European Air Quality Index methodology
Learn how hourly concentrations become European AQI categories, which pollutants count and how gaps affect the result.
How the hourly index works
The European Environment Agency groups each hourly pollutant concentration into one of six bands, from Good to Extremely poor. The overall category comes from the pollutant with the poorest band for that hour. The five index pollutants are PM2.5, PM10, nitrogen dioxide, ground-level ozone and sulphur dioxide. Carbon monoxide is shown separately where available.
The same mass concentration can fall into different bands for different pollutants. The table gives the upper limit of each band, in micrograms per cubic metre (µg/m³). Values above the final limit are Extremely poor.
| Pollutant | Good | Fair | Moderate | Poor | Very poor | Extremely poor |
|---|---|---|---|---|---|---|
| PM2.5 | 5 | 15 | 50 | 90 | 140 | >140 |
| PM10 | 15 | 45 | 120 | 195 | 270 | >270 |
| NO₂ | 10 | 25 | 60 | 100 | 150 | >150 |
| O₃ | 60 | 100 | 120 | 160 | 180 | >180 |
| SO₂ | 20 | 40 | 125 | 190 | 275 | >275 |
What makes a station index complete?
A traffic station needs nitrogen dioxide and at least one size of particulate matter. A background or industrial station also needs ozone. A station can meet these minimums while still missing some of the five index pollutants. Check the pollutant count beside the category before comparing two stations.
Where a station lacks sufficient coverage, individual pollutant values can still be useful. A missing result may reflect reporting delay, instrument maintenance or a pollutant that the station does not monitor. A poor pollutant band in an incomplete set is evidence of that pollutant at that time, even when the overall category is unavailable.
PiSpace Air presents measurements separately from modelled coverage. The EEA’s own index map can use downscaled model values to fill gaps, so its station category can differ from the category calculated from the available measurements here.
Comparing an hour with a longer period
Read the category alongside its location, time and data type. A roadside observation represents the station surroundings. A model estimate represents a grid cell. The European scale provides a common short-term reference when applied outside Europe, where an official local scale can use different pollutants or averaging periods.
An annual mean and an hourly category answer different questions. For long-term assessment or compliance, use the relevant authority’s statistics, averaging periods and completeness rules.
Worked example: one hour at a background station
Suppose an urban background station reports 12 µg/m³ of PM2.5, 18 µg/m³ of nitrogen dioxide and 115 µg/m³ of ozone for one hour. PM2.5 and nitrogen dioxide both fall in the Fair band. Ozone falls in the Moderate band, so it determines the overall category.
These three readings meet the minimum requirements for a background station. Coverage is still three of five because PM10 and sulphur dioxide are missing. If ozone is also missing, PiSpace Air shows the two reported concentrations and marks the overall index as unavailable. These values are illustrative.