India plans to spray water over its capital, New Delhi, to try to combat toxic smog that has triggered a pollution emergency, with conditions expected to get worse over the weekend.
Illegal crop burning in farm states surrounding New Delhi, vehicle exhaust in a city with limited public transport and swirling construction dust have caused the crisis, as they do year after year.
"Sprinkling water is the only way to bring down the dangerous pollution levels," said Shruti Bhardwaj, the environment ministry's senior most official in charge of monitoring air quality.
The 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 Delhi for the past four days. A US embassy measure of tiny particulate matter, called PM 2.5, showed a reading of 523 at 9am on Friday - 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: ordering a halt to all construction activities, restricting vehicular movement and raising 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 Delhi transport department said it re-introduced 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 on Thursday suggesting the city's government consider "cloud seeding" to induce rainfall artificially, a practice also used in Beijing.
Pronab Sarkar, president of the Indian Association of Tour Operators, said air pollution had also hit tourism.
"Many tourists are cancelling their bookings for Christmas holiday," he said.