India plans to spray water over its capital, New Delhi, to try to combat toxic smog that has triggered a pollution emergency, officials have said, with conditions expected to get worse over the weekend.

Illegal crop burning in farm states surrounding New Delhi, vehicle exhaust fumes in a city with limited public transport, and swirling construction dust have caused the crisis.

Sprinkling water is the only way to bring down the dangerous pollution levels, said Shruti Bhardwaj, the Indian environment ministry's most senior official in charge of monitoring air quality.

The Indian government is finalising the plans to spray the water from a height of 100 metres, which would be unprecedented, she said, without saying how much of the city of 22 million people would be covered.

The thick blanket of grey air and pollutants has enveloped the city for the past four days.

A US embassy measure of tiny particulate matter, called ‘PM 2.5’, showed a reading of 523 at 9am this morning - the outer limit of "good" air is 50.

PM 2.5 is about 30 times finer than a human hair. The particles can be inhaled deep into the lungs, causing heart attacks, strokes, lung cancer and respiratory diseases.

The air has remained consistently in the "hazardous" category or beyond those levels, despite a litany of government measures.

It has led to a halt on all construction activities, restricted vehicular movement and raised parking charges four times to push residents to use public transport.

Commercial trucks are now banned from the city unless they are transporting essential commodities and the city transport department said it reintroduced an odd-even scheme under which cars with licence plates ending in an odd number are allowed one day and even-numbered cars the next day.

The scheme was introduced in the Chinese capital a decade ago to fight traffic and pollution with mixed success.

The Delhi high court issued an order yesterday suggesting the city's government consider "cloud seeding" to induce rainfall artificially, a practice also used in Beijing.

Anumita Roychowdhury, executive director of the Centre for Science and Environment in New Delhi, said sprinkling water will help, but it will not solve everything.

"Each and every dust control measure has to be put in place to ensure that air quality improves as soon as possible," she said.

Satyendra Kumar Jain, Delhi’s health minister, said labourers living in urban slums or beneath road fly-overs were suffering the most.

Mr Jain said government hospitals in the city were treating thousands of patients with respiratory ailments.

"The only solution left is that we start spraying water right at the street level, especially along heavy traffic roads," he said.

Chief Minister Arvind Kejriwal, the leader of Delhi, called the capital a "gas chamber" earlier this week as his government sought urgent meetings with the federal government and neighbouring states to find solutions.

According to a 2016 World Bank report, nearly 1.4m people died in India due to air pollution in 2013, causing an economic loss worth 7.7 percent of the nation's GDP