Accredited official statistics

Background

Updated 27 June 2025

Accredited Official Statistics

Air quality statistics in the UK, 1987 to 2024 - Background

Updated 26th June 2025

1. Why measure UK air quality?

Air pollution is a local, regional and international problem caused by the emission of pollutants which either directly, or through chemical reactions in the atmosphere, lead to negative impacts on human health and ecosystems.

There are many sources of air pollution, including, but not limited to, power stations, transport, household heating, agriculture and industrial processes. The National Atmospheric Emissions Inventory (NAEI) provides estimates of the amount of different pollutants that are emitted to the air each year from human activity in the UK. However, the relationship between emissions and concentrations of air pollutants is not straightforward. Three main factors can have large, complex effects on air quality: the weather, transboundary movement, and secondary formation of pollutants (described below). As a result, it is important to measure pollutant concentrations, in addition to quantifying emissions. A more accurate picture of air quality in the UK aids the development of strategies to reduce air pollution from human activities and thereby helps reduce the impact of pollution on public health and the environment.

This publication covers UK concentrations of the following key pollutants thought to have the greatest health and environmental impacts resulting from exposure:

• nitrogen dioxide (NO2)

• particulate matter (PM10 and PM2.5)

• ozone (O3)

• sulphur dioxide (SO2) (This publication only includes statistics on the number of days when air pollution from this pollutant is moderate or higher. Concentration measurements of SO2 are not included, as concentrations of SO2 are very low across the UK, which reduces the requirement to monitor concentrations of this pollutant; and the national SO2 monitoring network sites are strategically placed next to point sources such as factories and are therefore not representative of levels experienced across the UK).

Please see the links at the bottom of this page for the statistics calculated for each pollutant.

2. The effects of air pollution

Air pollution has negative impacts on human health and the environment. For example, long term exposure to particulate matter contributes to the risk of developing cardiovascular disease and lung cancer. Particles can be inhaled and penetrate into the lungs. The smaller the particles, the deeper they can penetrate into the lungs and therefore health impacts are more strongly associated with the smaller PM2.5 fraction. As well as being emitted directly, particulate matter can be formed in the atmosphere from reactions between other pollutants, of which NO2, SO2, Non-Methane Volatile Organic Compounds (NMVOCs) and Ammonia (NH3) are the most important. More detail on the health impacts of other pollutants is provided within each pollutant section of this release. For more information about the effects of NH3, NMVOCs and SO2 see the relevant section in the latest emissions of air pollutants in the UK statistical release.

In order to better inform the public about short-term levels of outdoor air pollution and their potential health effects, the Daily Air Quality Index (DAQI) was developed following advice from the Committee on Medical Effects of Air Pollutants (COMEAP). This index, numbered 1-10, standardises the measured concentrations of individual pollutants by categorising them into 4 bands ("Low", "Moderate", "High", and "Very High"). These bands are linked to the latest understanding of the impacts on health of each pollutant. As such, the DAQI provides information about the health risk of different levels of pollutants in a simple and comparable format and is similar to the sun index or pollen index used in weather forecasts. Measured concentrations of the different pollutants should not be directly compared as their effects on health and the environment are very different.

Air pollution also damages ecosystems through:

• acidification (SO2, NO2 and NH3) – where chemical reactions involving air pollutants create acidic compounds which when deposited on land and aquatic systems can cause harm to soils, vegetation and buildings.

• eutrophication (NO2 and NH3) – where nitrogen can be deposited in soils or in rivers and lakes through rain, affecting the nutrient levels and diversity of species in sensitive environments, encouraging algae growth in lakes and water courses, for example.

• ground-level ozone (NO2 and NMVOCs) – where chemical reactions involving NO2 and NMVOCs produce the toxic gas ozone (O3) which can damage wild plants, crops, forests and some materials. Ozone is also a greenhouse gas contributing to global warming.

3. The links between emissions of air pollutants and air quality in the UK

While reducing UK emissions of air pollutants helps reduce atmospheric concentrations in the UK, the level of reduction in atmospheric concentrations is not always proportionate to the reduction in emissions.

Weather conditions can greatly affect local concentrations of air pollutants. At the most basic level, changes in wind speed or direction can increase dispersion, with settled conditions generally resulting in higher levels of ambient pollution.

Ozone is a secondary pollutant formed from a succession of reactions involving precursor pollutants (Nitrogen Oxides and volatile organic compounds) in the presence of sunlight. The levels of the precursors and the effect of the sun must be considered holistically when assessing ozone pollution, any change in the balance of precursors or in sunlight levels will affect the ozone production either to increase or decrease it.

The transboundary nature of some pollutants is another factor that contributes to the disconnect between emissions and concentrations. For example, emissions of the pollutants that lead to ozone formation have reduced considerably in the UK, but this is not reflected in the long-term trend in ozone concentrations. This may be partly explained by a proportion of the ozone experienced in the UK originating from releases of precursor pollutants that are transported across from mainland Europe and increased transport of ozone driven by increases in global hemispheric background ozone concentrations.

