Common questions about Delhi's air pollution

There is no single biggest cause. In winter, particles formed in the air from gases (secondary particles) are the largest part, followed by biomass burning and vehicles.^30,49 Among Delhi's own (local) sources, vehicles are the largest.⁵² Most of Delhi's PM2.5 comes from outside the city: only 36% of winter PM2.5 comes from Delhi's own emissions.³⁶ In October and November, crop smoke and Diwali add spikes.

In winter the air near the ground is cold and gets trapped under warmer air. In November 2026 the mixing layer over Delhi was about 50 m deep at night, and the ventilation index was below the 6,000 m²/s line on 9 of 10 days. In May, ventilation is about 3.6 times better than in November.^1,9,38

No. It is a major seasonal cause in October and November, adding on average 4.9% of Delhi's PM2.5 between mid-October and November 2025 and 15.5% in 2024, with much higher shares on some days.⁵⁵ In December 2025, after the fires had ended, Delhi's smog was even more intense.¹⁹ Local and regional sources matter more over the full year.

It can. From 3 to 10 November 2026, NASA satellites saw 4,567 fires in Pakistan's Punjab and 3,040 in India's Punjab. 71% of our 3-D air paths came from the north-west, over Punjab.^1,4

They add to the regional haze. Sentinel-5P satellite data shows NO₂ plumes over 16 of 18 coal plants within 320 km.^1,3 CREA estimates that coal units within 300 km released about 154 kilotonnes of SO₂ in 2025, 81% of it from plants exempted from installing sulphur controls.⁷²

About 1,052 within 50 km and 3,014 within 100 km, based on the SentinelKilnDB satellite dataset (images from Nov 2023–Feb 2024). Baghpat, Jhajjar and Bulandshahr have the most nearby.^1,13

A lot, but only for one to three days. Diwali 2026 had a 24-hour average AQI of 376, the highest Diwali level since 2021. Since 2016, Delhi's AQI on the day after Diwali has never been below 300.^44,53

GRAP (Graded Response Action Plan) is Delhi-NCR's emergency plan for bad air, with four stages linked to the AQI. Stage III is for "Severe" air (AQI 401–450), and since 2024 it can start early based on the forecast.³² In 2026, Stage III was invoked on 6 November. It banned construction and demolition work, except essential projects, and restricted BS-III petrol and BS-IV diesel cars.⁴⁷ In 2025, Stage III also let schools run classes up to Class 5 online where possible.⁵⁷

India's AQI scale: 0–50 Good, 51–100 Satisfactory, 101–200 Moderate, 201–300 Poor, 301–400 Very Poor, 401–500 Severe. The AQI is the highest of the sub-indices of the individual pollutants over the last 24 hours.²⁴ See what each level means for your health.

India's standard is 40 µg/m³ as a yearly average and 60 µg/m³ over 24 hours.²⁸ The WHO guideline is much stricter: 5 µg/m³ for the year and 15 µg/m³ over 24 hours.⁴⁰ Delhi's 2025 average was 96 µg/m³.⁷³

PM2.5 means particles smaller than 2.5 micrometres across. They go deep into the lungs. WHO links them with heart and lung disease, stroke and cancers.¹⁶ Delhi residents lose an estimated 8.2 years of life expectancy compared with air meeting the WHO guideline.²¹

A little. The Delhi Economic Survey called 2025 the cleanest year since 2015 apart from the lockdown year 2020, with PM2.5 at about 100 µg/m³.⁶⁹ It is still more than twice India's yearly limit.^28,73

Only a little. The January 2016 odd-even trial cut daytime PM2.5 by 14–16%, but the April 2016 trial showed no effect.²⁶ Smog towers help only within about 200 m.⁸¹ See the evidence scorecard.

On days with AQI above 200, the Health Ministry advises avoiding outdoor exercise in the early morning and late evening, keeping windows shut at those hours, and using a close-fitting N95 or N99 mask if you wear one. Paper or cloth masks do not work. An air purifier may help. Children, pregnant women, older people and people with heart or lung disease need extra care.²⁴

We used Sentinel-5P satellite NO₂ and SO₂, NASA FIRMS fire data, OpenAQ ground monitors, the CAMS air-quality model, Open-Meteo weather on eight pressure levels, WorldPop population, NFHS-5 district data, SentinelKilnDB brick kilns, OpenStreetMap and geoBoundaries. Details are on the Method page.