Cooler temperatures and slow-moving winds trap deadly pollutants.
At dawn on Wednesday, "hazardous" pollutant levels in parts of the sprawling urban area of more than 30 million people topped 806 micrograms per cubic metre, according to monitoring firm IQAir.
That is more than 53 times the World Health Organization recommended daily maximum of fine particulate matter -- dangerous cancer-causing microparticles known as PM2.5 pollutants that enter the bloodstream through the lungs.
Many in the city cannot afford air filters, nor do they have homes they can effectively seal from the foul smelling air.
The city is blanketed in acrid smog each year, primarily blamed on stubble burning by farmers in neighbouring regions to clear their fields for ploughing, as well as factories and traffic fumes.
But a report by The New York Times this month, based on air and soil samples it collected over a five-year period, revealed the dangerous fumes also spewing from a power plant incinerating the city's landfill garbage mountains.
Experts the newspaper spoke to said that the levels of heavy metals found were "alarming".
India's Supreme Court last month ruled that clean air was a fundamental human right, ordering both the central government and state-level authorities to take action.
But critics say arguments between rival politicians heading neighbouring states -- as well as between central and state-level authorities -- have compounded the problem.
The WHO says that air pollution can trigger strokes, heart disease, lung cancer and other respiratory diseases.
It is particularly punishing for babies, children and the elderly.
A study in The Lancet medical journal attributed 1.67 million premature deaths to air pollution in the world's most populous country in 2019.
pjm/rsc
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