Air pollution in New Delhi and surrounding towns has reached the worst levels so far this year, with authorities in the capital having declared a public health emergency and ordered the closure of schools.

Key points: New Delhi's Chief Minister said pollution levels had reached "unbearable levels"

New Delhi's Chief Minister said pollution levels had reached "unbearable levels" Particulate matter in the air hit 900 PM 2.5 — way over the "severe-plus" ceiling of 500

Particulate matter in the air hit 900 PM 2.5 — way over the "severe-plus" ceiling of 500 The smog has been exacerbated by seasonal crop stubble burning and humidity

The air quality index (AQI), measuring levels of PM 2.5 — tiny particulate matter in the air — deteriorated to above 900, way over the 500-level that qualifies as "severe-plus" on Sunday.

S***, I Smoke! — an app that uses pollution data to explain air quality via cigarette smoking — found that the Indian capital's present air quality was akin to smoking 33.2 cigarettes per day.

Aside from the harm it was doing to the lungs of some 40 million people living in the capital region, the smog was so bad more than 30 flights were diverted from Delhi airport due to poor visibility.

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Roads looked deserted as large numbers of people stayed home, rather than expose themselves to the noxious atmosphere outside.

"Pollution has reached unbearable levels across north India," Arvind Kejriwal, Delhi's chief minister wrote on Twitter.

The government environment monitoring agency SAFAR warned that no relief was expected for the next one to two days, as humidity resulting from unexpected light rains overnight had exacerbated pollution, already driven by higher than seasonal crop stubble burning by farmers in surrounding states.

"Wind speed is picking up and it could take 24 to 48 hours before the pollution level reduces to a level of around 500," Mahesh Palawat, vice president of private weather forecasting agency Skymet, said.

Anything above 400 on the AQI poses a risk for people with respiratory illness and can also affect even those with healthy lungs.

Authorities have declared a public health emergency

The thick smog has blanketed New Delhi and surrounding states in Northern India. ( Supplied: US National Oceanic and Atmospheric Association )

Doctors were reporting a spike in patients with respiratory related issues, according to Sachin Taparia, head of Local Circles, a Delhi-based private consultancy that conducts surveys on government policies and programs.

"Delhi has turned into a gas chamber as the pollution levels hit the severe category," Mr Taparia said.

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A survey of 17,000 people in the Delhi region by his consultancy found that 40 per cent want to get out of the area because of the failure to control pollution.

Authorities in Delhi on Friday declared a public health emergency and closed schools and all construction activity.

From Monday, the city government will also restrict the use of private vehicles on the capital's roads under an "odd-even" scheme based on license plates.

"The temporary restrictions on private vehicles will have a negligible impact as we face the most hazardous situation," said Skymet's Mr Palawat.

Delhi's Mr Kejriwal, and the chief ministers from the neighbouring states of Punjab and Haryana, urged the federal government to do more to combat the pollution.

According to a lawyer, the Supreme Court is likely to hear a petition on Monday from the environment agency, looking for ways to make state governments take tougher action against farmers to curb the stubble-burning.

Politicians have been reluctant to upset their farming constituencies.

According to SAFAR, satellite pictures had captured more than 3,000 incidents of stubble burning last week in neighbouring states, contributing to 44 per cent of Delhi's pollution.

ABC/Reuters