Indicator Assessment

Exposure of Europe's ecosystems to ozone

Indicator Assessment
Prod-ID: IND-30-en
  Also known as: CSI 005 , AIR 004
Last modified 31 May 2021
21 min read

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Ground-level ozone adversely affects not only human health but also vegetation and ecosystems across Europe, leading to decreased crop yields and forest growth, and loss of biodiversity. Much of Europe’s lands is exposed to ozone levels above the threshold and long-term objective values set in the EU’s Ambient Air Quality Directive (AAQD) for the protection of vegetation. For instance, after a 6-year period (2009-2014) of relatively low ozone values, the fraction of arable land exposed to levels above the AAQD threshold increased to 30% in 2015, falling to 19% in 2016, before increasing again to 26% in 2017 and 45% in 2018 and decreased only to 37% in 2019.


Exposure of agricultural area to ozone in EEA member countries and the United Kingdom

Data sources:
Data sources:

The pollution of air with ground-level ozone is a serious cause for concern in Europe, not only because of its harmful effects on human health but also because of its damaging effects on vegetation, leading to reduced crop yields and forest growth, and loss of biodiversity. The EU Ambient Air Quality Directive (EU, 2008) aims to protect vegetation from ozone and sets an accumulated ozone exposure threshold (AOT40) value, applicable from 2010, based on the sum of hourly ozone values that exceed 80 μg/m3 (40 ppb), of 18,000 μg/m3.hour.

The fraction of agricultural land in the EEA member countries exposed to ozone levels above the threshold is substantial and exceedances have been observed in central, southern and eastern Europe. Considerable variation from year to year makes trends difficult to identify but, if extremes in 2003 and 2006 are disregarded, the data do indicate a general decreasing trend between 2001 and 2017. The peak in 2003 can be explained by meteorological conditions favourable for ozone formation, resulting in exceptionally high concentrations; in June and July 2006, a large number of ozone episodes (EEA, 2007) resulted in the relatively high ozone values in this year.

In 2019, the fraction of land exposed to levels of ozone exceeding the threshold decreased by about 8%, compared with 2018, to 37%, amounting to a total area of 870 million km2 being exposed to levels above the air quality threshold. Ozone exposure values in 2019 were lower in northern countries and in central Europe than they were in 2018.

The Ambient Air Quality Directive also sets a long-term objective for the protection of vegetation from ground-level ozone: to reduce the exposure of vegetation to 6,000 μg/m3.hour or less. This long-term objective is in line with the critical level of ozone for the protection of crops defined by the United Nations Economic Commission for Europe (UNECE) Convention on Long-range Transboundary Air Pollution (CLRTAP) (UNECE, 1979). In 2019, this long-term objective was met for only about 14% of the total agricultural area of the EEA countries, increasing from 4% in 2018.

Exposure of forest area to ozone in EEA member countries and the United Kingdom

Data sources:
Data sources:

The UNECE CLRTAP (1979) defines a critical ozone exposure level for the protection of forests, of 10 mg/m3.hour. Between 2004 and 2019, large variations in the exposure of forested areas to ozone were observed. In 2004 and 2006, almost all forests were exposed to levels exceeding the critical level, whereas, in 2007, more than 40% of forests were exposed to levels lower than the critical level. The percentage of forests exposed to ozone below the critical level had been more or less stable since 2007 (with the exception of a decrease in 2008, 2018 and 2019), but, in 2018, decreased to the lowest value (13%) since 2007. In fact, in 2018, the proportion of forested areas exposed to levels in excess of 30 mg/m3.hour increased to 40% but decreased to 31% in 2019.

As for crop areas, the proportions of forested areas exposed to ozone above the critical threshold value were relatively low in northern Europe and highest in the countries around the Mediterranean. In 2019, the critical level was not exceeded in most of in Scandinavia, the United Kingdom, Ireland and Iceland, whereas, in southern Europe, North Macedonia, Greece and Turkey, levels above 50 mg/m3.hour were observed.


Supporting information

Indicator definition

This indicator shows the negative impact of ground-level ozone on ecosystems and vegetation in Europe. In particular, it shows exposure of areas covered with vegetation (crops and forests) to ground-level ozone.

