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

The declaration from the Indian Medical Association (IMA) came as the US embassy website said levels of the fine pollutants known as PM2.5, that are most harmful to health reached 703 micrograms per cubic metre – are over double the threshold of 300, which authorities consider to be hazardous to health.

“We have declared a state of public health emergency in Delhi since pollution is at an alarming level,” Krishan Kumar Aggarwal, head of the IMA, told AFP.

“Delhi authorities have to make every possible effort to curb this menace.”

Chief minister Arvind Kejriwal said he requested that education minister Manish Sisodia “consider closing schools for a few days”.

Considering high level of pollution, I have requested Sh Manish Sisodia, Education Minister, to consider closing schools for a few days — Arvind Kejriwal (@ArvindKejriwal) November 7, 2017

In 2014, Delhi was classed as the world’s most polluted capital by the World Health Organisation (WHO), with air quality levels being worse than in Beijing.

Since then, authorities have closed power plants temporarily and even taken some cars off the road.

It is known that the air quality in Delhi typically gets worse during the winter months, as cooler air traps pollutants closer to the ground.

Delhi has become a gas chamber. Every year this happens during this part of year. We have to find a soln to crop burning in adjoining states — Arvind Kejriwal (@ArvindKejriwal) November 7, 2017

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 made worse by the burning of crop stubble by farmers, after the harvest in Northern India, a practice which remains commonplace despite there being an official ban.

In some parts of Delhi, the air quality was so poor that it was even beyond the maximum level, according to the US embassy real-time air quality index.

In the RK Puram area, it stood at 999, beyond this no readings are available.

Waking up to Terriblly smogy Day in Delhi I just read the air quality index at 999 I think it’s the lowest I have seen 😰#delhipollution pic.twitter.com/nYJlZUC0mN — Rekha Kakkar (@Rekhakakkar) November 7, 2017

Dr Arvind Kumar, chairman for chest surgery at Sir Ganga Ram hospital, said that the level was equal to smoking 50 cigarettes a day.

“We are in a state of medical emergency, schools should be shut, we need to bring these levels down. We are all shortening our lives.”

The IMA had already written to Delhi’s chief minister calling for the 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 company Airtel threatened to pull its sponsorship of the event, due to be held on 19 November, if authorities failed to take action over air quality.

According to a report in the Lancet medical journal published last month, pollution in India was responsible for up to 2.5 million deaths in 2015, the highest number in the world.

“Total calm conditions, marked by the complete absence of wind, has led to the situation,” Dipankar Saha, a senior executive with the board, told the Press Trust of India news agency.

This time last year, unprecedented levels of pollution forced schools to shut, as authorities scrambled to contain the crisis.