NEW DELHI – Authorities have closed all the schools in the Indian capital, forcing as many as five million youngsters to stay home until Sunday as the city’s government attempts to tackle air pollution nearly 30 times higher than the recommended safe level.

The deputy chief minister, Manish Sisodia who had last evening ordered junior schools closed on Wednesday, comparing the national capital to a “gas chamber” as a deadly smog enveloped the city and hung low.

The US embassy website said that the concentration of PM 2.5 – a damaging particulate matter – topped 700 for a second day early on Wednesday, 28 times World Health Organisation safe level guidelines.

In some parts of New Delhi on Tuesday air quality was so poor that it was beyond the maximum reading of 999, a level equal to smoking 50 cigarettes a day.

The Delhi government has put out health advisories for high-risk groups, mainly the children and elderly, saying they should avoid outdoor activities, the NDTV reported.

Doctors called for the city’s half marathon, due to take place on 19 November, to be called off to protect runners and volunteers.

Manish Sisodia, Delhi’s deputy chief minister, tweeted: “Due to the deteriorating air quality in Delhi, the health of children cannot be compromised.

“We have ordered the closure of all the schools in Delhi until Sunday.”

The Indian Medical Association described the situation as a public health emergency and demanded administrators “curb this menace”.

The city, which has a population of 20 million, is said to be the world’s most polluted, often surpassing Beijing.

A Supreme Court-appointed panel has recommended emergency measures like four times the parking fees to discourage the use of cars, and reduced metro fares.

Delhi’s air quality typically worsens ahead of the onset of winter as cooler air traps pollutants near the ground, preventing them from dispersing into the atmosphere, a phenomenon known as inversion.

High levels of moisture in the air and a lack of wind mean emissions have been trapped in the environment, according to the Central Pollution Control Board.

Firecrackers set off to celebrate Diwali last month added to the toxic mix created by pollution from diesel engines, coal-fired power plants and industrial emissions. The problem has been further exacerbated by the burning of crop stubble by farmers after the harvest in neighbouring states, a practice that remains commonplace despite an official ban.