How we measure air quality
The scale we use
Every AQI on this site is on India’s National Air Quality Index, published by the Central Pollution Control Board (CPCB). It runs from 0 to 500 in six categories, from Good to Severe. We never mix in the US EPA AQI, which uses different cut-offs and often shows a higher number for the same air.
The CPCB AQI scale
India uses the CPCB National AQI. Apps using the US scale show different numbers.
Minimal impact
Minor breathing discomfort to sensitive people
Breathing discomfort to people with lung, asthma and heart diseases
Breathing discomfort to most people on prolonged exposure
Respiratory illness on prolonged exposure
Affects healthy people and seriously impacts those with existing diseases
Where readings come from
Our main source is CPCB’s real-time air quality feed on data.gov.in. It covers monitoring stations run by CPCB, state pollution control boards and bodies such as IITM and IMD. We fetch it every 15 minutes and keep an hourly history.
When a station is missing from that feed or its reading is late, we use readings from the same official monitors collected by OpenAQ (openaq.org). Those stations are marked OpenAQ on their pages. OpenAQ publishes pollutant concentrations, so we convert them to the CPCB scale ourselves, using CPCB’s breakpoints and averaging periods.
History before we started collecting (from October 2025) comes from OpenAQ’s public archive of the same monitors, converted the same way. The 30-day and month-by-month figures use one value a day per station: its 4 pm reading, which is the 24-hour average CPCB’s daily AQI bulletin uses. A city’s daily figure is the average of its stations.
How a station’s AQI is worked out
Each pollutant gets its own score on the 0–500 scale, called a sub-index. PM2.5, PM10, nitrogen dioxide, sulphur dioxide and ammonia use 24-hour averages; carbon monoxide and ozone use 8-hour averages. The station’s AQI is the highest sub-index, and that pollutant is shown as the main pollutant.
Following CPCB’s rule, we show an AQI only when at least three pollutants are measured and one of them is PM2.5 or PM10.
City and area pages
A city’s AQI is the average of its stations that reported in the last 3 hours, so one busy junction doesn’t stand for the whole city. Delhi NCR combines Delhi, Noida, Greater Noida, Gurugram, Ghaziabad and Faridabad.
Area pages, such as Andheri or Connaught Place, show the nearest monitoring station and its distance. When that station is more than 10 km away, the page says so.
When a station stops reporting
Stations sometimes go offline for maintenance or because of gaps in the feed. If a station hasn’t reported for 3 hours, its page shows the last reading greyed out with its time, and it is left out of rankings and city averages. After 24 hours without a reading we stop showing a number for it. We never present an old reading as the air right now.
Limits of station data
A monitoring station measures the air at one spot. Conditions on your street can differ, especially near traffic, construction or burning. Real-time readings are provisional, and CPCB may correct them later.
Use AQI247 as a guide, and follow official advisories and your doctor’s advice.
GRAP in Delhi NCR
The GRAP stage shown for Delhi NCR is updated by our team from official orders of the Commission for Air Quality Management (CAQM). Our lists of what each stage restricts are plain-language summaries; the CAQM order is always the authority.
Credit and reuse
Data: CPCB via data.gov.in, under the Government Open Data License – India; OpenAQ where marked. If you reuse numbers from AQI247, please credit “AQI247, CPCB data”. To report an error, write to contact@social-impact.in with the station name and time.