Delhi recorded only three days with an Air Quality Index (AQI) above 200 between September 15 and October 23 this year. During the same period in 2025, the number was 10. Its average AQI also fell from 169.3 to 142.6, a 15.8 per cent improvement.
But there is another number in the data. Delhi’s median AQI rose from 135 to 154 during the same period. This means that although the city experienced fewer days of severe pollution, the air quality on a typical day was worse than the previous year.
The difference matters when assessing the health effects of air pollution. People are exposed to the air they breathe every day, not just during episodes when pollution reaches severe levels. Prolonged exposure is associated with respiratory and cardiovascular diseases, while polluted air can also affect children’s lung development.
A new assessment by Climate Trends, covering Delhi’s air quality, crop-residue burning in Punjab and Haryana, and meteorological conditions between 2024 and 2026, examines this gap between seasonal averages and everyday exposure.
What the AQI Average Leaves Out
The mean AQI is calculated using daily readings across a given period. A few exceptionally polluted days can push this average upwards. The median, on the other hand, is the middle reading when all daily values are arranged in order. It offers a better indication of what a typical day looks like.
Climate Trends’ analysis found that Delhi’s mean AQI declined by 15.8 per cent between 2025 and 2026. However, the median increased by 14.1 per cent. The maximum AQI also came down from 353 to 245, while the number of days above 200 dropped from 10 to three.
The improvement in the mean, therefore, was largely associated with the reduction in extreme pollution days. It did not indicate an improvement in the typical daily reading.
Aarti Khosla, Director of Climate Trends, draws attention to this difference.
“The average AQI fell 15.8% this autumn, and that is the number that will get reported as improvement. But the typical day, the one most Delhiites actually live through, got worse, up 14.1%.”
She argues that air quality policies should be evaluated using the pollution levels people experience regularly, rather than seasonal averages alone.
Farm Fires and Delhi’s Pollution: Numbers Don’t Move Together
Every October, crop-residue burning in Punjab and Haryana becomes a major focus of Delhi’s air pollution debate. Smoke from these fires can travel towards the capital, contributing to particulate pollution. However, the extent of its impact depends on weather conditions and the movement of air.
The three-year assessment found that the relationship between farm fires and Delhi’s AQI varied considerably across the study period. Between September 15 and October 15, 2025, satellite observations recorded 201 crop-residue fire events in Punjab and Haryana. In 2026, the number was 193, a decline of just four per cent. Delhi’s mean AQI, meanwhile, fell by 15.8 per cent.
The statistical correlation between daily fire counts and Delhi’s AQI also weakened. The same-day correlation coefficient fell from 0.62 in 2025 to 0.17 in 2026. The previous year’s figures show a different pattern. Between 2024 and 2025, combined fire events declined by 88 per cent, but Delhi’s mean AQI fell by only 3.9 per cent.
These comparisons do not establish that agricultural burning has no effect on Delhi’s air. They show that the number of fires alone cannot explain the city’s year-to-year AQI changes.
Dr Palak Balyan, Research Lead at Climate Trends, explains that the movement of smoke depends on atmospheric conditions.
“Wind direction, wind speed, rainfall and atmospheric mixing determine whether smoke is transported towards Delhi, dispersed or removed from the atmosphere.”
The study also identifies traffic, industrial emissions, road and construction dust, and other regional combustion sources as contributors to Delhi’s pollution. However, it does not quantify how much each source contributed to the AQI changes recorded in 2026.
El Niño: Game Changer
The assessment also considers whether the strengthening El Niño conditions in 2026 could explain the changes in pollution and agricultural burning. El Niño refers to the periodic warming of sea-surface temperatures in the central and eastern equatorial Pacific Ocean. It influences large-scale atmospheric circulation and can affect rainfall and temperature patterns in different regions.
According to the NOAA Climate Prediction Center assessment cited in the report, the Niño-3.4 sea-surface temperature anomaly reached approximately +2.1°C by October 2026. NOAA also projected an 83 per cent or higher probability of strong-to-very-strong El Niño conditions continuing through January–March 2027.
Despite these conditions, the combined number of farm-fire events in Punjab and Haryana did not increase. Delhi’s mean AQI also declined.
The local meteorological data present a more complicated picture. Delhi’s accumulated rainfall between September 15 and October 23 fell from 46.70 mm in 2025 to 10.19 mm in 2026, a decline of 78.2 per cent. Its mean planetary boundary layer height (PBLH), which indicates the depth of the lower atmospheric layer where pollutants are mixed, decreased by 4.3 per cent.
Lower rainfall reduces the removal of airborne particles through precipitation. A lower boundary layer can also limit the vertical space available for pollutants to disperse.
Mean wind speed increased by 2.1 per cent, from 2.40 to 2.45 metres per second. Stronger winds can help disperse pollutants, but their effect depends on direction and the areas from which the air is moving.
Dr Sagnik Dey, Head and Vipula and Mahesh Chaturvedi Chair Professor in Policy Studies at the Centre for Atmospheric Sciences, IIT Delhi, cautions against using El Niño as a direct indicator of air pollution.
“El Niño should not be viewed as a direct predictor of air pollution. Its influence on regional air quality depends on how large-scale climate variability interacts with local rainfall, temperature, wind patterns and atmospheric stability.”
The assessment does not establish that El Niño caused the observed changes in Delhi’s AQI. It also cannot isolate the contribution of individual weather conditions or pollution sources to the decline in the city’s average reading.
Health Cost of an Ordinary Day
The distinction between average and median AQI has implications beyond how Delhi’s annual pollution figures are reported.
Dr Harshal Ramesh Salve, Professor at the All India Institute of Medical Sciences (AIIMS), Delhi, points out that the health burden of pollution develops through prolonged exposure, rather than only during a handful of severe pollution episodes.
He identifies impaired lung development in children and increased lifetime cardiovascular risks among adults as concerns associated with sustained exposure. He also recommends early screening for high-risk adults and treating clean air as a year-round public health priority.
The 2026 assessment does not show that Delhi has solved its pollution problem. Its findings show that a decline in the number of severe days can improve the seasonal average without improving the air quality people experience on most days.
The study points to a need to examine Delhi’s pollution beyond the seasonal AQI average. With the median AQI rising despite fewer severe pollution days, the improvement recorded in 2026 does not reflect cleaner everyday air. The contribution of traffic, industrial activity, construction dust and other local sources needs closer assessment, particularly since the study could not isolate their individual impact. Without that understanding, the reasons behind Delhi’s changing air quality will remain only partially explained.