In addition, secondary formation of pollutants may occur, contributing to final pollutant concentrations measured at a given location. For example, primary emissions of ammonia can react in the atmosphere with other substances to form secondary concentrations of particulate matter.

Local factors such as proximity to pollution sources also partially explain differences in trends between emissions and concentrations of air pollutants. For example, emissions of nitrogen oxides at a national level have been decreasing in the long-term, however at some roadside locations corresponding decreases in nitrogen dioxide concentrations may not be observed because of significant local emissions of nitrogen oxides.

Restrictions related to the Coronavirus (COVID-19) pandemic in 2020 and 2021 influenced emissions of pollutants from many sources, in particular road transport, when the volume of traffic was reduced significantly. Overall, emissions of air pollutants were mostly lower in 2020 and 2021 compared to 2019. This in turn influenced the concentrations of pollutants, although, as discussed above, the level of reduction in atmospheric concentrations is not always proportionate to the reduction in emissions.

Defra produce Accredited Official Statistics annually in February on emissions, which are estimated by combining the amount of pollutant emitted from a certain activity with estimates of the duration of that activity. These statistics analyse national levels and trends in emissions. The latest emissions estimates for the UK can be downloaded from the NAEI.

For further information on air quality data and information, please refer to the air quality and emissions statistics GOV.UK webpage.

4. Compliance with air quality limit values

The Air Quality Standards Regulations (2010) (AQSR) outline the air quality target values, long-term objectives and legally binding limit values for concentrations of major air pollutants that impact public health in the UK. For the protection of vegetation, critical target values and levels are also provided for ozone and nitrogen dioxide (not including urban areas), respectively, although these are not legally binding.

The UK is divided into 43 zones for air quality assessment which includes 28 agglomeration zones (large urban areas). For each zone, limit values apply to annual mean concentrations of each pollutant covered in this release used to measure long-term exposure, except for ozone which has targets and long-term objectives (for both the protection of human health and ecosystems). Similarly, there are additional limit values for daily mean concentrations of PM10 and sulphur dioxide, and hourly mean concentrations of nitrogen dioxide and sulphur dioxide. A summary of the air quality objectives and target and limit values used for assessment across all pollutants can be found on the UK-AIR website. For historical assessments, please see the European Environment Agency website.

The air quality data obtained from the statutory monitoring networks is supplemented by modelling and additional low-cost monitoring and is assessed against the air quality target values, long-term objectives and legally binding limit values in the AQSR.

For an overall summary of compliance, see the relevant years in the Air Pollution in the UK report, published each September.

5. How is air quality measured?

Monitoring data is combined with modelled data for annual statutory reporting of pollutant concentrations to provide data that covers the entire UK. The UK-AIR website provides further information and the most up-to-date data for all air pollutants measured by the AURN, modelling and other monitoring networks.

5.1 Monitoring - the Automatic Urban and Rural Network

In accordance with the AQSR, the concentrations of the key pollutants are measured via a UK-wide network of approximately 195 monitors, managed by the Environment Agency. This network, the Automatic Urban and Rural Network (AURN), captures continuous ambient concentrations of these pollutants on a near-hourly basis. This is the monitoring network that provides the data used in this Accredited Official Statistics release. The monitoring method employed varies by pollutant:

• NO2 – Chemiluminescence

• O3 – UV absorption

• SO2 – UV fluorescence

• PM – a range of methods – The methods for measuring particulate matter approved by Defra under MCERTS certification are listed on the UK-AIR website

In addition to the AURN, in 2020 a new national network of diffusion tubes was deployed to monitor NO2 concentrations at 177 roadside sites, increasing to 300 roadside sites in 2021. Because historical data does not exist for this network, data from the network are not included in this Accredited Official Statistics Publication.

5.2 Modelling - Pollution Climate Mapping

To compliment the data collected by instrument monitors, the UK uses Pollution Climate Mapping (PCM) models. PCM is a collection of models designed to fulfil part of the UK’s statutory reporting requirements. PCM consists of one model per pollutant and models background concentrations on a 1 km² scale plus around 9,000 representative roadside values.

The model estimates concentrations of each pollutant for each limit value set by the UK’s Air Quality Standards Regulations 2010, except for hourly limit values which are based entirely on monitoring data.

Although ratified results from monitoring are available in June (see section 6 below), the modelled data are not ratified until September, which means the modelled data are not used in this Accredited Official Statistics publication.

6. Monitoring data Quality Assurance and Quality Control (QAQC)

All monitoring data collected by the AURN undergoes a comprehensive ratification process before it is used in this publication and subsequent compliance reporting under the AQSR. First, the data is reviewed and 'cleaned’. Second, a quarterly review is conducted incorporating QAQC audits of monitoring stations and comparing recent data to local and national trends. Only once both stages are completed is the data classified as “Ratified” and ready for use in reporting. Occasionally corrections need to be made to ratified data and these are logged on UK-Air.