Ground level ozone is one of the most prominent air pollution problems in Europe, mainly due to its effects on human health, crops and natural ecosystems. When absorbed by plants, it damages plant cells, impairing their ability to grow and reproduce, and leading to reduced agricultural crop yields, decreased forest growth and reduced biodiversity.

The risk is estimated by reference to either a target value and a long-term objective, or to the 'critical level' for ozone for each location. The target value and the long-term objective are levels fixed with the aim of avoiding, preventing or reducing harmful effects on the environment. For ozone, the critical level is a concentration in the atmosphere, above which direct adverse effects on receptors, such as human beings, plants, ecosystems or materials, may occur according to present knowledge (ICP on Modelling and Mapping, 2015; UNECE, 2004).


Ozone concentrations: micrograms of ozone per cubic meter (µg/m³) or parts per billion (ppb). Note: 1 ppb ~ 2 µg/m³. 

    • AOT40 (Accumulated ozone exposure over a threshold of 40 parts per billion): the sum of the differences between hourly concentrations greater than 80 µg/m3 (= 40 parts per billion) and 80 µg/m3 accumulated over all hourly values measured between 08:00 and 20:00 Central European Time. For crops, the accumulation period is defined as 1 May to 31 July (growth period until harvest). For forest, the accumulation is defined as 1 April to 30 September (vegetation/growth period). AOT40 is expressed in (μg/m3)·hours.
    • AOT40 estimate: in cases where, due to missing values, all possible measured data are not available, the AOT40 values are calculated according to the following formula:

AOT40estimate = AOT40measured x [(total possible number of hours)/(number of measured hourly values)]

Where 'total possible number of hours' is the number of hours within the time period of the AOT40 definition, (i.e. 08:00 to 20:00 CET from 1 May to 31 July each year, for vegetation protection and from 1 April to 30 September each year for forest protection)

    • For the risk of ozone damage due to ozone exposure, a percentage (%) of the total agricultural/forest area in each country in excess of the reference level is calculated.


Policy context and targets

Context description

This indicator provides relevant information for the EU's Seventh Environmental Action Programme (7th EAP) and the Clean Air Programme for Europe proposed by the European Commission at the end of 2013. The long-term strategic objective and core of the new air package is to attain 'air quality levels that do not give rise to significant negative impacts on, or risks for, human health and the environment'. The 7th EAP kept the intermediate objectives already set in the 6th EAP and the 2005 Thematic Strategy on Air Pollution to further reduce air pollution and its impacts on ecosystems and biodiversity by 2020. This will be accomplished by achieving full compliance with existing legislation. Furthermore, the long-term objective to not exceed critical levels remains in place.

Internationally, a first step to address air-pollution related impacts on health and the environment was the 1979 United Nations Economic Commission for Europe (UNECE) Geneva Convention on Long-range Transboundary Air Pollution (LRTAP Convention).

A centrepiece of the convention is the 1999 Gothenburg Protocol to Abate Acidification, Eutrophication and Ground-level Ozone, subsequently amended in 2012. Critical ozone levels for vegetation were also defined under the LRTAP Convention.

The Air Quality Directive (EU, 2008) sets both a target value (to be met in 2010) and a long-term objective for ozone for the protection of vegetation. The long-term objective is largely consistent with the long-term critical level of ozone for crops (UNECE, 2004), as defined in the UNECE LRTAP Convention.


  • UNECE CLRTAP Gothenburg Protocol (1999; amended in 2012)

Using a stepwise approach and taking into account advances in scientific knowledge, the long-term target under the amended protocol is that atmospheric depositions or concentrations do not exceed for parties within the geographical scope of EMEP, the critical levels of ozone, as given in Annex I.

Annex I of the amended protocol includes a short definition of critical levels for ozone.

Critical levels for the protection of crops and forests (AOT40f) have also been defined under the LRTAP Convention (UNECE, 2004). The critical level for crops is consistent with the EU long-term objective for vegetation. The critical level for forests relates to the accumulated sum during the growing season (considered as April to September) and is set at 10,000 μg/m3·h.

