AQI data sources

Readings from government monitors run by the Central Pollution Control Board (CPCB) and state pollution control boards, and from sensor networks. They're collected by Open Air Quality, a Rainmatter Foundation project. We fetch them every hour.

OfficialReference-grade monitors run by CPCB and the state boards. India's official AQI comes from these.

SensorLow-cost devices. They cover more places, often closer to you, but read less accurately, especially in humid air.

City numbers, Rankings and the colours on the map use official monitors only. Sensors show as rings on the map, and as “Also nearby” when one is closer to you than the official monitor.

We follow India's National Air Quality Index (NAQI). Each pollutant gets a score from its average over the last 24 hours (8 hours for ozone and carbon monoxide), counted only when at least two-thirds of those hours have readings. The AQI is the highest of these scores, and that pollutant is the one we name, as in “Mostly PM2.5”.

Above 500: India's scale officially stops at 500, and CPCB reports anything worse as 500. We keep counting on the same scale instead, so you can see how far past the limit the air is.

It needs at least three pollutants, one of them PM2.5 or PM10. A new monitor shows its latest hourly reading until a day of data has built up, and says so. The US AQI is shown next to it for comparison.

• Good0–50
• Satisfactory51–100
• Moderate101–200
• Poor201–300
• Very poor301–400
• Severe401+

Live readings aren't checked by CPCB before they're published, so we check them. We keep, but don't use, a reading that is:

• Negative, or impossibly high for that pollutant
• More than 3 hours old
• Stuck on the same value for 6 hours
• A sudden jump to over 4 times the previous hour

City number: the average of the city's official monitors, the same way CPCB's daily bulletin works out a city's AQI. Each city also lists its monitors, worst first.

Best time to go out: each monitor's usual daily pattern over the last two weeks, adjusted for how today compares.

GRAP stage: read from the Commission for Air Quality Management's orders and news every hour, never guessed from the AQI.

Many apps show the US AQI, which rates the same air higher than India's scale. Some use the latest hour instead of a 24-hour average, so they jump around more. Others mix in sensors or use satellite and model estimates. We show India's official method, from official monitors.