NDTV has uncovered a big contributor of the smoke that pulls down Delhi's air quality, makes the capital's air among the world's most polluted, and one that has gone unchecked.
It is called furnace oil.
Furnace oil is the thick black residue left at the bottom of the barrel when relatively cleaner fuels such as petrol, kerosene and petrol are removed from crude petroleum during the refining process.
And evidence of how the authorities had shut its eyes came on Monday when the Centre told the Supreme Court that no furnace oil was being used in Delhi.
An investigation by NDTV that covered several industrial zones across Delhi, however, showed how far off-the-mark the authorities were.
Pretending to shop for furnace oil for a fictitious factory, NDTV tracked dozens of trucks supplying furnace oil and tyre oil, another completely illegal fuel being brought to the gates of factories.
One driver showed an invoice from an oil refinery at Panipat and bills to show that the furnace oil was meant for a factory in Haridwar. But the fuel had made its way to Delhi where it is sold for around 30-32 rupees a litre, a price that makes
Shot on hidden camera, another driver spoke how a large number of factories were illegally using the fuel in Delhi.
"There are probably only a few factories where we have not delivered our furnace oil. Our truck regularly supplies to Wazirpur, Jhangirpuri, Badli, & Jhilmil," the driver said, reeling off names of localities in the capital from where his long list of clients operate.
A team of the Supreme Court-appointed Environment Pollution Control Authority on air pollution (EPCA) led by chairman Bhure Lal on Tuesday promptly carried out checks at various industries and found the evidence too.
"It's just not possible without the connivance of government officials. We could find so many trucks involved but it seems no one has bothered to check," Bhure Lal later told NDTV.
Sunita Narain who heads the Centre for Science and Environment said she found the NDTV probe's findings "shocking". Anumita Roy Chowdhury, her colleague at CSE explained why furnace oil was such a dirty fuel. "This is a concentrate of not only high levels of sulphur but heavy metals which bind with sulphur and are cancer causing" she said.
Ms Narain said a sample of furnace oil tested by them had a sulphur content ranging from 24,000 to 40,000 ppm (parts per million). This means the sulphur content of this banned fuel was 500 to 800 times more than the sulphur content of fuels such as diesel and petrol (50 ppm).
When asked about the NDTV findings, Union Environment Secretary AN Jha said that the Centre would take action if any proof is brought to them. Delhi, however, is not holding its breath on that.