  • Air Quality Directive (2008/50/EC)

For the protection of vegetation from ozone exposure, the Air Quality Directive (EU, 2008) defines:

a) the target value for the protection of vegetation as AOT40-value (calculated from hourly values from May to July, considering the growing season) of 18,000 (μg/m3)·h, averaged over 5 years. This target value should be met in 2010 (2010 being the first year from which data will be used in the calculation over the following 5 years).

b) a long-term objective as AOT40-value (calculated from hourly values from May to July) of 6,000 (μg/m3)·h, with no defined date of attainment.

In the assessment part of the indicator, the target value threshold is also considered. This is the target value considered only for 1 year and not for the averaged period of 5 years.

  • The Clean Air Programme for Europe

New air policy objectives for 2030 are specified in the Clean Air Programme for Europe proposed by the European Commission in 2013, in line with the long term objective of reaching no exceedance of the critical levels.

Related policy documents

  • 7th Environment Action Programme
    DECISION No 1386/2013/EU OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 20 November 2013 on a General Union Environment Action Programme to 2020 ‘Living well, within the limits of our planet’. In November 2013, the European Parliament and the European Council adopted the 7 th EU Environment Action Programme to 2020 ‘Living well, within the limits of our planet’. This programme is intended to help guide EU action on the environment and climate change up to and beyond 2020 based on the following vision: ‘In 2050, we live well, within the planet’s ecological limits. Our prosperity and healthy environment stem from an innovative, circular economy where nothing is wasted and where natural resources are managed sustainably, and biodiversity is protected, valued and restored in ways that enhance our society’s resilience. Our low-carbon growth has long been decoupled from resource use, setting the pace for a safe and sustainable global society.’
  • 1999 Protocol to Abate Acidification, Eutrophication and Ground-level Ozone
    Convention on Long-range Transboundary Air Pollution 1999 Protocol to Abate Acidification, Eutrophication and Ground-level Ozone, amended on 4 May 2012.
  • A Clean Air Programme for Europe
    Communication from the Commission to the Council, the European Parliament, the European Economic and Social Committee and the Committee of the Regions - "A Clean Air Programme for Europe", COM(2013) 918 final
  • Directive 2008/50/EC, air quality
    Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on ambient air quality and cleaner air for Europe.
  • Thematic Strategy on Air Pollution
    Communication from the Commission to the Council and the European Parliament - Thematic Strategy on air pollution (COM(2005) 0446 final)
  • UNECE Convention on Long-range Transboundary Air Pollution
    UNECE Convention on Long-range Transboundary Air Pollution.


Methodology for indicator calculation

AOT40 estimated values are calculated from hourly data (EU, 2008) at all rural background stations available in the Air Quality e-reporting database (former AirBase). Only data series with more than 75 % valid data are considered.

The AOT40 maps have been created by combining measurement data from the rural background stations combined with the results of the EMEP dispersion model (EMEP, 2014), altitude field and surface solar radiation in a linear regression model, followed by the interpolation of its residuals by ordinary Kriging (Horalek et al, 2013). For altitude, dataset GTOPO30 (Global Digital Elevation Model) at a resolution of 30 x 30 arcseconds has been used. The solar radiation has been obtained from ECMWF's Meteorological Archival and Retrieval System (MARS). Kriging is a method of spatial statistics (N. Cressie, 1993) that makes use of spatial autocorrelation (the statistical relationship between the monitoring points expressed in the form of variograms). Kriging weights the surrounding measured values to derive an interpolation for each location. The weights are based (i) on the distance between the measured points and the interpolated point, and (ii) on the overall spatial arrangement among the measured points. The type of Kriging at its parameters (in particular the parameters describing the semivariogram) is chosen in order to minimise the RMS error.

The AOT40 maps have been overlayed in a geographical information system with the land cover CLC2006 map. The resolution was 500 x 500 m2 to generate maps for the agricultural area at risk due to ozone exposure. Exposure of the agricultural area (defined as the land cover level-1 class 2 Agricultural areas encompassing the level-2 classes 2.1 Arable land, 2.2 Permanent crops, 2.3 Pastures and 2.4 Heterogeneous agricultural areas) and forest areas (defined as the land cover level-2 class 3.1. Forests) have been calculated at the country-level.

