Authorities in the Indian capital, Delhi, have shut all schools and colleges indefinitely amid the worsening levels of air pollution.

Construction work has also been banned until November 21 but an exception has been made for transport and defence-related projects.

Only five of the 11 coal-based power plants in the city have been allowed to operate.

A toxic haze has smothered Delhi since the festival of Diwali.

The levels of PM2-point-5 – tiny particles that can clog people’s lungs – in Delhi are far higher than the World Health Organization’s safety guidelines.

Several parts of the city recorded figures close to or higher than 400 on Tuesday, which is categorised as severe.

Some schools had already shut last week because of pollution and the Delhi government said it was mulling over a lockdown to improve air quality as dense clouds of smog engulfed the city.

A mix of factors like vehicular and industrial emissions, dust and weather patterns make Delhi the world’s most polluted capital.

The air turns especially toxic in winter months as farmers in neighbouring states burn crop stubble.

And fireworks during the festival of Diwali, which happens at the same time, only worsen the air quality.

Low wind speed also plays a part as it traps the pollutants in the lower atmosphere.