Air
Why is Delhi's air so bad?
Three things at once. Delhi sits in a landlocked basin where winter air stops moving; it is ringed by a dense industrial, agricultural and vehicular region whose emissions arrive regardless of what the city itself does; and a large share of its particulate matter forms in the atmosphere rather than at any chimney. No single source explains it, and the two government-commissioned studies that tried to apportion it do not agree with each other.
A basin, a season and a region
The emissions are large. The reason they concentrate is meteorological, and it is annual.
Delhi lies inland in the Indo-Gangetic plain with the Himalaya to the north and the Aravalli to the south-west. There is no coastal breeze to flush the air. Through late October and November the surface cools quickly after sunset, a temperature inversion forms, and the layer of air available for mixing collapses from over a kilometre to a few hundred metres. The same daily emissions are then diluted into a fraction of the volume.
The airshed is regional, not municipal. Emissions from the whole National Capital Region and well beyond it — brick kilns, thermal power, industry, transport corridors, and in autumn crop-residue burning across Punjab and Haryana — move into the basin on prevailing winds. A measure applied inside Delhi's boundary acts on part of the problem by construction.
And much of the fine fraction is secondary. Sulphur dioxide, nitrogen oxides and ammonia released hundreds of kilometres away react in the air to form sulphate, nitrate and ammonium particles that arrive as PM2.5. This is why the city's air can worsen with no local change at all.
- Inversion
- A layer of warmer air sitting above cooler air near the ground, which stops vertical mixing. The single biggest reason Delhi's winter readings are worse than its summer ones at similar emission levels.
- Airshed
- The area whose emissions actually reach a place. Delhi's is regional; its administrative boundary is not.
- Mixing height
- The depth of atmosphere available to dilute emissions. It collapses on winter nights.
What is actually being broken
"Bad air" is a description. These are the published figures the air is judged against, and they are what makes a breach a fact rather than an impression.
PM2.5, 24-hour mean
60 µg/m³
CPCB, NAAQS 2009
PM2.5, annual mean
40 µg/m³
CPCB, NAAQS 2009
PM10, 24-hour mean
100 µg/m³
CPCB, NAAQS 2009
The National Clean Air Programme, announced in 2019, set city-level reduction targets against a 2017 base year. Those targets are policy commitments rather than notified standards — a city can miss them without breaking the law, which is a distinction worth holding on to when a headline uses the two interchangeably.
How to read the argument
The honest answer to "what causes it" is a range, and the range is wide.
Two studies commissioned by government have tried to apportion Delhi's PM2.5 by source. IIT Kanpur's 2016 study reports vehicles at between 6 and 29 per cent depending on site and season. The TERI-ARAI study published in 2018 reports transport at 17 to 28 per cent using a dispersion model. A spread that wide is not a rounding difference — it is the state of the knowledge.
Stubble burning is the most-quoted source and the most seasonal. Its contribution is measured in days, peaks in late October and November, and is negligible for most of the year. A figure quoted from a peak week and applied to the annual average is one of the commonest misreadings of Delhi's air.
Read any single-cause claim about Delhi's air against the season it was measured in and the method used to attribute it. Source apportionment sets out why two honest studies land so far apart.
What this number cannot tell you
- No monitoring network can attribute a reading to a source. Attribution is modelled, and the models disagree.
- A reading cannot separate what Delhi emitted from what arrived. The airshed does not respect the boundary the policy is written for.
- A fall in the number between two winters cannot be read as a policy result without controlling for wind, rain and inversion depth.
- None of this measures indoor exposure, which for most residents is the larger share of the day.
What has actually changed
Delhi has moved its public transport fleet to compressed natural gas, shut the Badarpur coal plant, adopted Bharat Stage VI fuel nationally in 2020 and built a graded emergency response that closes construction and restricts traffic at declared thresholds. Each of these is a real intervention. None of them addresses a regional airshed, and the winter readings are the evidence of that.
Where this comes from
Every figure above is read out of one of these, at the address printed beside it.
- Source apportionment study — Delhi TERI and ARAI, for the Department of Heavy Industry, Government of India · The 2018 dispersion-model split
- National Clean Air Programme Ministry of Environment, Forest and Climate Change / CPCB · City targets and the PRANA dashboard
- National Ambient Air Quality Standards, 2009 Central Pollution Control Board · The limits in the table above
Reuse freely — CC BY 4.0. The grant covers this page. Each source above keeps its own terms, which is why every one of them is named.
Next
The reading this explains, the explanations next to it, and the one place to stand in it.
show different numbers? Four reasons two honest sources give you a different AQI for the same city at the same minute. Stand in it CityScapes One day, four sites — the landfill, the river, the forest and the street — with the readings in hand.