The temporal trends have been estimated using a Mann-Kendal statistical test. This test is particularly useful since missing values are allowed and the data need not conform to any particular distribution. Moreover, as only the relative magnitudes of the data rather than their actual measured values are used, this test is less sensitive to incomplete data capture and/or special meteorological conditions leading to extreme values (Gilbert, 1987).

AOT40 is used to be in line with the Air Quality Directives. However, in considering the latest scientific knowledge concerning vegetation ozone exposure, it should be noted that, at present, ozone impacts on vegetation are better modelled by fluxes of ozone into stomatal openings of vegetation (Mills et al., 2011a and 2011b).

Methodology for gap filling

In the AOT40-mapping, Turkey has to be excluded due to the lack of reported measurements at rural background stations. In the exposure estimates, the number of countries has been growing since 2004. Figures show the situation in individual years and not a trend.

Methodology references

  • Cressie, N., 1993 Statistics for spatial data, Wiley, New York.
  • ETC/ATNI, 2020, forthcoming, European air quality maps for 2018 European air quality maps for 2018 — PM 10 , PM 2.5 , ozone, NO 2 and NO x spatial estimates and their uncertainties, Eionet Report ETC/ATNI 2020/10, European Topic Centre on Air pollution, transport, noise and industrial pollution
  • Gilbert, R. O., 1987 Statistical Methods for Environmental Pollution Monitoring, Van Nostrand Reinhold, New York.
  • Horálek, J. et al, 2013, European air quality maps of PM and ozone for 2011 and their uncertainty, ETC/ACC Technical Paper 2013/13.
  • ICP on Modelling and Mapping, 2015, ICP on Modelling and Mapping critical loads and levels approach.
  • Mills, G., et al, 2011a Evidence of widespread effects of ozone on crops and (semi-)natural vegetation in Europe (1990–2006) in relation to AOT40-and flux-based risk maps, Global Change Biol., 17, 592–613, doi:10.1111/j.1365-2486.2010.02217.x, 2011a.
  • Mills, G., et al, 2011b, New stomatal flux-based critical levels for ozone effects on vegetation, Atmos. Environ., 45, 5064–5068, doi:10.1016/j.atmosenv.2011.06.009, 2011b.
  • UNECE, 2004 Manual on Methodologies and Criteria for Modelling and Mapping Critical Loads and Levels and Air Pollution Effects, Risks and Trends Mapping Manual, UNECE Convention on Long-range Transboundary Air Pollution, ICP Modelling and Mapping. 


Methodology uncertainty

This indicator provides information on the area for which monitoring information is available. In previous years, yearly changes in monitoring density influenced the total monitored area. By using interpolated maps, this problem is largely solved; maps are less sensitive for changes in the central part of the network (though more sensitive for changes in the number of stations in the outskirts).

The indicator is also subject to year-to-year fluctuations as it is mainly sensitive to episodic conditions, and these depend on particular meteorological situations, the occurrence of which varies from year to year. When averaging over Europe, this meteorologically induced variation may be less, provided spatial data coverage is sufficient. Methodology uncertainty is also given by uncertainty in mapping AOT40 based on the interpolation of point measurements at background stations. The mean interpolation uncertainty of the map of AOT40 for crops is estimated to be about 35 %.

Data sets uncertainty

Most data have been officially submitted to the Commission under the Exchange of Information Decision (EU, 1997), the Implementing Decision on exchange of information and reporting (EU, 2011) and/or to EMEP under the LRTAP Convention. Air quality monitoring station characteristics and representativeness may not be well documented, which may imply that stations that are not representative for background conditions have been included, probably leading to a slight underestimation of the indicator. Coverage of territory and time may be incomplete. 

Rationale uncertainty

This indicator is rather sensitive to the precision at the reference level (40 ppb or 80 micrograms per m3)

Data sources

Other info

DPSIR: State
Typology: Performance indicator (Type B - Does it matter?)
Indicator codes
  • CSI 005
  • AIR 004
Frequency of updates
Updates are scheduled once per year
EEA Contact Info


Geographic coverage

Temporal coverage





Filed under:
Filed under: ozone, aot40, csi, air pollution


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