The Air Quality Index (AQI) is an indicator which is used to inform people about air quality in a clear and simple way.

In order to obtain the Air Quality Index (AQI) in the Basque Country, there is a surveillance and monitoring network which measures a series of criteria in real time, such as the pollutants SO2, NOx, CO, PM10 and O3, at stations spread out across different areas. The set of values that can be taken by the AQI is grouped into intervals which are linked to a descriptor or colour representing the quality of the air in a particular area.

It is divided into five categories, which define the state of the air quality as: Very Good, Good, Average, Bad and Very Bad.

The criteria used to calculate the index are as follows:

• An AQI is established per pollutant and per station. The latter is defined by the worst AQI for all of the pollutants.
• The hourly and daily AQI is provided per pollutant and per station.
• In order to establish the concentration ranges, the values set out in Annex I of Royal Decree 102/2011 must be taken into account.