A U.S. embassy measure of tiny particulate matter, called PM 2.5, showed a reading of 523 at 9 a.m. today. 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.
Illegal crop burning in neighbouring states Punjab and Haryana, vehicle exhaust in a city with limited public transport and swirling construction dust have caused the crisis, as they do at the onset of winter every year.
"Sprinkling water is the only way to bring down the dangerous pollution levels," said Shruti Bhardwaj, the environment ministry's most senior official in charge of monitoring air quality
The government was finalising 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.
For the past four days, the air has remained consistently in the "hazardous" category or beyond, despite a litany of government measures. All construction activities have been ordered to a halt and parking charges have been raised 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 Delhi government said the "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 - will be back for next week.
The Delhi high court issued an order on Thursday suggesting the city's government consider "cloud seeding" to induce rainfall artificially.
"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, New Delhi's health minister, said labourers living in urban slums or beneath road fly-overs are suffering the most. Government hospitals in the city, Mr Jain said, 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," Mr Jain said.
Chief Minister Arvind Kejriwal called the capital a "gas chamber" earlier this week as he sought urgent meetings with the Centre and the Chief Ministers of Punjab and Haryana to find solutions.
(Additional reporting by Manoj Kumar, Writing by Rupam Jain, Editing by Tom Lasseter and Nick Macfie)