Critical loads have been defined as: "the highest load that will not cause chemical changes leading to long-term harmful effects in the most sensitive ecological systems". Critical loads are the maximum amount of pollutants that ecosystems can tolerate without being damaged. The definition has been redrafted in order to fit specialist areas of interest, most particularly the acidification of freshwater and soils.
The concept behind critical loads is based upon a dose-response relationship where the threshold of harmful response (within the ecosystem) is triggered by a certain load of pollutant - the critical load. However, it is not always easy to apply without careful consideration of the nature of the affected ecosystem and the threshold effects of harmful pollutants. In order for critical loads to be used, target loads need to be set for different areas in order to try and halt the acidification processes. Target loads have been defined as "the permitted pollutant load determined by political agreement". Therefore, target loads can be either higher or lower than the critical load values. For example, the target load may be lower so as to give a safety margin or the target load may be higher for economic reasons. The reasoning behind this is that critical loads only show where there is an acidification problem and to what degree damage is occurring. Target loads are used in order that emissions can be reduced accordingly to meet the targets and limit the amount of damage.
The critical loads for total acidity of sulphur and nitrogen need to be determined so that a coherent international agreement can be reached with regard to abatement policies. There are numerous methods that are available for obtaining critical loads. In order to obtain values for the critical loads, an ecosystem has to be chosen and then a suitable indicator species is selected to represent the ecosystem. A chemical limit is subsequently defined as the concentration at which the indicator species will die. In forests the indicators are trees, and in freshwaters they are fish.
Through the Convention on Long-Range Transboundary Pollution, member countries of the United Nations Economic Commission for Europe (UNECE) agreed that the critical loads approach provided an effective scientific approach for devising strategies for the abatement of acid deposition. The UK Government accepted that the critical loads approach was the best way to establish abatement strategies in relation to sulphur dioxide and nitrogen oxides emissions. It was recognised that critical loading maps were essential in providing information on the geographical distribution of pollutant-sensitive locations. The Critical Loads Advisory Group (CLAG) was set up to produce critical load maps for the United Kingdom. The National Centre for Critical Loads Mapping was subsequently established at the Institute for Terrestrial Ecology, now the Centre for Ecology and Hydrology.
Critical load maps of soil acidity have been produced for the UK at a grid resolution of 1 km squares. Maps for the critical acidity of freshwater environments are based on a single water sample from a single site in each of the 10 km squares used, assumed to be the most sensitive surface water within the grid square. These critical loads maps, when combined with acid deposition values, produce exceedance maps which show where and by how much the critical loads are being exceeded. Maps are available for soils in the UK relating to acidity and sulphur deposition showing areas that are sensitive to acidification. These correspond to areas where there have been reports of acidification. In the UK a national target load map for the year 2005 has been produced for soils on a 20km by 20km grid system, showing the target loads that need to be met for such areas.
In 1997, critical loads for acidification were exceeded in 71% of UK ecosystems. As sulphur deposition continues to fall, this value is expected to fall to below half by 2010, when nitrogen deposition will dominate. Critical loads for eutrophication (nutrient depletion) in 1997 were exceeded in about a quarter of UK 1km by 1km squares with sensitive grasslands and a little over half with heathland. Again, this is expected to decline over the next 10 to 15 years.