B5. Pressure from pollution

B5a. Air pollution

i. Area affected by acidity

ii. Area affected by nitrogen


Type: Pressure Indicator


Indicator Description

The air pollutants sulphur dioxide, nitrogen oxides and ammonia can contribute to acidification, and nitrogen oxides and ammonia can contribute to terrestrial eutrophication. These pollutants arise mainly from burning fossil fuels and from livestock waste. Around a third of UK land area is sensitive to acidification, and a third to eutrophication (with some areas sensitive to both). Critical loads are thresholds for pollutant load above which significant harmful effects may occur on sensitive habitats, so statistics on critical load exceedance indicate the risk of damage.




The area of sensitive UK habitats that exceeds the critical load for acidification has continued to decline since 1996, but there has been less change in the area that exceeds the critical load for eutrophication. Acid deposition exceeded critical load in 42% sensitive habitats in 2015, and nitrogen deposition exceeded critical load in 62% of sensitive habitats in 2015.


Figure B5ai. Area of sensitive UK habitats exceeding critical loads for acidification and eutrophication, 1996 to 2015.


  1. Each column represents critical load exceedances based on a three-year average of deposition data to reduce year-to-year variability.
  2. Since 2002, nitric acid has been included in the estimates of nitrogen deposition, and since 2003 aerosol deposition loads of sulphate, nitrate and ammonium have also been included.  This additional deposition led to some increases in critical load exceedance compared with earlier periods.
  3. There was a revision to the calculation of deposition data for the period 2004 to 2013 in 2015, which means the exceedance results for this period are not directly comparable to those previously published.

Source: Centre for Ecology & Hydrology.


Assessment of change in area of sensitive habitat exceeding critical loads


Long term

Short term

Latest year

Area affected by acidity



Decreased (2015)

Area affected by nitrogen



Decreased (2015)



Critical loads are thresholds for the deposition of pollutants causing acidification and/or eutrophication above which significant harmful effects on sensitive UK habitats may occur.  Approximately 78,000km2 of UK terrestrial habitats is sensitive to acid deposition.  About 73,000km2 is sensitive to eutrophication; much of this is sensitive to both.

In 1996, acid deposition exceeded critical loads in 73% of the area of sensitive habitats.  This declined to 42% in 2015.  There has been a slight decrease in the area affected over the short term, since 2010, when the figure was 47%.

In 2015, nitrogen deposition exceeded critical loads in 62% of sensitive habitats.  This was a decrease from a level of 75% in 1996.  However there was little change in the short term.

Based on these figures the habitat areas at risk from acid and nitrogen deposition has declined over the long term (1996 to 2015), however, reducing deposition below the critical loads does not necessarily mean that ecosystems have recovered, as there can be a time-lags before the chemical environment and the flora and fauna recover.


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Last updated: July 2018

Latest data available: 2015 (2014 – 2016)

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