The World Health Organisation in 2014 classed Delhi as the world’s most polluted capital, with air quality levels worse than Beijing. A 2015 study showed about half the Indian capital’s 4.4 million schoolchildren had compromised lung capacity and would never totally recover. Research found that about 2.5 million Indians die each year from pollution, the highest number in the world.
In November 2017, air quality in Delhi plummeted to levels were alike to smoking at least 50 cigarettes in a single day. Slow winds and colder temperatures have been blamed for a surge in airborne pollutants beyond what instruments in the city could measure with some recording an Air Quality Index (AQI) maximum of 999. Levels of PM2.5 reached 710 micrograms per cubic metre, with levels of 0 to 50 suggesting 'good' and levels exceeding 500 considered 'hazardous'.
PM2.5 constitute fine pollutants smaller than 2.5 micrometers – small enough to evade the body’s natural filters and permeate the blood-brain barrier.
Delhi’s air quality is extremely poor for most of the year due to road dust, open fires, vehicle exhaust fumes, industrial emissions and the burning of crop residues, known as stubble burning) in neighbouring states to clear them for the next season. But conditions worsen in winter months when slow winds and cool temperatures trap pollutants closer to the ground.
Though Delhi gets most attention, toxic air afflicts the entire north Indian plain, including parts of Pakistan. A 2017 study found the holy city of Varanasi had among the worst air in the country, resulting in increased mortality.
 The Indian Express: At 2.5 million, India tops list of pollution-linked deaths: Study – October 20, 2017
 Jain et al: Ambient PM2.5 exposure and premature mortality burden in the holy city Varanasi, India in Environmental Pollution - 2017