Where Delhi sits on India's air quality scale.
This morning Delhi's air was rated "poor", one point short of "very poor", and the forecast says no rain this week. After a while you stop noticing the number. I noticed that I had stopped noticing, and it bothered me more than the number did.
So I went to the source studies instead of the headlines. Three things surprised me. The biggest slice of Delhi's winter pollution is not smoke from anything. More than half of what Delhi breathes in a bad week is made outside Delhi. And the inside of a well-off home is not the refuge most of us assume it is.
What is in the air
The particles that do the most harm are PM2.5: fine enough to pass deep into the lungs and into the blood. The most detailed recent breakdown I found comes from an IIT Kanpur-led consortium with IIT Delhi and TERI, which tracked Delhi's PM2.5 by source, hour by hour, through the winter of 2022 to 2023.
What made up Delhi's winter PM2.5.
The top bar is the one I did not expect. About a third of Delhi's winter PM2.5 is never emitted as a particle at all. It starts as gas: sulphur dioxide, mostly from coal power plants, nitrogen oxides from engines and furnaces, and ammonia from fertiliser and livestock. In cold, humid, still air these gases react and condense into fine salts such as ammonium sulphate and ammonium nitrate. The chimney may be 200 kilometres away. The particle is made over your roof.
That changes how I think about blame. The visible culprits, a smoking truck or a burning field, are real. But a large part of the problem is invisible, chemical and regional, and no one standing in Delhi can see it being made.
Where it comes from
IITM Pune runs a decision support system that estimates, day by day, how much of Delhi's PM2.5 was made inside the city and how much drifted in. For the first eight days of this November, the Centre for Science and Environment summarised it like this.
Most of Delhi's air is made outside Delhi.
Stubble gets the headlines every November, and on some days it is the largest single input. But on most days, most of the problem is the wider region: Gurugram, Faridabad, Ghaziabad, Noida, Sonipat and beyond, with their own traffic, industry, generators and construction. Delhi is one room in a house whose windows all open onto the same courtyard.
Why November and why here
Delhi sits on the Indo-Gangetic plain, a flat, crowded basin walled by the Himalaya to the north. In winter the ground cools fast at night and a layer of warmer air settles on top of the cold air, like a lid. Scientists call it an inversion. The wind drops. The height to which pollution can mix shrinks. The same emissions that disperse in May are packed into a much smaller box of air in November. Fog adds moisture, which speeds up the chemistry that turns gases into particles.
None of this is new. What is new is how much we now put into that box.
How India got here
No single decision made Delhi's air. A series of sensible ones did, each solving a different problem.
- 1970 TO 2010Rice goes from about 7% of Punjab's gross cropped area to about 36%, pulled by assured procurement at the minimum support price. Rice was not a traditional Punjab crop. Food security was the goal, and it worked.
- 1990s ONWARDCombine harvesters replace hand harvesting of paddy. They are faster and cheaper, but they leave standing stubble, and the window before wheat sowing is short. A match is the cheapest way to clear a field.
- 1998The Supreme Court orders Delhi's buses, autos and taxis onto CNG. It was resisted at the time. It is still the clearest proof that Delhi's air responds to firm, enforced rules.
- 2009Punjab and Haryana ban early paddy transplanting to save groundwater. The harvest moves later, and the burning moves into colder weeks, when the air clears slowly. I wrote about this in the previous letter.
- 2019The National Clean Air Programme launches. Its target for 131 cities is now a 40% cut in particulate levels by 2025 to 2026.
- JULY 2025The sulphur rule for coal plants is relaxed. About 78% of India's coal plants no longer need sulphur scrubbers. Within 300 kilometres of Delhi there are 12 coal plants with 37 units; by CREA's estimate, about 90% of their sulphur dioxide comes from units without scrubbers.
What it costs
Delhi's average PM2.5 in 2023 was 88.4 micrograms per cubic metre. India's own standard is 40. The WHO guideline is 5. Even meeting India's standard would give back about 4.7 of those 8.2 years.
What one person can do
I wanted this part to be honest. You cannot fix a regional airshed from your balcony. But exposure is not the same as pollution, and exposure is something you have real control over. Here is what the evidence supports, in three rings, from the one you control most to the one you control least.