To enhance the accuracy of this Accredited Official Statistics release, monitoring data are only included if the annual capture rate is at least 75 per cent; this aligns with the UK’s Air Quality Standards Regulations (2010) (AQSR) Data Quality Objectives where a monitoring site must have data available for at least 75 per cent of the year to be used.

Uncertainty in measurements is accounted for when producing these Accredited Official Statistics by averaging concentrations across multiple monitoring sites and presenting confidence intervals to show how much the average value could vary due to differences in monitoring site locations. These confidence intervals reflect how well the sample of monitoring sites represents air pollution levels across the UK. However, the intervals do not include measurement uncertainty – the possible error in each instrument’s readings.

Only ‘Type Approved’ analysers are currently included in the AURN network, which excludes many monitors used by Local Authorities such as diffusion tubes. Type Approval of analysers is carried out in the UK under the MCERTS scheme managed by the Environment Agency. Type Approval is undertaken for analysers to adhere to European Committee for Standardisation (CEN) methods, established to meet the minimum performance requirements based on the AQSR Data Quality Objectives. In adherence to the AQSR, monitors also undergo periodic maintenance (e.g. calibration, audits and servicing) to ensure continuation of proper functioning.

An annual technical report summarises the QAQC activities carried out for the AURN network each year. The approach taken for reporting on the quality and traceability of the assessment methods applied, follows guidance set out in the Implementing Provisions on Reporting (IPR) guidance on air quality reporting, and is in line with the AQSR.

Monitors are sited according to specific ‘macroscale’ and ‘microscale’ criteria outlined in Schedule 1 of the AQSR. The ‘macroscale’ siting criteria are intended to ensure that the station’s location is representative of the exposure of the population, in the context of the averaging period of the limit values (see section 4 above). This includes locations where the highest concentrations are likely to occur to which the public are exposed. As such, the AQSR specifies that all monitoring stations must be in areas where there is public access and/or fixed habitation. Exceptions include monitoring around industrial areas where health and safety at work regulations apply, or around roads where normal pedestrian access is not possible. Based on these criteria, the UK classifies monitoring sites according to the type of area (urban/suburban/rural) and the predominant sources of pollution (traffic/industrial/background). The ‘microscale’ siting criteria are aimed at ensuring the station is sampling air that is representative of its immediate vicinity. For example, the air intake must be in an open area, with no obstructions around it, and the opening where air is sampled should be between 1.5 and 4 meters high.

7. Local air quality monitoring

In addition to the monitoring networks used for national assessment of air quality, local authorities conduct their own monitoring in accordance with the Local Air Quality Management Guidelines. This guidance is not as stringent as the requirements for the national network (although some sites on local networks will meet the national criteria).

Monitoring at a locally managed level has a large focus on nitrogen dioxide concentrations, primarily monitoring pollution using diffusion tubes. This is known as non-automatic monitoring, as data are manually collected on a monthly basis from each site. Many local authorities also choose to operate automatic monitoring stations that can measure concentrations of multiple pollutants much in the same way that the AURN operates. Data quality will be variable dependent on site positioning, instruments and techniques used, and quality control procedures.

Locally managed data are directly disseminated through multiple platforms, including UK-AIR, Air Quality England and London Air.

Since data from locally managed monitoring sites may not meet the same quality criteria as data from the national networks, they are not used in these Accredited Official Statistics; instead, it should be seen as supplementing the high-quality measurements from the AURN, by greatly increasing the sampling points for air quality in the UK.

8. Limitations of statistics covered by this publication

These Accredited Official Statistics largely focus on mean changes to concentrations in the short- and long-term as measured by the Automatic Urban and Rural Network (AURN). There are monitoring stations that will differ significantly in pollution levels and trends compared to the national mean. The accompanying tables provide station-level figures for the entire analysis, so that users can construct their own analysis based on the monitoring stations they are interested in. Measured data from each station are available from the UK-AIR website.

Much of the analysis in this publication displays time series for concentrations of air pollutants, but the individual stations that feed into the calculations will change from year-to-year. This may be because old stations are shut down and new ones become operational, or an operational issue with a station meant that data capture of 75 per cent was not achieved. Values for each year are broadly comparable as the network is representative of UK air pollution as defined by domestic legislation. As there are approximately 195 monitoring stations in the UK affiliated to the AURN, there will be areas of the country that are better or worse represented than others. Nevertheless, monitoring stations that are part of the AURN are spread geographically and according to population density such that the impact of changes in siting should be minimal when comparing different years.

Where reasons are given for changes in the level or trend of air quality, these are usually based on correlation with supplementary data on known factors for levels of that pollutant. Other factors may influence the level of air pollution other than those stated. Even when looking at yearly averages, the data can still be affected by natural variability, like changes in weather from year to year.

9. Sections in this release

Summary

Concentrations of nitrogen dioxide

Concentrations of particulate matter (PM10 and PM2.5)

Concentrations of ozone

Days with ‘Moderate’ or higher air pollution (includes sulphur dioxide)

Compliance with the Code of Practice for Statistics and Defra group Statistics quality principles, and recent changes to the publication

Statistical tables (ENV02 – Air quality statistics)