Ground-level ozone

Click in the image to access the four-day ground-level ozone forecast by the Copernicus Atmosphere Monitoring Service

Health issues

Ground-level ozone affects human health by impairing respiratory and cardiovascular function, which leads to more hospital admissions, school and work absences, medication use, and even premature mortality. Short-term exposure to ozone is associated with respiratory symptoms, reduced lung function and airway inflammation; long-term exposure with aggravated asthma and an increased incidence of strokes. Contrary to the detrimental impacts of tropospheric or ground-level ozone – the ozone we breathe – stratospheric ozone is beneficial for human health by blocking UV radiation.

Observed effects

Ground-level ozone formation and its meteorological sensitivity

Surface ozone (O₃) is a secondary pollutant produced in the atmosphere in the presence of sunlight and chemical precursors. The main precursors of ozone are nitrogen oxides (NOx) and volatile organic compounds (VOC), which originate primarily from transportation and industrial activities that are largely associated with urban areas. Carbon monoxide (CO) and methane (CH₄) emitted by residential and agricultural sources tend to play a minor role in ozone formation. Ozone precursors can also have a natural origin, such as biogenic emissions of VOC, soil emissions of NOx, wildfire emissions of CO and biosphere emissions of methane (Cooper et al., 2014; Monks et al., 2015).

Maximum ozone concentrations generally occur dozens of kilometres away from the urban areas where the main sources of ozone precursors are, in contrast to other air pollutants (such as particulate matter and nitrogen dioxide) that largely concentrate in cities. Because the photochemical formation of ozone takes several hours, winds can transport the pollution plume before ozone is formed. In addition, certain NOx species degrade ozone in specific conditions (i.e., close to the emission sources, at night or in winter), which results in generally lower ozone concentrations over city centers where NOx is emitted. Once formed, ozone can be maintained in the atmosphere for days to weeks, often undergoing long-range or transboundary transport. Nevertheless, also in urban - and particularly suburban - areas, high ozone levels can be observed.

Because ozone generation requires solar radiation, ozone concentrations typically reach a daily maximum a few hours after midday. Concentrations also follow a pronounced seasonal cycle that in Europe peaks between early spring and late summer. The dependence on sunlight makes ozone very sensitive to meteorological and climatic variability. The fluctuation of ozone from one year to another depends largely on how warm and dry the summer is; intense heatwaves can lead to peak ozone values. The relationship with sunlight means that southern Europe tends to have higher ozone concentrations than northern Europe (EEA, 2022a).

Concentrations and population exposure

The annual ozone concentrations were found to have increased slightly in Europe between 2005 to 2019, while the highest ozone peaks had declined (Solberg et al., 2022). In 2020, only 19% of all ground-level ozone monitoring stations across Europe achieved the long-term objective set in the 2008 Ambient Air Quality Directive that the maximum daily eight-hour mean may not exceed 120 micrograms per cubic meter (µg/m³) within a calendar year. Across Europe, 21 countries, including 15 EU Member States, registered ozone concentrations in excess of the EU target value for the protection of human health (the maximum daily eight-hour mean of 120 µg/m³) (EEA, 2022a). The proportion of the population exposed to surface ozone above EU target levels has fluctuated between a 64% peak in 2003 to 9% in 2014 (EEA, 2022b). The proportion of the population exposed to concentrations above the 2021 WHO short-term guideline value (the maximum daily eight-hour mean of 100 µg/m³) fluctuated between 93% and 98% in the period 2013-2020, with no decreasing trend over time.

Health impacts

High-levels of ozone cause breathing problems, trigger asthma, reduce lung function and cause lung disease (WHO, 2008). In 2019, 12 253 people across 23 European countries were hospitalized with respiratory diseases caused or exacerbated by acute exposure to ozone. The burden of mortality and morbidity caused by exposure to ozone levels is typically lower in the Northern European countries compared to the rest of Europe (EEA, 2022a). In 2020, an estimated 24 000 people across the 27 EU Member States died prematurely due to acute exposure to ozone above 70 µg/m³. The countries with the highest rates of mortality in 2020 due to exposure to ozone were Albania, Montenegro, Greece, Bosnia and Herzegovina and North Macedonia, in order of decreasing rank (EEA, 2022a). Since 2005 there has been no specific trend in ground-level ozone-related mortality, and the year-to-year variability depends mostly on summer temperatures (Solberg et al., 2022).

Besides the direct health effects, surface ozone is absorbed through the stomata of plants and can adversely impact crops and forestry yields, which affects food supply. Wheat yields were estimated to be reduced in Europe up to 9% in 2019. In terms of economic losses, 1,4 billion EUR was lost across 35 countries (EEA, 2022c).

