Air quality · Methodology
How our air quality numbers are made
Every number on our air-quality pages is one of two things, and the page always says which: an official measurement published by a government agency, or our modelled estimate. We never present a model value as a measurement.
Two scales
Indian cities use India’s National Air Quality Index (NAQI), the scale the Central Pollution Control Board (CPCB) publishes and Indian news reports quote. Every other city uses the US Environmental Protection Agency’s AQI, the most widely used scale worldwide. A page never mixes the two; the world rankings put Indian cities on the US scale so the comparison is like for like.
The scales disagree on the same air, mostly because the US scale is stricter on fine particles (PM2.5). Computed with our engine for PM2.5 alone, 24-hour average:
| PM2.5 (µg/m³) | India NAQI | US EPA AQI |
|---|---|---|
| 12 | 20 | 56 |
| 35 | 57.758620689655174 | 99 |
| 60 | 100 | 154 |
| 150 | 323.25581395348837 | 225 |
India’s PM2.5 bands: 0–30, 31–60, 61–90, 91–120, 121–250, 251–380 µg/m³ for Good to Severe. US bands (May 2024 revision): 0–9.0 Good, 9.1–35.4 Moderate, 35.5–55.4 Unhealthy for sensitive groups, 55.5–125.4 Unhealthy, 125.5–225.4 Very unhealthy, above that Hazardous.
- Good 0–50
- Satisfactory 51–100
- Moderate 101–200
- Poor 201–300
- Very poor 301–400
- Severe 401–500
- Good 0–50
- Moderate 51–100
- Unhealthy for sensitive groups 101–150
- Unhealthy 151–200
- Very unhealthy 201–300
- Hazardous 301–500
India: the official bulletin and our hourly estimate
Official. CPCB publishes a daily AQI bulletin at 4 PM for about 260 cities. It is the only figure we call “official”. We read it from CPCB’s PDF each evening and link the PDF on every city page.
Live estimate. CPCB’s hourly station feed stopped working in September 2026, so our hourly number between bulletins is modelled: the Copernicus Atmosphere Monitoring Service (CAMS) forecast for the city, averaged over CPCB’s own windows (24 hours; 8 hours for CO and ozone), scored with CPCB’s sub-index formula and validity rule (at least three pollutants, one of them PM2.5 or PM10), then scaled per city to match its recent bulletins. Raw CAMS reads about twice CPCB over India, a known dust bias.
How close it is. Measured against CPCB’s bulletin over 539 city-days to 26 September 2026, the corrected 4 PM estimate was within 10% of the official figure about 28% of the time, within 25% about 72% and within 50% about 92%. That is a model’s accuracy, not a monitor’s; when the two differ, trust the bulletin.
See India’s cities.
The rest of the world
We cover 570 cities in 160 countries: each country’s capital and largest city, and its cities of 600,000 people or more (up to 25 per country), from GeoNames. Places closer than 20 km to a bigger covered city are left out, because the model would give them the same number.
For each city we take the CAMS forecast at its coordinates (the European domain at about 11 km over Europe, the global one at about 45 km elsewhere) and score every hour the way the US EPA’s AirNow scores monitors:
- PM2.5 and PM10: EPA’s NowCast, a weighted average of the last 12 hours that reacts quickly when air gets worse or better.
- Ozone and carbon monoxide: the 8-hour average.
- Nitrogen dioxide and sulphur dioxide: the hour itself, on EPA’s 1-hour breakpoints.
- The AQI is the highest sub-index; PM2.5 or PM10 must be present.
CAMS issues two forecast runs a day. We fetch every six hours and keep 24 hours of the past and 48 of the forecast, so each page shows the current hour and the outlook. Outside India there is no official figure to correct against, so world values are uncorrected model output.
What a model cannot tell you
- A grid cell is a city-wide average. Next to a busy road, a factory or a construction site, air can be much worse.
- Wildfire smoke, dust storms and festival fireworks can come in stronger or weaker than forecast.
- Other apps may show different numbers: they use monitoring stations, low-cost sensors, another scale or another averaging window.
Sources and licences
- CPCB daily AQI bulletin, Central Pollution Control Board, Government Open Data License – India.
- Copernicus Atmosphere Monitoring Service forecasts, via the Open-Meteo API, CC BY 4.0. Generated using Copernicus Atmosphere Monitoring Service information; neither the European Commission nor ECMWF is responsible for any use made of it.
- City names, coordinates and populations: GeoNames, CC BY 4.0.
- Health statements: CPCB (India) and the US EPA’s AirNow Air Quality Guide.
Questions or a correction: support@risescribe.com.