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O3 is formed in the atmosphere when heat and light cause chemical reactions between nitrogen oxides (NOX) and volatile organic compounds (VOCs), including methane, which is also a powerful greenhouse gas (EEA, 2025d). Emissions of these gases come from anthropogenic sources; VOCs can also come from biogenic sources. O3 and its precursor pollutants are also carried to Europe from other parts of the northern hemisphere and from the stratosphere into the lower troposphere.
Meteorology plays an important role in air pollution and interannual variations in pollutant concentrations. This effect is especially significant for O3. The complexity of O3 formation is discussed further in a recently published EEA briefing on addressing O3 pollution in Europe (EEA, 2026a).
The EU and WHO have set air quality standards and AQG levels for O3 for both short- and long-term concentrations. They are given in Table 3. The EU target value is defined as a three-year average, but here the situation for individual years is also presented for what is called the target value threshold (a maximum daily eight-hour mean of 120 micrograms per cubic metre (µg/m3) not to be exceeded on more than 25 days (current) or 18 days (2030) over only one year).
Table 3. EU air quality standards and WHO AQG levels for O3
Current EU standard | 2030 EU standard | WHO 2021 AQG level | ||||
|---|---|---|---|---|---|---|
Averaging period | Standard | Not to be exceeded more than | Standard | Not to be exceeded more than | AQG level | Comments |
Maximum daily 8-hour mean | Target value: 120μg/m3 | 25 days /year averaged over 3 years | Target value: 120μg/m3 | 18 days /year averaged over 3 years | 100μg/m3 | 99th percentile (3-4 days/year) |
Long-term objective: 120μg/m3 | N.A. | Long-term objective: 100μg/m3 | 3 days/year (99th percentile) | 100μg/m3 | 99th percentile (3-4 days/year) | |
Peak season | 60μg/m3 |
Notes: N.A.: not applicable. The annual peak season value is an average of the daily maximum eight-hour mean O3 concentration in the six consecutive months with the highest six-month running average for O3 concentration.
The EEA has received valid 2024 O3 data from 37 of the reporting countries (all, excluding Albania and Liechtenstein). Preliminary data for 2025 have been received from all the countries except Albania, Kosovo, Liechtenstein, Serbia and Türkiye.
Target values
In 2024, 15.6% of the reporting stations registered concentrations above the current target value (TV) threshold. They were located in 17 Member States (Austria, Bulgaria, Croatia, Cyprus, Czechia, France, Germany, Greece, Hungary, Italy, Poland, Portugal, Romania, Slovakia, Slovenia, Spain and Sweden) and six other reporting countries (Bosnia and Herzegovina, Kosovo, North Macedonia, Serbia, Switzerland and Türkiye) as shown in Map 6.
Levels above the TV (averaged over the 2022-2024 period) were registered in 16.9% of the reporting stations, located in 16 Member States (Austria, Croatia, Cyprus, Czechia, France, Germany, Greece, Hungary, Italy, Malta, Poland, Portugal, Romania, Slovakia, Slovenia and Spain) and five other reporting countries (Bosnia and Herzegovina, North Macedonia, Serbia, Switzerland and Türkiye) as shown in Map 7 and Figure 6.
In addition to data collected from monitoring stations, countries can also use modelling applications to assess compliance with the standards. Based on assessment models, Italy and Poland reported current O3 TV exceedances for the protection of health for two and three air quality zones, respectively.
In 2025, 22.4% of the reporting stations registered concentrations above the TV threshold. They were located in 18 Member States (Austria, Belgium, Bulgaria, Croatia, Cyprus, Czechia, France, Germany, Greece, Hungary, Italy, Malta, Poland, Portugal, Romania, Slovakia, Slovenia and Spain) and two other reporting countries (Bosnia and Herzegovina and Switzerland) (Map 6).
Map 6. Concentrations of O3 in 2024 and 2025 in relation to the current EU target value threshold
Map 7. O3 concentrations in 2024 by country in relation to the current EU target value
Figure 6. O3 concentrations in 2024 by country in relation to the current EU target value
Overall, 78.2% of the reporting stations registered concentrations below the 2030 target value threshold in 2024. All the reporting stations in ten Member States (Belgium, Denmark, Estonia, Finland, Ireland, Latvia, Lithuania, Luxembourg, Malta and the Netherlands) and those in Andorra, Iceland, Montenegro and Norway had concentrations below the 2030 target value threshold (Map 8).
Map 8. Concentrations of O3 in 2024 in relation to the 2030 EU target value threshold
Long-term objectives
In 2024, 77.4% of the reporting stations registered concentrations above the current long-term objective (LTO). They were located across all the EU-27 Member States (except Ireland) and all the other reporting countries (except Iceland and Montenegro). Based on assessment models, Italy and Poland reported O3 LTO exceedances for the protection of health in 8 and 44 air quality zones, respectively.
In 2025, 85.4% of the reporting stations registered concentrations above the LTO. The stations were located in all the reporting countries except Iceland and Kosovo.
Figure 7 provides a summary of the O3 stations above EU air quality standards and WHO AQG levels.
Figure 7. Percentage of reporting monitoring stations registering O3 concentrations above the EU standards and the WHO AQG levels
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