Research and analysis

Sulphur dioxide (SO2) compliance assessment 2025

Published 9 October 2026

This compliance assessment is part of Air pollution in the UK 2025. There is a page for each of 12 pollutants.

This page provides the UK’s compliance assessment for 2025 against the:

• sulphur dioxide (SO2) limit values set for the protection of health
• SO2 critical level set for the protection of vegetation

The compliance criteria is set in the Air Quality Standards Regulations (2010).

The Air Quality Standards Regulations (2010) require reporting of compliance on an annual basis. This report continues the series of annual compliance reporting.

1. SO2 pollution and its impacts

SO2 is an acid gas that is formed when fuels containing sulphur impurities are burned.

SO2 is a respiratory irritant that can cause constriction of the airways, and people with asthma are considered to be particularly sensitive to its effects. Health effects can occur rapidly, making short-term exposure to peak concentrations important, as reported by the Committee on the Medical Effects of Air Pollutants.

SO2 deposition is harmful to plants at high concentrations. It contributes to acidification of terrestrial and aquatic ecosystems, damaging habitats and contributing to biodiversity loss. SO2 is also a precursor to the formation of secondary sulphate particles in the atmosphere.

2. Sources of SO2 pollution

The release of pollutants into the atmosphere is known as emissions.  Annual UK emissions of SO2 are estimated each year and are presented in Defra’s Accredited Official Statistics publication, Emissions of Air Pollutants. The latest estimates currently available are for 2024.

The National Atmospheric Emissions Inventory reports emissions of SO2 from human activities in the UK, but there are a few exceptions included as memo items, such as international shipping and aviation. The information presented here does not include memo items.

Emissions of SO2 have decreased substantially since 1990, largely driven by a decline in coal use in energy industries. The decline has also been supported by stricter limits being placed on the sulphur content of liquid fuels, and by the closure or adaptation of many coal-fired power stations.

In 2024, the largest sources of SO2 emissions in the UK were fuel burning in commercial, industrial and residential settings.

While SO2 emissions within the UK have decreased substantially, volcanic eruptions can release significant quantities of SO2 into the atmosphere, which can then be transported to the UK. Historically, for example, eruptions that occurred in Iceland caused episodes of elevated SO2 concentrations in the UK. Emissions of SO2 from volcanic eruptions are not estimated in the National Atmospheric Emissions Inventory.

3. Trends in SO2 pollution

The change in SO2 concentrations over time since 1990 is shown in Figure 1. Average measured SO2 concentrations have decreased from an annual mean of 39.4 µg/m3 in 1990, to 0.8 µg/m3 in 2025.

The average concentration has been calculated from all stations in the Automatic Urban and Rural Network (AURN) that had an annual data capture greater than or equal to 75 per cent. The shaded areas represent the 90 per cent confidence interval. The intervals narrow over time because of an increase in the number of monitoring stations and a reduction in the variation between annual means at monitoring stations. Trends over time can be influenced by changes to the number of monitoring sites and their locations. The pattern shown here will therefore exhibit some effect of changes to the structure of the monitoring network over time.

Figure 1: Annual mean concentrations of SO2 in the UK, 1990 to 2025

The annual air quality statistics published for a range of pollutants identifies days with ‘moderate’ or higher air pollution. Very few days have been identified as ‘moderate’ for concentrations of SO2 at either urban or rural monitoring sites over the last decade.

4. Spatial variation

Figure 2 shows the modelled 99.18th percentile of 24-hour means (which corresponds to the 4th highest day in a full year) in 2025 for each 1x1km grid square of the UK. The data comes from the CMAQ-Urban Model used for supplementary assessment against the SO2 limit values and critical level set out in section 6.

If the 99.18th percentile (that is, the 4th highest day) is greater than the 24-hourly mean limit value of 125 µg/m3, this would indicate that there were more than the permitted 3 exceedances in the year. There were no areas of the UK where this was the case in 2025.

The areas of highest SO2 concentrations in Figure 2 are around the Derbyshire Dales, Southampton, and Peterborough. These concentrations correspond to localised areas of industrial activities, including material recovery facilities, fuel refineries, and brickworks.

Figure 2: Modelled 99.18th percentile of 24-hour mean SO2 concentrations for 2025

Note that Figure 2 shows the modelled concentration of SO2 for each 1x1 km grid square of the UK. Within the grid squares, there may be local areas of higher concentration close to specific emission sources. These will not be visible in the background maps.

