Background

The European Union has established the European Air Quality Index, accounting for five pollutants: PM_2.5, PM₁₀, O₃, NO₂, and SO₂. An hourly AQI value is reported, as well as forecasting for the following 24 hours.

The EU has adopted policies on air quality since the 1980s ¹. The European Air Quality Index was developed by the Commission’s Directorate General for the Environment and the European Environment Agency ². The European Commission has established European Standards regarding air quality index calculation, reporting, and documentation ¹.

The European Air Quality Index is calculated for over 3,500 stations across Europe and EEA member countries, reporting ‘up-to-date’ air quality data officially reported every hour. Modeled air quality data from the European Union’s Copernicus Atmosphere Monitoring Service (CAMS) is used to supplement measured data in index calculations when there is a lack of data values.

Note that the AQI map provided by the European Environment Agency depicts the air quality for 3 hours ago by default. The AQI for the past 48 hours and forecast for the next 24 hours can also be displayed.

The European Environment Agency (EEA) has updated the methodology used to calculate the European Air Quality Index (EAQI) to improve scientific consistency and public communication of air quality risks.

The updated index aligns with the 2021 World Health Organization (WHO) Air Quality Guidelines and elements of the revised EU Ambient Air Quality Directive (EU 2024/2881) ³. It introduces pollutant bands that are designed to represent comparable health risks across pollutants, making the index easier to interpret.

The EAQI continues to be reported at the monitoring station level, while maps using modelled concentrations may also be provided to show broader spatial patterns.

Color scale

The European Air Quality Index is comprised of six categories:

Note: Adapted from “European Air Quality Index” (n.d.), airindex.eea.europa.eu/AQI/index.html². Accessed 23 July 2024.

Methods

1980s-2024

NO₂, O₃, and SO₂ use hourly concentrations, while PM_2.5 and PM₁₀ use 24-hour running means for the previous 24 hours when calculating the European Air Quality Index. A 24-hour running mean is calculated if there is corresponding data for at least 18 hours out of the previous 24. There are no rounding conventions stated for concentration or sub-index values.

Using the breakpoint concentration table below, a sub-index is determined for each pollutant with provided data. The overall European Air Quality Index for a station corresponds to the poorest sub-index of that station.

Note: Adapted from European Air Quality Index” (n.d.), airindex.eea.europa.eu/AQI/index.html². Accessed 23 July 2024.

Note: We have assumed a convention of rounding to one significant figure to avoid overlapping limit values within the breakpoint concentration table. This was used to determine the lower limit as the map released by the European Union to provide public information uses pollutant concentrations to one significant figure ².

Air quality measurements that exceed the upper bound of the ‘extremely poor’ category are not taken into account as they are typically found to be erroneous ².

The index for traffic stations only accounts for NO₂ and PM (either PM_2.5 or PM₁₀ or both). SO₂ concentrations can skew the AQI as they can run especially high around these areas, while O₃ typically remains very low.

The index for industrial and background stations requires data to be either measured or modeled for at least three pollutants for calculation: O₃, NO₂, and PM (either PM_2.5 or PM₁₀ or both).

Updated 2024 Methodology

The AQI is calculated using hourly concentration data from five key pollutants: PM_2.5, PM₁₀, Nitrogen Dioxide (NO₂), Ozone (O₃), and Sulfur Dioxide (SO₂). All pollutant concentrations are expressed in micrograms per cubic meter (µg/m³).

Each pollutant’s hourly concentration is compared against predefined concentrations corresponding to the six qualitative air quality categories: Good, Fair, Moderate, Poor, Very Poor, and Extremely Poor.

Air quality categories are now based on equal relative risk of short-term mortality from air pollution exposure. The system uses PM_2.5 as the reference pollutant, with its concentration thresholds defined using WHO guideline levels and interim targets. Thresholds for other pollutants are then calculated so that each band corresponds to a similar level of health risk.

Another important change is that the index is now updated hourly for all pollutants, including particulate matter (PM), allowing the index to better reflect short-term changes in air quality.

Unlike breakpoint-based systems such as the U.S. Air Quality Index, the European Air Quality Index does not calculate a numeric sub-index using linear interpolation. Instead, measured pollutant concentrations are directly classified into predefined air quality categories.

The overall air quality level at a monitoring station is determined by the pollutant with the highest severity category at that time.

It is important to note that the index reflects short-term (hourly) air quality conditions only. It is intended as a public information tool and does not represent long-term regulatory compliance with EU air quality limit values.

For all monitoring stations, the index is calculated if there is at least one pollutant with adequate data. For stations that do not report adequate data or if data cannot be filled for the minimum pollutants, the data is depicted as semi-transparent circles on the map provided by the EEA. For stations that have no data, a gray dot is used ².

References

“Air Quality.” European Commission, environment.ec.europa.eu/topics/air/air-quality_en.

Baran, Paulina. “Guidelines for the demonstration and subtraction of exceedances attributable to natural sources.” European Commission, circabc.europa.eu/ui/group/cd69a4b9-1a68-4d6c-9c48-77c0399f225d/library/dfbc1f99-f763-4a14-aff8-a8c597a65103/details.

“European air quality forecast plots.” Copernicus Atmosphere Monitoring Service, atmosphere.copernicus.eu/european-air-quality-forecast-plots.

“European Air Quality Index.” European Environment Agency, airindex.eea.europa.eu/AQI/index.html.

“European Air Quality Index Calculation.” Copernicus Atmosphere Monitoring Service, ecmwf-projects.github.io/copernicus-training-cams/proc-aq-index.html.

“ETC HE Report 2024/17: EEA´S Revision of the European Air Quality Index Bands.” Eionet Portal, 2024, www.eionet.europa.eu/etcs/etc-he/products/etc-he-products/etc-he-reports/etc-he-report-2024-17-eeas-revision-of-the-european-air-quality-index-bands.