Delhi's PM2.5 readings went blank on Diwali night just as they neared 1,000

Delhi's air was already bad on Diwali. Then, on the night itself, a lot of the numbers simply stopped showing up.

An analysis of records from the Central Pollution Control Board (CPCB, India's national pollution watchdog) shows that PM2.5 readings at many Delhi monitoring stations climbed towards 1,000 micrograms per cubic metre and then went blank. PM2.5 refers to fine particles small enough to get deep into your lungs.

What happened?

Across all 59 stations in Delhi, 43 had a gap in PM2.5 data between 10pm and 6am. At 14 of those 43, the last reading before the gap was 900 or more, within 100 of the 1,000 mark.

The gaps were not limited to one agency. Stations run by all three operators went blank: the CPCB, the IITM (a government meteorology institute) and the Delhi Pollution Control Committee (DPCC), the city's own pollution regulator.

Here is the odd part. The same stations kept sending readings for gases. Only the PM2.5 numbers disappeared.

Why does it matter?

Those overnight hours are exactly when firecrackers and smoke tend to pile up, so a missing stretch of data is a missing stretch of the story. Without it, the official picture of how bad the night really got is incomplete.

The official numbers we do have were already grim. Delhi's average AQI (air quality index) hit 376 on Diwali day, in the "very poor" range. On Monday the city woke up to very poor air again, with some areas above 400.

Loni, about 30 km from Delhi, also ranked among the world's most polluted places.

Where does this leave things?

A year ago, in October 2025, the Supreme Court allowed limited green firecrackers in Delhi-NCR (the capital region) for Diwali. This year's night-time readings suggest the conversation about what that meant on the ground is far from over.

The analysis shows what the records contain: the gaps, their timing and the last values before them. What it does not settle is what the true PM2.5 levels were while the stations were silent.