CAQI - Common Air Quality Index


Not everybody is an expert in the field of air quality and knows all there is to know about ozone, nitric oxides and particulate matter. So the Common Air Quality Index (CAQI), covering the most relevant air pollutants, was defined by the Citeair project ( and To present the air quality situation in European cities in a comparable and understandable way, all detailed measurements are transformed into a single relative figure: the Common Air Quality Index (or CAQI).


What does the CAQI consider?

  • The CAQI converts the concentrations of pollutants into one air quality index. The higher the CAQI, the worse the experienced air quality.
  • The CAQI considers relevant atmospheric pollutants that cause unfavorable conditions (ozone (O3), nitrogen dioxide (NO2), carbon monoxide (CO), sulphur dioxide (SO2), particulate matter (PM10), fine particles (PM2.5)).
  • The CAQI is determined by the pollutant with the highest concentration. Bad air quality is indicated by high CAQI values.
  • Elevated CAQI values are due to enhanced concentrations of at least one pollutant.


How is the CAQI computed exactly?

The Common Air Quality Index (CAQI) is computed applying observed or forecasted values for the relevant trace gases ozone (O3), nitrogen dioxide (NO2), particulate matter (PM10) and fine particles (PM2.5) at a certain location.

The concentration of each constituent determines the index class as shown in Table 1. At first, the CAQI is computed for every single chemical. The aggregated CAQI value is determined by the worst (most enhanced) of the single chemical CAQI values.



CAQI - A European Development

The CAQI was developed by the Citeair project, which was co-funded by the INTERREG IIIC and INTERREG IVC programs. For further information you may consult and


Welcome to obsAIRve!

Take a deep breath wherever you go! obsAIRve provides you with real-time information on air quality. You can find out about the air quality in your city via a web service or on the go using the smartphone app.

The obsAIRve service is part of the GMES (Global Monitoring for Environment and Security) program and funded by the European Commission. All air quality data disseminated by obsAIRve is derived mainly from participating cities as well as from the relevant GMES projects.

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