Check your nearest station, not the city average
PM2.5 varies widely across Delhi at the same hour. The CPCB's SAMEER app shows station-level readings. Evening rush hour is often the worst window: this November, nitrogen dioxide peaked around 7 pm and stayed high until midnight as still night air trapped traffic exhaust.
Realistic: this changes when you walk or run, not whether you go out.
If you wear a mask, wear one that seals
An N95 or a well-made KN95 is designed to filter at least 95% of fine particles, if it fits: nose clip pressed firm, no gaps at the cheeks or chin. Surgical and cloth masks do not seal and offer much less protection. Beards break the seal.
Realistic: real-world health evidence for masks in the general public is still limited. Fit matters more than brand.
Move hard exercise indoors on very poor days
During exertion you breathe several times more air per minute. On the worst days, the workout is better done in a filtered room or skipped.
Realistic: children, older people, pregnant women and anyone with heart or lung conditions face the most risk; their doctor is the right person to set limits.
Do not assume indoors is safer
A study of about 1,500 Delhi households by EPIC India found indoor PM2.5 well above what the nearest government monitor showed, in better-off and poorer homes alike. Homes that owned an air purifier were only about 10% cleaner than those that did not.
Realistic: owning a purifier is not the same as being protected by one.
Make one clean room rather than a slightly cleaner house
Pick the bedroom, where you spend eight hours. Close the door and windows, seal obvious gaps, and run a purifier sized for the room continuously. The industry rule of thumb (AHAM): its smoke CADR should be at least two-thirds of the floor area in square feet. A 12 by 15 foot bedroom is 180 square feet, so look for a smoke CADR of 120 or more.
Realistic: a small purifier in a big open living room, with the door ajar, does very little.
Stop adding smoke indoors
In the EPIC study indoor levels climbed fastest in the morning and evening, around cooking time. Use the exhaust when cooking, and on bad days skip incense, mosquito coils and candles.
Realistic: small, free, and entirely in your hands.
Report what you can see
Open garbage burning, uncovered construction sites and road dust are local and fixable. The Delhi government's Green Delhi app logged 84,765 complaints in 2024; the MCD resolved about 84% of the ones sent to it.
Realistic: this reaches the visible, local slice. It does nothing about sulphate from coal plants 200 kilometres away.
Shift one regular trip
Transport is about half of Delhi's own emissions. Emergency curbs this month barely moved congestion: the city's travel time index went from 2.38 to 2.40. Volume is the problem. One car trip a week moved to the Metro, multiplied across an office or a housing society, is a real number.
Realistic: small alone, meaningful as a habit others copy.
Ask for the rules that worked
Two of the clearest improvements in Delhi's history, the move to CNG and the 2018 closure of the Badarpur coal plant, came from firm rules, enforced. The largest open questions now are sulphur controls on the coal plants around Delhi and making it cheaper for a farmer to manage stubble than to burn it. RWAs, letters to elected representatives and public comment on draft rules are slow, but they are how those changes have happened.
Realistic: years, not weeks. That is the honest timescale.
What I am taking from this
The second-order lesson for me is about where to spend effort. Most of us spend our worry on the part we cannot control, the city's AQI, and almost nothing on the part we can, our own exposure. A well-sealed bedroom and a fitted mask do more for your family this winter than any amount of outrage at a headline.
It is the same split I see in money. We fret about the market, which nobody controls, and neglect the costs, habits and exposures that are entirely ours to set.
Start with the levers that are yours.
| Lever | Who controls it | How fast it helps | How much it helps you |
|---|---|---|---|
| Clean bedroom, sized purifier | You | Tonight | Large, for eight hours a day |
| Fitted N95 outdoors | You | Immediately | Meaningful if it seals |
| Timing outdoor activity | You | Today | Moderate |
| Reporting local burning and dust | You and the city | Days | Small, local |
| Fewer car trips | Many people | Months | Small alone, real together |
| Coal plant controls, stubble economics | Governments | Years | The largest, eventually |
One question
Which part of your exposure this winter is fully in your control, and have you acted on it?
Breathe carefully this week,Ajitesh