Ozone (O3)
Updated 30 June 2026
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Accredited Official Statistics
Air quality statistics in the UK, 1987 to 2025 - Ozone (O3)
Updated 30th June 2026
1. Why measure O3?
Ozone (O3) is a gas which is damaging to human health and can trigger inflammation of the respiratory tract, eyes, nose and throat, as well as asthma attacks. In addition, ozone can have adverse effects on the environment through oxidative damage to vegetation, reducing growth, flowering and seed-setting, including for crops. Ozone can also react with other chemicals in the air to form smog. Therefore, it is important to measure ozone concentrations to identify whether they pose risks to public health and the environment.
In contrast to many air pollutants measured in the UK, there are no major emission sources of ozone itself. The vast majority of ozone is instead formed in the air from reactions between other pollutants. For example, non-methane organic volatile compounds (NMVOCs) can photochemically react with other air pollutants outdoors in the presence of sunlight (via ultraviolet radiation) to produce ground-level ozone. However, O3 concentrations reduce when it reacts with nitric oxide (NO), producing oxygen and nitrogen dioxide (NO2).
The complex interdependence between concentrations of ozone and its precursor pollutants makes both assessing and tackling exposure to ozone difficult. For example, modelling suggests that, as NO concentrations decrease in urban areas, there is an associated increase in ozone concentrations. This is likely to continue until there is a significant reduction of both ozone and its precursor pollutants like NO. Another layer of complexity in assessing exposure to ozone is that, once formed, ozone can travel long distances and can reach high concentrations far from the original sources of pollution, meaning UK concentrations can be influenced by emissions in other countries.
These characteristics mean that estimating emissions or measuring concentrations of ozone precursor pollutants alone is insufficient to assess potential exposure to ozone in the UK. Therefore, not only is measuring ozone concentrations one of the only methods of effectively assessing ozone pollution generally in the UK, but it is also important to help quantify any perverse outcomes of tackling exposure to other pollutants like NOx.
The Air Quality Standards Regulations (2010) set the target for ozone to a maximum daily eight hour mean concentrations of 120mg. This target value is not to be exceeded on more than 25 days per calendar year averaged over three years.
2. Trends in concentrations of O3 in the UK
2.1 Annual mean concentrations of O3 in the UK, 1987 to 2025
The O3 index shows the annual mean of the daily maximum 8-hour mean, averaged over all stations that had annual data capture greater than or equal to 75 per cent. The shaded areas represent the 95 per cent confidence interval for the annual mean concentration for urban background stations and rural background stations. Annual means for individual stations can be found in the O3 statistical tables which accompany this report.
Figure 13: Annual mean concentrations of O3 in the UK, 1987 to 2025
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Urban background O3 pollution has shown a long-term increase.
The annual average daily maximum eight hour mean concentration of ozone at urban background stations has shown an increasing trend since 1992. The highest concentration - 68.3 µg/m3 – was reached in 2025. This was an increase of 3 per cent compared to 2024.
Ozone concentrations can vary substantially between years because of inter-annual variability in meteorology. Some variation from year-to-year is expected due to fluctuations in the occurrence of hot summer weather conditions which are associated with high ozone concentrations.
From the start of the time series in 1992 to the mid-2000s, urban background ozone concentrations have increased. This may have been due to the reduction in emissions of nitrogen oxides in the UK and Europe, which can inhibit the formation of ozone in urban areas. Concentrations fluctuated from the mid-2000s to 2016 with no obvious trend.
Since 2017, urban background ozone concentrations have followed an upward trend, with 2025 being the highest in the time series. Particularly hot summer weather occurred in 2022, which possibly led to more ozone formation than usual. Since 2018, ozone concentrations at urban background stations have consistently been higher than at any other point since monitoring began.
Rural background O3 pollution has been higher in recent years.
The average daily maximum eight hour mean concentration of ozone at rural background stations has fluctuated since the start of the time series in 1987. It was 72.7 µg/m3 in 2025, an increase of 2 per cent from 2024 but slightly down from the highest levels on record in 2022. Since 2018, rural background concentrations of ozone have been consistently amongst the highest on record. Some variation from year-to-year is expected due to fluctuations in the occurrence of hot summer weather conditions which are associated with high ozone concentrations. The future trend in concentrations may be increasingly dependent on global emissions of ozone precursor substances.
Ozone is formed through a complex chemical reaction involving NOx and volatile organic compounds (VOCs) in the presence of sunlight. Ozone concentrations are typically higher in rural areas compared to urban areas. This is partly due to there being more pollutants in the air in urban areas, due to traffic, industrial activities and other sources, which can react with each other and can degrade ozone or inhibit its formation (for example nitric oxide (NO)). Overall, air pollutant concentrations in urban areas are on a long-term declining trend, leading to less degradation and increased formation of ozone, which contributes towards ozone concentrations increasing in these areas.
