This year was no different, as air pollution increased sharply. After-effects are likely to persist as pollutants remain in the air and slowly dissipate. The bigger problem for Delhi, indeed much of northern India, is that pollution rises after Diwali and persists into the winter months.
Courtship disaster
This year’s Diwali was different for the National Capital Region of Delhi (NCR) as the Supreme Court allowed the use of ‘green crackers’, effectively overturning the total ban it had enforced in 2020. It allowed the burning of crackers during 6-7 am and 8-10 pm on 19 October and 20 October.
The evening slot was punishable on both these days, as it was on October 21, although crackers were not allowed.
Air pollution levels have been rising in Delhi NCR in the evening hours, as shown by the plotting of PM2.5, a key pollutant, at 15-minute intervals. As expected, PM2.5 levels began to rise sharply in the evening, around 5:30 p.m., and did not begin to subside until the early hours of the following morning.
The sheer scale of the increase in pollution is staggering: a 7-fold increase on Diwali between 6:30 pm and 12:45 am in both Delhi and Gurugram. In other cities of Delhi NCR it was 2-4 times and in toxic territory.
It hangs in the air
If the 2024 data is anything to go by, toxic air is likely to persist even after the Diwali blast, not just in Delhi NCR but also in other leading Indian cities. The Air Quality Index (AQI) is a single value designed to show a composite picture of air quality across multiple pollutants.
We looked at the AQI data for four leading Indian cities during the 10 days before and 10 days after Diwali. (Diwali that year was October 31).
Strikingly, even after Diwali, AQI remains high in major cities as pollutants persist in the atmosphere. Among the four cities, the average AQI for the 10 days after Diwali in 2024 fell only in Bengaluru compared to the 10-day run before Diwali.
In comparison, it was 17% higher in Delhi and a whopping 55% higher in Mumbai. But there is no comparison in absolute numbers – Delhi’s pollution levels were much higher than other cities and remained in the “very poor” classification throughout this period.
Fine edges
This is true not only for the composite AQI. This also applies to one of its important sub-components: PM2.5, which are fine particles of dust, soot or smoke suspended in the air that are around 2.5 micrometres in size, about 30 times finer than a human hair.
The particles are dangerous because they are easily inhaled and can enter the lungs and bloodstream.
Similar to the overall AQI, PM2.5 spikes two to three days before Diwali. But unlike AQI, it is retreating, except in Delhi. At its peak, Delhi’s PM2.5 levels are about 1.7-3.3 times higher than the other four cities studied here.
In a study of PM2.5 in northern India published in the journal Nature Communications in 2024, authors Deepika Bhattu and others found that “ammonium chloride and organic aerosols from traffic exhaust, residential space heating, and unsaturated vapor oxidation from fossil fuels are the dominant sources of PM in Delhi.”
Winter horror
In Delhi this year, while all pollutants have shown an increase, PM2.5 and PM10 (particles around 10 micrometers in size) have been particularly pronounced. But for Delhi and its environs, the worst is yet to come.
After Diwali comes the season of crop burning in northern India, when farmers burn agricultural residue in their fields to prepare for the new planting season. The resulting smoke contributes to air pollution in the region.
Although an important contributor to pollution in November, air pollution levels in Delhi NCR remain high throughout the winter months – long after crop burning has ended.
Air quality only starts to improve in February. In other words, crop burning is one of several factors that contribute to the winter pollution levels in Delhi NCR. This type of pollution is the main cause of respiratory diseases, especially asthma. And this year it looks no different.
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