Once again the air quality in New Delhi has plunged. The city of 16 million people ranks among the world's most-polluted cities.

The trigger for the smog is India's northeasterly monsoon, which has now set in across all parts of the country. This signals an end of the summer rains, and a change to drier weather for most of the country.

The reason for this is the moisture feed is cut off as the winds are now draining in cool air from the Himalayas. This stable set up is ideal for the development of fog.

Delhi imposes car restrictions to curb rising pollution

The fog is mixed with smoke from burning crops in agricultural states across the northern plains and traffic emissions. Add to that firecrackers set off to mark the festival Diwali and it's a perfect recipe for a thick blanket of smog.

Weather scientists said that the Diwali firecracker smoke caused the air pollution to hit "severe" levels. The pollutants breached the 1,000 microgram mark in the Indian capital as it shot up nearly 10 times above normal levels.

It is standard practice at this time of year for farmers in New Delhi's neighbouring Punjab and Haryana states to set fire to paddy stubble in their fields after cultivating crops as part of slash-and-burn efforts.

According to the Centre for Science and Environment, New Delhi is facing a health emergency amid the worst smog in 17 years.

Authorities said they would crack down on those found burning waste, including leaves, to curb pollution.

"The situation today is not confined to New Delhi but also to its surrounding areas of Gurgaon and Noida, and a major contributing factor to the pollution is the smoke coming from the burning of crops in Punjab, which is still going on," said New Delhi Environment Minister Imran Hussain.

Delhi's cars and the odd-even formula

The government has come under sharp criticism from courts and environmentalists for not doing enough to deal with the problem.

Initiatives to clean up New Delhi's air have hit roadblocks in the past.

Authorities have responded with measures such as a ban on old lorries from entering the city, and briefly introduced a scheme that limited private vehicle usage to alternate days. But experts say those efforts have done little to reduce pollution.