Accredited official statistics

Air pollution

Updated 10 December 2024

Applies to England

© Crown copyright 2024

This publication is licensed under the terms of the Open Government Licence v3.0 except where otherwise stated. To view this licence, visit nationalarchives.gov.uk/doc/open-government-licence/version/3 or write to the Information Policy Team, The National Archives, Kew, London TW9 4DU, or email: psi@nationalarchives.gov.uk.

Where we have identified any third party copyright information you will need to obtain permission from the copyright holders concerned.

This publication is available at https://www.gov.uk/government/statistics/england-biodiversity-indicators/air-pollution

Last updated: 2024

Latest data available: 2021 (mid-year of the 3-year moving window 2020 to 2022)

Introduction

This indicator shows the proportion of sensitive habitat in England that is affected by two major impacts of air pollution: (i) acidification; and (ii) the excessive richness of nitrogen (eutrophication). The air pollutants sulphur dioxide, nitrogen oxides and ammonia can contribute to acidification; nitrogen oxides and ammonia can also contribute to terrestrial eutrophication. These pollutants arise mainly from livestock waste and from burning fossil fuels in industry and road transport. Approximately 23,000 km2 of terrestrial habitat areas in England are sensitive to acidification and about 26,000 km2 are sensitive to nutrient nitrogen deposition (eutrophication); many areas (almost 14,000 km2) are sensitive to both.

Critical loads are thresholds for pollutant load above which significant harmful effects may occur on sensitive habitats; statistics on critical load exceedance indicate the risk of damage. To reduce the effects of variation in meteorology, exceedance statistics are reported as the mean of three years of data and time periods are referred to using the middle year of the three. For example, “2020” means the period 2019 to 2021.

Data for this indicator can be found in the published datafile. We also have a similar indicator at the UK level.

Type of indicator

Pressure indicator

Type of official statistic

Official statistic

Assessment of change

Assessment of change in in area of sensitive terrestrial UK habitat exceeding critical loads

Measure Assessment Time period Result
Area affected by acidity Long term 2003 to 2021 Improving
Area affected by acidity Short term 2016 to 2021 Improving
Area affected by nitrogen Long term 2003 to 2021 Little or no change
Area affected by nitrogen Short term 2016 to 2021 Little or no change

Notes on indicator assessment

Long- and short-term assessments are based on a 3% rule of thumb. Where possible, the base years for these assessments use a three-year average. See Assessing Indicators.

Key results

In 2003, acid deposition exceeded critical loads in 77% of the area of sensitive terrestrial habitats in England. This declined to 67% in 2021. The short-term trend between 2016 and 2021 was a 3% decrease in the area affected by acidity.

In 2021, nitrogen deposition exceeded the critical load for eutrophication in 99% of sensitive habitats. This percentage was 100% in 2003. In the short term, the area where nitrogen deposition exceeded critical load showed little or no change between 2016 and 2021.

Figure 1: Percentage area of sensitive terrestrial habitats in England exceeding critical loads for acidification and eutrophication, 2003 to 2021

Source: UK Centre for Ecology & Hydrology

Notes about Figure 1:

  • Each bar represents a rolling three-year average of deposition data. To reduce the effects of year-to-year variation in meteorology, exceedance statistics are reported as the mean of three years of data. In Figure 1, time periods are referred to using the middle year of the three. For example, “2020” means the period 2019 to 2021.

Further detail

For information on the methods used in this indicator, please see the technical annex.

Relevance

The air pollutants sulphur dioxide, nitrogen oxides and ammonia can contribute to acidification, and nitrogen oxides and ammonia can contribute to terrestrial eutrophication, both of which adversely affect semi-natural ecosystems. Exceeding the critical load for acid deposition is likely to cause low soil pH and high aluminium availability, making the habitat unsuitable for many species. Excess nitrogen as a nutrient can also affect species composition, for example, by triggering accelerated growth of some species at the expense of others. These species and habitats tend to be of high conservation value and are often the first ones to be affected. This in turn can lead to loss of ecosystem function.

Acknowledgements

Thank you to the many people who have contributed by providing data and to the many colleagues who have helped produce this indicator, particularly the UK Centre for Ecology and Hydrology.

