Decoding Delhi’s Smog Season: Fewer Bad Days, Higher Baseline

On 21 October, Delhi’s Air Quality Index (AQI) touched 245, highest in 100 days, and the Commission for Air Quality Management (CAQM) invoked Stage I of the Graded Response Action Plan across the National Capital Region. According to the Centre for Research on Energy and Clean Air (CREA), this was the latest date on which Delhi’s air entered the “poor” category since at least 2015.

A late start is welcome. But the autumn data show two trends running side by side.

Fewer Bad Days, A Higher Baseline

An analysis by Climate Trends, released on 28 October, compares Delhi’s daily AQI between 15 September and 23 October in 2024, 2025 and 2026. Over this period, the average AQI fell from 169.3 in 2025 to 142.6 this year, a decline of 15.8%. AQI crossed 200 on three days this year, compared with ten last year, and peaked at 245, against 353 a year earlier.

The median AQI, the middle value when all days are ranked and a closer measure of a typical day moved the other way, rising from 135 to 154, an increase of 14.1%. The improvement in the average came from fewer extreme days, while the ordinary day became more polluted.

CREA’s data for the first week of October point in the same direction. Delhi’s average AQI for those seven days was 171, the second highest for that week in six years, compared with 117 last year.

“We cut the extremes, we didn’t clean the everyday air,” said Aarti Khosla, director of Climate Trends.

What The Fire Data Show

Crop-residue burning in Punjab and Haryana is a recurring feature of Delhi’s autumn. Between 15 September and 15 October, satellite-detected fire events in the two states numbered 193 this year, against 201 last year, a decline of 4%.

The same-day correlation between fire counts and Delhi’s AQI was 0.41 in 2024, 0.62 in 2025 and 0.17 in 2026, the weakest of the three.

On 20 October, the Decision Support System run by the Indian Institute of Tropical Meteorology estimated stubble burning’s share of Delhi’s pollution at about 8%. The burning season typically intensifies from late October into November, and Punjab had recorded 542 farm fires by 23 October, according to CAQM figures.

“Crop-residue burning emissions do not translate into the same air-quality impact every year,” said Dr Palak Balyan, research lead at Climate Trends.

What The Weather Data Show

Weather conditions this autumn were not favourable for dispersing pollution. Modelled data for 15 September to 23 October, compared in the Climate Trends analysis, show Delhi’s boundary layer, the layer of air in which pollution is trapped, was 4.3% lower than last year, and rainfall was 78.2% lower. Wind speed was 2.1% higher.

This has coincided with a strengthening El Niño. Dr Sagnik Dey of IIT Delhi noted that El Niño “should not be viewed as a direct predictor of air pollution”.

A Year-round, Regional Problem

Delhi’s pollution comes from many sources that operate throughout the year. A Centre for Science and Environment analysis of the first half of December 2025 found that transport accounted for about 46% of PM2.5 generated within Delhi, and that about 65% of the PM2.5 in the city’s air that fortnight came from outside its borders. Delhi’s environment minister, Manjinder Singh Sirsa, said this month that about 30% of the city’s pollution originates within Delhi, while 60–65% comes from or is influenced by surrounding areas.

Together, these figures describe an airshed in which vehicles, industry, dust and waste burning across the region shape the everyday baseline, with farm fires adding to it seasonally.

The Cost Of The Ordinary Day

The everyday baseline matters because health effects accumulate. “It is that cumulative exposure, far more than any single severe day, that drives the long-term disease burden,” said Dr Harshal Ramesh Salve of AIIMS, Delhi.

A 2024-study in The Lancet, covering 2008–2019, attributed about 11.5% of Delhi’s annual deaths to short-term PM2.5 exposure, the highest share among 10 Indian cities studied.

There is an economic cost too. A 2021 Dalberg study for the Clean Air Fund estimated using 2019 data that air pollution costs Indian businesses about $95 billion a year, roughly 3% of GDP, through lost productivity, absenteeism and reduced consumer spending.

Measuring What Matters

Delhi’s pollution response is organised around thresholds, with restrictions tightening as AQI rises. Fewer days above those thresholds are a real gain. This autumn’s data suggest that the typical day deserves equal attention. As the burning season peaks and colder air settles over the city, both measures will be worth watching.

Disclaimer: The views expressed are personal.