Projected effects

Future ground-level ozone concentrations

The year-to-year variability in ozone concentrations and its peak values are affected by the ongoing and future changes in the key atmospheric parameters in a complex way (Table 1). Higher probability of heatwaves will likely lead to increases in ground-level ozone concentration peaks. Increased solar radiation and summertime temperatures will also accelerate the chemical process of ozone formation. Emission of VOC (the ozone precursor) will be increased by warmer summers (Langner et al., 2012), but also reduced by higher levels of CO₂ in the atmosphere (Szopa et al., 2021). More frequent summertime wildfires will act as a source of both VOC and CO emissions (Parrington et al., 2013). The removal of ozone from the atmosphere via absorption by vegetation – itself harmful to plants – can be reduced by heat and water stress on plants (Szopa et al., 2021). At the same time, increased humidity will increase ozone destruction in areas that are low in NOx, such as maritime areas in Scandinavia (Colette et al., 2015).

Table 1: Selection of meteorological parameters that may increase under future climate change and their impact on ozone levels

^{Source: Adapted from Jacob and Winner (2009), The Royal Society (2008) and Lin et al. (2020)}

Future climate change is expected to increase ozone concentrations, but this increase should not exceed 5 µg/m³ in the daily maximum by the middle of the century and would therefore likely be outweighed by reductions in ozone levels due to planned future emissions reductions of ozone precursors. However, the end of the century projections suggest an increase of up to 8 µg/m³ in ozone concentrations. Decreases are projected only over oceanic and northernmost areas (British Isles, Scandinavian, and Baltic countries) (Figure 1).

Figure 1. Modelled future change in summertime ground-level ozone concentrations (daily maxima) over Europe at the middle of the century (left) and at the end of the century (right). Source: ETC/ACM (2015)

Health impacts

The mortality related to acute ozone exposure is expected to increase due to climate change by 2050, especially in Central and Southern Europe (Orru et al., 2019; Selin et al., 2009). Geels et al. (2015) estimated that climate change alone will lead to a 15 % increase in the total number of ozone-related acute premature deaths in Europe towards the 2080s under the RCP 4.5 climate scenario. Net economic welfare losses (including mortality costs and leisure losses) due to ozone-related health impacts from climate and precursor emission changes could accumulate to 9.1 billion EUR between 2000 and 2050. The effect on the costs of projected changes in emissions would largely overrun the climate impact (Selin et al., 2009).

Policy responses

Monitoring, targets and warnings

Under the 2008 Ambient Air Quality Directive, the European Member States are responsible for the monitoring and reporting of ground-level ozone data to the European Environment Agency. Monitoring of hourly ozone concentrations is performed at almost 2000 stations throughout Europe, including rural, suburban and urban background stations – to document the exposure of the population. Ozone concentrations are also measured at industrial and traffic stations, located in close proximity to a major road or an industrial area/source.

The 2008 Ambient Air Quality Directive sets a target value and a long-term objective value for ozone for the protection of human health. An overview of the legal standards for ground-level ozone set in the Directive to protect human and environmental health are given in Table 2.

Table 2 : Overview of threshold and target values and long-term objectives for atmospheric ground-level ozone

^{* AOT40 (µg/m3 x hours) is the sum of the difference between hourly concentrations greater than 80 µg/m3 and 80 µg/m3 over a given period using only the 1-hour values measured between 8.00 and 20.00 Central European Time (CET) each day}

The 2008 Ambient Air Quality Directive also includes regulatory obligations to inform the population about high concentrations of ground-level ozone (Table 2). The Information threshold reflects a 'level beyond which there is a risk to human health from brief exposure for particularly sensitive sections of the population'. When the threshold is exceeded, national authorities are required to inform the public. The Alert threshold reflects a 'level beyond which there is a risk to human health from brief exposure for the general population'. National authorities are required to inform the public, give advice and implement short-term action plans when this threshold is exceeded. Exceedance of both thresholds should be reported by the Member States to the European Commission.

Information on annual ozone concentrations is available at the EEA’s air quality statistics viewer. Up-to-date air quality information is available at the EEA’s UTD air quality viewer and through the European Air Quality Index. The Copernicus Atmosphere Monitoring Service provides a 4-day forecast of ground-level ozone concentrations. In several European countries ozone concentration levels are included in heat-health action plans. See an example from Belgium here.

