What is AQI?
AQI is an index that converts a pollutant concentration into a number from 0 to 500 and a matching category. The number and colour provide a quick overview, while the concentration shows what was actually measured.
On AirQuality.city, the overall AQI is based on particulate matter, PM2.5 and PM10.
How the website determines the overall AQI
A separate AQI is calculated for PM2.5 and PM10. The higher value becomes the station's overall result.
| Indicator | Example |
|---|---|
| PM2.5 AQI | 82 |
| PM10 AQI | 117 |
| Overall AQI | 117 |
In this example, PM10 determines the overall AQI because its value is higher.
NO₂, O₃, SO₂, and CO may be shown for official stations that measure them, but they are not currently included in the website's overall AQI.
AQI is not the same as concentration
PM2.5 and PM10 are normally displayed in micrograms per cubic metre, µg/m³. This is the measured concentration.
AQI is calculated from that concentration using a defined scale. It is useful to check both:
- concentration tells you how much particulate matter was measured
- AQI tells you which category that value falls into
How to read the result
Do not rely only on the colour or one isolated number. Check:
- the overall AQI
- whether PM2.5 or PM10 is dominant
- the time of the latest measurement
- the trend over the previous few hours
- the station location
A brief spike at one location may not describe conditions across an entire city. A rise lasting several hours, or similar readings at several nearby stations, is a stronger signal.
Why another website may show a different result
Air-quality websites do not necessarily use the same method. They may differ in:
- averaging period
- update time
- pollutants included
- monitoring station used
- data processing and later validation
AirQuality.city uses a scale and concentration breakpoints based on the U.S. EPA methodology for PM2.5 and PM10. Its display is near-current, so it is not the same as an official daily AQI.
You can find the category ranges on the AQI scale page and a technical overview on the air-quality methodology page.