5. Episodes of SO2 pollution in 2025

Air pollution is communicated on the Daily Air Quality Index (DAQI), a simple scale that explains the short-term health risk of air pollution in words, numbers and colours.

See the guidance on pollutant concentrations for the DAQI.

Figure 3 provides a calendar plot of the DAQI for SO2 concentrations in 2025. Each day in the year is represented by a coloured square. All the squares are coloured green, denoting that every AURN monitoring site on every day of the year recorded an SO2 concentration that fell within the ‘low’ pollution band. The full classification of pollutant concentrations and their bandings is available online. There were no short-term episodes of SO2 pollution in 2025 to investigate further.

While direct emissions of SO2 in the UK are now very low, as outlined above, there can be occasions when SO2 concentrations are elevated due to transboundary sources, such as volcano eruptions. Examples of these episodes occurred in 2024, due to eruptions that occurred in Iceland, but there were no such instances in 2025.

Figure 3: Visual summary of the DAQI for SO2 at AURN monitoring sites in 2025

Note 1: The colour of each square represents the maximum DAQI across the national network.

Defra issues public information alerts via the Check Air Quality service when pollutant concentrations from monitoring stations breach certain alert thresholds.

The alert threshold for SO2 is when concentrations exceed 500 µg/m3 for 3 consecutive hours over a 100 km2 area or an entire zone, whichever is smaller.

There were no breaches of the alert thresholds for SO2 in 2025.

6. SO2 limit values and critical level

This section outlines the limit values and the critical level set for SO2 concentrations.

Compliance against these criteria are assessed for each zone of the UK. For further information on the definition of agglomeration and non-agglomeration zones see the methods for 2025 Air Quality Standards Regulations (2010) compliance assessment.

Table 1: SO2 compliance criteria

Description	Averaging period	Metric
SO2 limit value for health	1-hour	350 µg/m3 not to be exceeded more than 24 times a calendar year
SO2 limit value for health	Daily	125 µg/m3 not to be exceeded more than 3 times a calendar year
SO2 critical level for vegetation	Annual mean and winter (1 October – 31 March)	20 µg/m3 for non-agglomeration zones

The rules for assessing compliance are set out in the Air Quality Standards Regulations (2010), as amended.

7. Evidence used for assessing compliance

See the methods for 2025 Air Quality Standards Regulations (2010) compliance assessment.

The evidence base for the annual assessment of compliance against the above SO2 limit values and critical level is underpinned by a combination of measurements and the results of modelling assessments.

The use of models enables air quality to be assessed at locations without monitoring sites. It has the added benefit of providing additional information on source apportionment and projections to support the development and implementation of air quality policies.

For further information on the measurements and the models used in the 2025 compliance assessment, and to download the data, see the air quality compliance assessment 2025: results and data tables.

Monitoring

In 2025, there were 21 monitoring stations from the AURN network which met the data quality requirements for assessment against the SO2 limit values. This includes:

• 12 background stations
• 2 roadside monitoring stations
• 4 industrial monitoring stations
• 3 rural background monitoring stations

Modelling

The CMAQ-Urban Model was used for compliance assessment in 2025.

For assessment against the limit values for SO2, one model output was calculated:

• annual average SO2 concentrations for each 1x1km grid square of the UK

For the assessment against the SO2 critical level, one model output was calculated:

• annual average SO2 concentration for each 1x1km grid square of the UK that is at least 20 km away from agglomerations or more than 5 km away from other built-up areas, industrial installations or motorways or major roads with traffic counts of more than 50,000 vehicles per day

For more information on the modelling methods and for access to the data, see the air quality compliance assessment 2025: modelled data and reports.

Modelled data is also made available in accessible formats within air quality compliance assessment 2025: results and data tables. 

8. Compliance assessment for 2025

All zones were compliant with the limit value for 1-hour mean SO2 concentrations in 2025.

All zones were compliant with the limit value for daily mean SO2 concentrations in 2025.

All non-agglomeration zones within the UK also complied with the critical levels for annual mean and winter mean SO2 concentrations in 2025, set for protection of ecosystems (these are not applicable to built-up areas).

The results of the assessment are also made available within the air quality compliance assessment 2025: results and data tables.

9. Changes in compliance over time

This section provides information on non-compliances in previous years from 2008 onwards. 2008 is the year that limit values for pollutants were set in the EU Air Quality Directive, which was subsequently transposed into UK legislation by the Air Quality Standards Regulations (2010).

The UK has been compliant with the limit values and critical level set for SO2 since 2008.