Air pollution in Delhi has hit hazardous levels, prompting officials to announce on Wednesday a temporary shutdown of the education system until next week.

The move came a day after doctors declared a "public health emergency" in the world's most polluted city.

"Delhi has become a gas chamber. Every year this happens during this part of year," its Chief Minister Arvind Kejriwal said on Twitter.

As winter approaches, a thick blanket of poisonous air particles envelops Delhi, mainly due to burning of crop stubble by farmers in the neighboring states, dust from construction sites, vehicle emissions and burning of coal and garbage. Low wind speeds and low temperatures further aggravate the problem.

The Indian Medical Association (IMA) has advised residents to stay indoors and avoid any physical activities.

The IMA called on the authorities to halt sports and other outdoor activities in schools. The organizers of the Delhi Half Marathon, scheduled to take place on November 19, have also been asked to postpone the event.

"Every possible step required to tackle the situation has been already identified, and the need of the hour is to put them into action," India's Environment Minister Dr. Harsh Vardhan said on Twitter.

"If Government of Delhi thinks that sprinkling of water through helicopter is the most cost effective measure, it's free to do so," he said.

Recurring problem

The readings on the government's air quality index showed that the concentration of poisonous particulate matter in some places peaked at the most "severe" level of 500.  According to the Central Pollution Control Board's (CPCB) index any measure above 100 is considered unhealthy.

Levels of fine particulate matter (PM 2.5) - small particles that pose the greatest threat to lungs – in some places were well over double the threshold of 300 that is classed as "hazardous." People took to social media to show the extent of the problem.

The heavy smog resulted in a huge pile-up of vehicles on an expressway just outside Delhi, Indian broadcaster NDTV reported. At least 24 vehicles were damaged and several of the drivers were injured, it said.

Top polluter

The alarming level of smog has once again underscored Delhi's reputation as one of the worst polluters. The World Health Organization (WHO) in May 2014 found Delhi to be the most polluted city in the world.

The dubious distinction has prompted authorities to take some remedial measures such as a temporary ban on the sale of firecrackers ahead of the Hindu festival of Diwali and taxing trucks that pass through the city. Last year, the Delhi government also experimented with road-space rationing based on registration numbers. But these measures have had little effect.

The air quality in neighboring cities of Faridabad, Ghaziabad, Bhiwadi and Noida was also found to be "severe," which according to the CPCB index is not only harmful for people with lungs and heart diseases but also for healthy people.

A study by the Lancet medical journal last month found that pollution claimed as many as 2.5 million lives in India in 2015, the highest in the world.