Accredited official statistics

Background

Updated 30 April 2024

Accredited Official Statistics

Air quality statistics in the UK, 1987 to 2023 - Background

Updated 30 April 2024

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. Air quality is the term we use to describe how polluted the air we breathe is.

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. The main factors which can have significant and complex effects on air quality are weather, the transboundary movement of air pollutants, the interactions and reactions between pollutants after being emitted, and the secondary formation of pollutants (described below). As a result, it is important to measure ambient air 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 to public health and the environment.

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

• Nitrogen dioxide (NO2)
• Particulate matter (PM10 and PM2.5)
• Ozone (O3)
• Sulphur dioxide (SO2). SO2 is only included in the ‘number of days when air pollution was moderate or higher’ statistic. This is because measurements of SO2 using UK monitors are not representative of air pollution across the whole UK area since SO2 monitors are strategically placed next to point sources such as factories. Concentrations of SO2 are very low across the UK, which reduces the requirement to monitor concentrations of this pollutant.

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, volatile organic compounds (VOCs) and ammonia (NH3) are the most important. More detail on the health impacts of pollutants is provided within each pollutants section of this release. For more information about the effects of ammonia, VOCs 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 values of different pollutants by categorising them into 4 bands (“Low”, “Moderate”, “High”, and “Very High”). These bands are linked to the perceived impacts to 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 values 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 VOCs) – where chemical reactions involving NO2 and VOCs 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. For example, ozone is formed in the presence of sunlight. Changes in wind speed or direction can influence dispersion with settled conditions generally resulting in higher levels of ambient air pollution.

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 trends in global hemispheric background ozone concentrations.

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

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 volume of traffic was reduced significantly. Overall, emissions of air pollutants were mostly lower in 2020, 2021 and 2022 than they were in 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 UK levels and trends in emissions. The latest emissions estimates can be downloaded from the NAEI website.

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) 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 for which an 8-hour mean is used to gauge short-term impacts. Similarly, there are additional limit values for daily mean concentrations of PM10 and hourly mean concentrations of nitrogen 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, (pre 2003) 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 Air Quality Standards Regulations (2010).

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 UK’s Air Quality Standards Regulations (2010), the concentrations of the key pollutants are measured via a UK-wide network of approximately 175 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. The monitoring methods 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 April, 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)

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 Monitoring Certification Scheme (MCERTS) scheme managed by the Environment Agency. Type Approval is undertaken in order for analysers to adhere to European Committee for Standardisation (CEN) methods, established to meet the minimum performance requirements based on the Data Quality Objectives in the UK’s Air Quality Standards Regulations (2010) (AQSR). In adherence to the AQSR, monitors also undergo periodic maintenance (e.g. calibration, audits and servicing) to ensure continuation of proper functioning.

Despite Type Approval and regular maintenance, each monitor is associated with a certain degree of error. If capture rates and/or pollutant concentrations are low, negative values may be recorded. Values are only kept if they are greater than or equal to the negative detection limit. This approach is compliant with the AQSR (2010). To help ensure accuracy of Accredited Official Statistics, monitoring data is only included for assessment if the annual data capture rate is not below 75% following the AQSR (2010) Data Quality Objectives. A capture rate of 90% is required for fixed evidence, but a reduction of 5% is permitted to allow for maintenance, and a further reduction of 10% is permitted for indicative evidence. QA/QC reports are published on the UK AIR website, with the latest AURN technical QA/QC report found here.

Monitors are also sited according to specific criteria outlined in Schedule 1 of the AQSR (2010). Schedule 1 of the AQSR (2010) sets ‘macroscale’ and ‘microscale’ siting criteria. 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. This includes locations where the highest concentrations are likely to occur to which the public are exposed. As such, the AQSR (2010) 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 monitor 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 representative of its immediate vicinity. For example, the air flow around the inlet must be unrestricted, and the inlet height must be in the range 1.5 – 4.0m.

7. Local authority 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 monitoring on 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 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 either on standalone websites operated by the local authorities or on platforms developed to hold local data (examples include Air Quality England and London Air). In 2020, local data from these networks were introduced to the UK-AIR website to allow users to access both nationally and locally-managed data from one location. This includes the majority of locally-managed automatic monitoring stations.

Locally-managed data are not likely to meet the same quality criteria as data from the national AURN network; however it could 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. Hourly-measured and/or daily-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 the requirement for annual 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 175 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 so 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; weather effects in particular can introduce noise into the time series even when considering annual statistics.

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)