More than two million farmers burn 23 million tonnes of crop residue on some 80,000 sq km of farmland in northern India every winter. Farmers burn crop residue because the stubble left behind after the combine harvesters have done their job is sharper and taller than it otherwise would be, potentially injurious to farmers and not good fodder for animals. If they do not remove the stubble, straw gets stuck in the machines that plant the rice crop. The stubble smoke is a lethal cocktail of particulate matter, carbon dioxide, nitrogen dioxide, and sulphur dioxide.
Harvard University researchers estimated that nearly half of Delhi's air pollution between 2012 and 2016 was due to stubble burning. Another study attributed more than 40,000 premature deaths in 2011 to air pollution arising from crop residue burning alone.
Levels of tiny particulate matter (known as PM 2.5) that enter deep into the lungs reached as high as 700 micrograms per cubic metre in some areas. The WHO recommends that the PM2.5 levels should not be more than 25 micrograms per cubic metre on average in 24 hours. Last winter Air Quality Index (AQI) recordings consistently hit the maximum of 999 - exposure to such toxic air is akin to smoking more than two packs of cigarettes a day.
The "green revolution" allowed a country wracked by famines, hobbled by un-irrigated farms and dependency on food aid to produce enough grain to feed its people. The northern states of Punjab and Haryana turned into breadbaskets, producing enough rice and wheat for the country. Wheat is sown and harvested during winters, and rice to coincide with the monsoon season in July and August. The "green revolution" was an unqualified success in giving India much-needed food security. It led to vast increases in wheat and rice production, but also ended up polluting air and depleting groundwater.
There are some 26,000 combine harvesters in use in India, most of them in northern India. They are responsible for a practice that is now a major contributor to air pollution.
The government has tried to solve this with "happy seeders", which are attachments mounted on tractors that help plant wheat seeds without getting jammed by rice straw stubble from the previous crop. But they are expensive - upwards of 130,000 rupees (£1,363; $1,769) and diesel-guzzlers - and remain out of reach for most farmers, who own small plots of land. During the smog season last year, according to Mr Singh, there were about 2,150 of these machines in Punjab and Haryana as against an estimated requirement of more than 21,000. Another machine called the "super straw management system" which chops and spreads the stubble evenly is also effective but expensive for the majority of farmers.
If crop stubble burning is to be stopped fully within five years, 12,000 "happy seeders" will need to be purchased every year. India, he believes, needs a second "green revolution which would be a technological one - one that adequately deals with agricultural shock to air quality". Until that happens, Delhi's foul air will continue to poison its 18 million people.
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