The morning after Diwali, millions of people across North India woke up to the worst air pollution levels of the season. Over the last week, air quality levels had been dipping in the region but the firecracker smoke on Sunday night further exacerbated the crisis.

On Monday morning, the air quality index of the Ministry of Earth Sciences showed PM 2.5 levels had crossed the 700-mark near Delhi University. These are particles that are small enough to lodge themselves in human lungs. An index reading above 100 is considered unhealthy and above 300 hazardous. All monitoring stations in Delhi recorded levels above 400, visible in red-coloured markers on the map below.

Air quality status in Delhi, according to the System of Air Quality and Weather Forecasting And Research, Ministry of Earth Science and Indian Institute of Tropical Meteorology.

Another measure of the toxicity of North India’s air comes from satellite imagery. When pollution figures from NASA’s and European earth observation systems are converted into colour they show a red swirl enveloping North India, in stark contrast to the rest of Asia and the world.

The red indicates high concentrations of PM 2.5 in the range of 90-130 micrograms per cubic metre. This is 4-5 times the concentrations recommended by the World Health Organisation. Its guidelines say concentrations must be limited within 25 micrograms per cubic metre in a 24-hour cycle.

What is causing this toxic haze over North India?

“Cars, trucks, industry, coal plants, trash burning, crop residue burning, construction, even traditional stoves and household fires to keep warm: air pollution is caused by a complex list of sources,” writes economist Dean Spears in his new book on North India’s air pollution crisis. The exact contribution of each source, however, is still a matter of contention because India lacks high-quality data on air pollution, he explains in this excerpt from the book.

“Many questions remain unanswered about the details of the source of north India’s air pollution, even to the experts,” notes Spears.

Still, there is no doubt that crop residue burning contributes to the polluted air. Satellite imagery shows farm fires are raging in North India even as the air quality in the region deteriorates.

Credit: Fire Information for Resource Management System

Further zooming into this image, taken from NASA’s Fire Information for Resource Management System, which provides near real-time active fire data from satellite images, it is clear where the densest concentration of farm fires lies: the states of Punjab and Haryana.

Credit: Fire Information for Resource Management System

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