Concentration reductions

In 2021, the World Health Organization (WHO) published new air quality guidelines to protect human health, updating the 2005 air quality guidelines on the basis of a systematic review of the latest scientific evidence of how air pollution damages human health. The European Commission published a proposal for a revision of the Ambient Air Quality Directive in October 2022, which aligns EU air quality standards more closely with 2021 WHO recommendations, and introduces limit values for all air pollutants currently subject to target values, except for ozone. Ozone is exempted from this change from target to limit value due to the complex characteristics of its formation in the atmosphere which complicate the task of assessing the feasibility of complying with strict limit values.

The impact of climate change exacerbating ozone formation could partly offset the efforts to reduce emissions of ozone precursors. This is referred to as ozone climate penalty. Compensating this climate penalty over Europe’s mainland would require ambitious mitigation measures (30 to 50% cuts in NOx and VOC emissions). In the long term, reductions of methane emissions can also efficiently reduce ozone formation. Since methane is also an important greenhouse gas, its reduction also benefits climate change mitigation (UNEP, 2021; JRC, 2018).

References

• Colette, A. et al., 2013, European atmosphere in 2050, a regional air quality and climate perspective under CMIP5 scenarios, Atmos. Chem. Phys. 13, 7451-7471. https://doi.org/10.5194/acp-13-7451-2013
• Colette, A. et al., 2015, Is the ozone climate penalty robust in Europe?, Environmental Research Letters 10(8), 084015. https://doi.org/10.1088/1748-9326/10/8/084015
• Cooper, O.R. et al., 2014, Global distribution and trends of tropospheric ozone: An observation-based review, Elementa 2, 000029. https://doi.org/10.12952/journal.elementa.000029
• EEA, 2022a, Air quality in Europe 2022, Briefing No 05/2022. European Environment Agency web report
• EEA, 2022b, Exceedance of air quality standards in Europe. European Environment Agency
• EEA, 2022c, Impacts of air pollution on ecosystems, European Environment Agency web report
• ETC/ACM, 2015, Modelled future change in surface summertime ozone concentrations
• Geels, C. et al., 2015, Future premature mortality due to air pollution in Europe–sensitivity to changes in climate, anthropogenic emissions, population and building stock, International Journal of Environmental Research and Public Health 12, 2837-2869. https://doi.org/10.3390/ijerph120302837
• Jacob D.J. and Winner D.A., 2009, Effect of climate change on air quality, Atmospheric Environment 43, 51-63. https://doi.org/10.1016/j.atmosenv.2008.09.051
• JRC, 2018, Global trends of methane emissions and their impacts on ozone concentrations, Joint Research Centre, European Commission.
• Langner, J., et al., 2012, A multi-model study of impacts of climate change on surface ozone in Europe, Atmospheric Chemistry and Physics 12, 10423-10440. https://doi.org/10.5194/acp-12-10423-2012
• Lin, M. et al., 2020, Vegetation feedbacks during drought exacerbate ozone air pollution extremes in Europe, Nature Climate Change 10, 444-451. https://doi.org/10.1038/s41558-020-0743-y
• Monks, P.S., et al., 2015, Tropospheric ozone and its precursors from the urban to the global scale from air quality to short-lived climate forcer, Atmospheric Chemistry and Physics 15, 8889-8973. https://doi.org/10.5194/acp-15-8889-2015
• Orru, H., et al., 2019, Ozone and heat-related mortality in Europe in 2050 significantly affected by changes in climate, population and greenhouse gas emission, Environmental Research Letters 14, 074013 https://doi.org/10.1088/1748-9326/ab1cd9
• Parrington, M., et al., 2013, Ozone photochemistry in boreal biomass burning plumes, Atmospheric Chemistry and Physics 13, 7321-7341. https://doi.org/10.5194/acp-13-7321-2013
• Selin, N.E., et al., 2009, Global health and economic impacts of future ozone pollution, Environmental Research Letters 4, 044014. https://doi.org/10.1088/1748-9326/4/4/044014
• Solberg, S., et al., 2021, Long-term trends of air pollutants at national level 2005-2019, ETC/ATNI Report 9/2021
• Szopa, S., et al., 2021, Short-Lived Climate Forcers. In: Masson-Delmotte V. et al., 2021, Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change Intergovernmental Panel on Climate Change.
• The Royal Society, 2008, Ground-level ozone in the 21st century: future trends, impacts and policy implications, The Royal Society Policy Document
• UNEP, 2021, Global Methane Assessment: Benefits and Costs of Mitigating Methane Emissions. UNEP CCAC
• WHO Europe, 2008, Health Risks of Ozone from Long-range Transboundary Air Pollution. World Health Organization Regional Office for Europe