ExplainedDelhi air
Delhi's AQI explained: why the index stops at 500 and what the Diwali-night readings mean
Delhi's pollution authorities published hourly readings on Monday that peaked at 3,744 micrograms of PM2.5 per cubic metre, while the official index showed 370. How the AQI is worked out, why it cannot show that gap, and why monitors going blank matters.
The short answer
India's Air Quality Index turns pollutant readings into a score from 0 to 500, and for fine particles (PM2.5) it reaches the top of the scale at 380 micrograms per cubic metre, using a 24-hour average. Anything above that still reads 500, and a long average dilutes short spikes. That is why Delhi's official 24-hour AQI was 370 after Diwali while hourly readings at Mundka touched 3,744, and why the hours when monitors went blank matter.
What does the Air Quality Index actually measure?
It converts the concentration of pollutants in the air into a single score from 0 to 500 that people can read at a glance. India’s national index, run by the Central Pollution Control Board (CPCB), sorts the score into six bands: Good (0 to 50), Satisfactory (51 to 100), Moderate (101 to 200), Poor (201 to 300), Very Poor (301 to 400) and Severe (401 to 500).
In Delhi in winter the pollutant that drives the score is almost always PM2.5, particles under 2.5 micrometres across, small enough to reach deep into the lungs. Smoke from firecrackers, farm fires, vehicles and burning waste is rich in them.
How is the score worked out?
Each pollutant’s measured concentration is mapped onto the scale, and for fine particles the reading is a 24-hour average. For PM2.5, a day’s average of up to 30 micrograms per cubic metre counts as Good and up to 60 as Satisfactory. That 60 is also India’s national 24-hour standard. The Severe band begins at 250.
The average is the catch. Smog that sits at 900 for four hours in the small hours is blended with twenty cleaner hours, so the published figure is far lower than what people breathed at the worst moment. A reading at 9 am covers the previous 24 hours, not the last one.
Why does the index stop at 500?
Because the scale was built with a ceiling. PM2.5 reaches the top score of 500 at 380 micrograms per cubic metre. Anything above that still reads 500, so an AQI of 500 does not say whether the air held 380 or several times more.
Other systems differ. The US scale runs into four-digit values, and many phone apps use hourly or shorter averages and combine several monitors, which is why one city can show three different numbers on the same morning.
What happened on Diwali night in 2026?
The hourly readings, once released, were far above anything the index shows. The Delhi Pollution Control Committee (DPCC) published them on Monday evening, November 9, after they were missing from the central database. They put PM2.5 at 3,744 at Mundka at 2 am, more than 60 times the national standard. JNU rose from 161 at 8 pm to 786 by 11 pm.
Against that, the 24-hour AQI to 9 am on Monday was published as 370. With the restored hours it works out to about 384, a provisional figure not yet in CPCB’s records at the time of reporting. Both numbers sit under the cap and the averaging, which is why they look so different from the hourly peaks.
Delhi's air, November 6 to 9, 2026
- November 6The Commission for Air Quality Management brings in Stage III of GRAP as the city's average AQI reads 382.
- November 7 and 8The Supreme Court allows only green crackers in fixed windows; bursting continues past them.
- Night of November 8Monitors at several stations stop reporting for hours while readings rise.
- November 9, about 8 pmThe DPCC publishes the missing hourly values.
Why do monitors go blank, and does it matter?
It matters because the worst hours are the ones that vanish. An official has attributed the gaps to “some technical glitches” and said all 43 DPCC stations were giving data. In the CPCB’s database 36 of Delhi’s 59 stations were blank overnight. A newspaper analysis found 42% of expected hourly PM2.5 readings missing from 39 stations between 10 pm and 6 am.
Stations at R K Puram, Nehru Nagar, Mundka, Sonia Vihar and Dwarka were among those reported to have stopped for several hours. Similar interruptions were reported on earlier Diwali nights.
The Aam Aadmi Party alleges the stations were shut on purpose and that water is sprayed around monitors. No source read by this desk confirms either charge, and the authorities have not said the shutdowns were deliberate.
What does the smoke come from?
Mostly local sources on a night like this, with farm fires adding a share. Punjab logged 1,152 fires on Diwali day, the season’s highest single day, according to Commission for Air Quality Management (CAQM) figures. Even so, stubble burning was estimated at 3.5% of Delhi’s PM2.5 on Sunday, against 19% from transport.
What should a reader take from the number?
Read the hourly PM2.5 value, not only the AQI band, when the air is bad. The band tells you the day’s average category. The hourly figure, where it exists, shows the peak your lungs took. Children, the elderly and people with heart or lung disease are advised to stay indoors during Very Poor and Severe spells.
When the index sits near 500, remember it has stopped counting. Delhi’s early warning system expects severe air for several more days, and GRAP Stage III stays in force until CAQM reviews it. As of November 10, 2026, CPCB is expected to enter the DPCC hourly values into its records, which would lift the published average.