pollution Control Board. A reading over 400 affects healthy people, with serious impacts on those with existing diseases, the federal government says.

The index was over 800 in some pockets of the city, according to data from the Delhi Pollution Control Committee.

“What is happening with air pollution in Delhi is nothing short of a crime against humanity!” author and socialite Suhel Seth wrote on Twitter. “There's a total collapse of accountability!”

The world’s most polluted capital is blanketed in smog every winter as cold, heavy air traps construction dust, vehicle emissions and smoke from the burning of crop stubble in the neighbouring states to clear the fields for the next crop.

Lower temperatures, calmer winds and their changing direction worsen the air quality from time to time.

Parents and environmentalists on social media demanded schools to be closed.