3. Average hours spent in ‘Moderate’ or higher O3 pollution
This metric measures the annual trend in the number of hours on average that concentrations are recorded at levels that may impact human health. For O3, ‘Moderate’ air pollution is triggered when the latest 8-hour running mean concentration is greater than 100 µg/m3. This is the level where citizens who are vulnerable to the health impacts of air pollution should take action to reduce activity and their outdoor exposure. More advice on the Daily Air Quality Index (DAQI) is published on gov.uk. The coloured areas of figure 14 relate to the categories of the DAQI (see Table 20 in the statistical tables that accompany this release).
Figure 14: Mean hours when O3 pollution was ‘Moderate’ or higher for rural background stations, 1987 to 2025
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Figure 15: Mean hours when O3 pollution was ‘Moderate’ or higher for urban background stations, 1992 to 2025
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Since 2018, urban and rural background monitoring stations recorded an increased number of hours of ‘Moderate’ or higher O3 air pollution, after a decade of a relatively low number of hours.
At urban background stations, between 2017 and 2020, there was a large increase in the number of hours for which the mean O3 concentration over the previous 8 hours exceeded 100 µg/m3. This led to the highest value in over a decade with 134 hours occurring in 2020. Overall, the time series remains volatile, with a considerably lower mean number of hours in 2021 (40 hours) followed by relatively high levels since, with 135 hours in 2025.
There was a similar rise in rural background concentrations between 2017 and 2019. This was followed by a modest decrease in the number of hours of ‘Moderate’ or higher O3 pollution between 2019 and 2020 at rural background stations, though values remained relatively high (166 hours). Despite a dip in 2021, the figure was still relatively high in 2025 at 155 hours per station.
The overall trend in the rural indicator shows a long-term decrease interrupted by several years in the early 2000s where moderate pollution was more common. The long-term decrease in peak concentrations is likely driven by reductions in global emissions of substances that lead to the formation of ozone such as nitrogen oxides and volatile organic compounds.
The overall trend in the urban indicator is less clear. Ozone concentrations are strongly influenced by weather, which likely contributes to the high variability over time and peaks such as in the hot, sunny summers of 2003, 2006, 2018, 2019, 2020, 2022 and now 2025 which was the hottest UK summer on record. This means that long time monitoring series are required to distinguish between weather effects and the effect of changes in pollutant emissions. In 2020, reductions in NOx concentrations in urban environments as a result of COVID-19 restrictions were probably also a large contributing factor to increased ozone concentrations, since less NO will have been available to react with ozone and inhibit its formation.
4. Temporal variations in concentrations of O3 in the UK, 2025
4.1 Monthly variations
The O3 index shows the monthly mean, averaged over all stations that had monthly data capture greater than or equal to 75 per cent in a given year. The shaded areas represent the 95 per cent confidence interval for the monthly mean concentration for urban background stations and rural background stations.
Figure 16: Monthly mean of the daily maximum rolling 8-hour mean concentrations of O3 in the UK, 2025
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For O3, the highest average concentrations tend to occur during the spring and summer months. Ozone is a secondary pollutant, meaning it is created in the atmosphere through a cycle of reactions of its precursors nitrogen oxides and VOCs, with sunlight as a key part of the ozone production cycle. In the summer, lower wind speeds lead to the build-up of these precursor compounds, while more sunlight and higher temperatures increase the rates of reactions that generate ozone.
In 2025, the month that had the highest ozone concentrations was May for both rural and urban monitoring stations (with monthly mean concentrations of 90.1 µg/m3 and 89.2 µg/m3 respectively). May 2025 was sunny, warm and dry with above average temperatures, for more detail, see the Met Office blog. This sunny spell likely increased the rate of chemical reactions between ozone precursor pollutants that built up in the atmosphere.
Ozone exhibits strong seasonal and diurnal cycles. Monthly average ozone concentrations in the UK are usually at their maximum in April and May, in contrast to most of continental Europe where monthly ozone has a maximum in June and July. The range of the annual cycle is between 50-90 µg/m3, being slightly greater at urban stations, which likely reflects increased rates of chemical removal from increased NO emissions in winter. There is some evidence that the annual amplitude in ozone has been decreasing slightly - see AQEG report Ozone in the UK.
5. Sections in this release
Background to concentrations of air pollutants
Concentrations of nitrogen dioxide
Concentrations of particulate matter (PM10 and PM2.5)
Methodology for calculating progress towards the PM2.5 targets
Days with ‘Moderate’ or higher air pollution (includes sulphur dioxide)