Technical annex

Methodology

Critical loads are thresholds above which significant harmful effects on sensitive habitats may occur, according to current levels of scientific understanding. Critical loads have been established separately for acidification and nutrient nitrogen (eutrophication effects). The pollutants causing acidification and eutrophication mainly arise as a result of emissions from livestock waste and from burning fossil fuels in industry and road transport.

There are three main steps in the assessment of the area of sensitive habitat that exceeds critical loads:

  • calculation of critical loads for each of the sensitive habitats;
  • mapping of the habitats; and
  • identification of the area of habitat where deposition exceeds the critical load.

Several methods are used to calculate critical loads for terrestrial habitats in England, based on either empirical (observational or experimental) evidence or mass-balance (input/output) data. Both types of method can be used to calculate critical loads for acidity and eutrophication, the choice of which method to use is determined by the habitat type.

Critical loads are reviewed and updated periodically as new research data becomes available. Critical loads for nutrient nitrogen were established in 2003, and revised in 2011. For all years, exceedance is calculated using the 2011 values for nutrient nitrogen critical loads. Details of the revision can be found in the 2011 UK Status Report and the 2015 Methods Report, available on the Critical Loads and Dynamic Modelling website. The latest revision of nutrient nitrogen critical loads was published in October 2022 and has been included since the 2023 update of this indicator. Most of the critical loads have been reduced (become stricter) and in most cases, the reliability of these values has increased. Details can be found in the Review and revision of empirical critical loads of nitrogen for Europe. The method for calculating acidity critical loads remain unchanged from those published earlier see Air Pollution Trends Report 2024. The trends in critical loads exceedances are calculated using deposition maps based on the Concentration Based Estimated Deposition (CBED) methodology. To identify the area exceeding critical loads, deposition maps based on a 5km x 5km grid covering the UK are produced based on the sum of wet deposition, dry deposition and cloud deposition. These are available on the UKCEH website. These deposition data are overlain on maps of critical loads for each habitat to calculate critical load exceedances and the areas of habitat exceeded.

Average Accumulated Exceedance

The percentage area of habitat with exceedance of critical loads is a useful metric but it can be insensitive to changes between years, since the area exceeded can remain the same even if there is a change in the magnitude of the exceedance. The “Average Accumulated Exceedance” (AAE) averages the exceedance across the entire habitat area and so gives an indication of change in the magnitude of exceedance.

AAE is calculated as: (exceedance × exceeded habitat area) ÷ (total sensitive habitat area)

The trend results show that the area of sensitive habitats in England exceeding acidity critical loads decreased from 77.3% in 2003 to 66.7% in 2021; over this timescale the magnitude of exceedance (AAE) has fallen by 54%, from 1.04 keq ha-1 year-1 in 2003 to 0.48 keq ha-1 year-1 in 2021. During the same period, the area of sensitive habitats where eutrophying pollutants (nutrient nitrogen) exceed critical loads fell slightly from 100.0% to 99.3% and the AAE in England decreased by almost 26%, from 19.2 kg ha-1 year-1 to 14.3 kg N ha-1 year-1. Note that the results include some changes reported as percentages, for example, “The short-term trend between 2014 and 2019 showed a 17.3% decrease in the area affected by acidity”. These percentages do not refer to absolute change, in which going from 50% of land area exceeded to 25% exceeded would be considered a 25% decrease. They refer to relative change, for example, going from 50% of land area exceeded to 25% exceeded is considered a 50% decrease. This example has used data from a previous version of this indicator.

Table 1: The 14 habitats considered sensitive to acidification and/or eutrophication for which critical loads are calculated

Habitat Critical loads calculated for acidification Critical loads calculated for eutrophication
Acid Grassland Yes Yes
Calcareous Grassland Yes Yes
Dwarf Shrub Heath Yes Yes
Bog Yes Yes
Montane Yes Yes
Coniferous Woodland Yes Yes
Beech Woodland No Yes
Oak Woodland on acid soil No Yes
Scots pine No Yes
Dune Grassland No Yes
Saltmarsh No Yes
Mixed Woodland No Yes
Freshwaters (1752 sites) Yes No
Broadleaved and mixed woodland Yes No

Development plan

Since our previous publication we have adapted the language and visualisations used in this indicator. We are keen to hear from our users about these changes, as well as our published development plan, please email us.

Back to top