India plans to spray water over its capital, New Delhi, to combat toxic smog that has triggered a pollution emergency, officials said on Friday, with conditions expected to worsen 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 every year.

“Sprinkling water is the only way to bring down the dangerous pollution levels,” said Shruti Bhardwaj, an environmental official charged with monitoring air quality.

The government was 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.

An Indian family crosses a road amid heavy smog in front of the Red Fort in New Delhi on Friday. Photograph: Prakash Sing/AFP/Getty Images

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, cancer and respiratory diseases.

The air has remained consistently in the “hazardous” category or beyond those levels in recent days, despite a litany of government measures - ordering a halt to construction, restricting car use and raising parking charges fourfold to persuade 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.

Chief minister Arvind Kejriwal, the leader of Delhi, called the capital a “gas chamber” earlier this week. Affluent residents and large private companies have been placing bulk orders for air purifiers and filtration masks.

“I have installed six air purifiers in my home and closed all the windows, but my eyes are still watering,” said Ranveer Singhal, a commodity trader living in leafy neighbourhood of the city.

Organisers of music festivals and open air parties have cancelled events after the Central Pollution Control Board said air quality could deteriorate further during the weekend.

Pronab Sarkar, president of the Indian Association of Tour Operators, said air pollution had also hit tourism.

The National Green Tribunal directed the Delhi government and neighbouring states to stop farmers from burning crop residue, but many have no choice, unable to afford machinery to do the job instead.

For the 2017/18 fiscal year, the federal government allocated 1.33 billion rupees (€17m) to Punjab, Haryana, Rajasthan and Uttar Pradesh to help them in crop residue management and farm mechanisation.

Government data showed that Punjab had not utilised the allocated fund and Rajasthan had used only a third.

Punjab, known as India’s grain bowl, disputed the claims of the federal government. A state government official said the fund was specifically meant for farm mechanisation, not for stopping the burning of straw. –Reuters