A public health emergency has been declared in New Delhi as a choking blanket of smog descended on the world's most polluted capital city.

The declaration on Tuesday came as readings of the most harmful type of pollutant soared to more than double the level classed as hazardous.

The US embassy website reported the amount of PM2.5 in the air has reached 703mg – when 300mg is deemed highly dangerous.

The head of the Indian Medical Association (IMA) Krishan Kumar Aggarwal told AFP: ‘We have declared a state of public health emergency in Delhi since pollution is at an alarming level.

‘Delhi authorities have to make every possible effort to curb this menace.’

Delhi's air quality typically worsens ahead of the onset of winter because cooler air traps pollutants near the ground

‘Delhi is becoming unliveable’

The World Health Organization in 2014 classed New Delhi as the world's most polluted capital, with air quality levels worse than Beijing.

Since then authorities have closed power plants temporarily and experimented with taking some cars off the road.

But on Tuesday many residents said too little was being done to tackle the scourge.

‘It's a recurring problem and we need to find a quick and permanent solution to it before it is too late,’ said real estate agent Vipin Malhotra.

‘Delhi is becoming unliveable, particularly for children, as the pollution has reached uncontrollable levels.’

The World Health Organization in 2014 classed New Delhi as the world's most polluted capital

Low visibility caused by the smog forced the closure of one of the runways at Delhi airport, delaying some flights.

It’s a situation expected to get worse, as the city’s air quality typically reduces ahead of the onset of winter. This is because cooler air traps pollutants near the ground, preventing them from dispersing into the atmosphere – a phenomenon known as inversion.

Firecrackers set off to celebrate the Diwali festival of lights in the city add to the toxic mix created by pollution from diesel engines, coal-fired power plants and industrial emissions.

The problem is further exacerbated by the burning of crop stubble by farmers after the harvest in northern India, a practice that remains commonplace despite an official ban.

Schools may be shut

Delhi's Chief Minister Arvind Kejriwal said he had asked the state's education authorities to consider shutting schools, a step the IMA has backed.

Unprecedented pollution levels this time last year forced schools to shut for several days.

The IMA had earlier written to Kejriwal calling for an upcoming half marathon to be cancelled, saying the health consequences for runners could be ‘disastrous’.

Running in such conditions could trigger asthma attacks, worsen lung conditions and increase the risk of heart attacks and strokes, it said.

On Monday telecoms giant Airtel threatened to pull its sponsorship of the event, due to be held on November 19, if authorities failed to improve the situation.

A report in the Lancet medical journal last month said pollution had claimed as many as 2.5 million lives in India in 2015, the highest in the world.