• Critical load maps have been developed to give a national picture of the relative sensitivity of soils, broad ecosystem types and freshwaters to acidification and/or eutrophication. 
  • National critical load maps and data sets were never intended for small, local scale, or site-specific assessments of risk.
  • In general, dose-response relationships have not been developed in the UK for the application of critical load models to specific individual species.
  • By contrast, acidity critical loads for freshwaters are based on the protection of brown trout populations only. While dose-response functions are sparse for other aquatic organisms, work under the Defra Freshwaters Umbrella has developed functions for acid-sensitive diatom and macro-invertebrate species. At present dose-response functions do not exist for most other aquatic species of flora and fauna. 
  • The original ‘mapping dataset’ for acidity critical loads for freshwaters were mapped at 10x10 km resolution and based on critical load values for a single site (high altitude lake or stream), considered to be the most sensitive site, within a 10x10 km grid square in acid-sensitive areas, and within a 20x20 km square in non-sensitive areas.  The dataset now incorporates data from other studies, so some new sites have been added, and where multiple datasets for a site were available, the most recent, best estimate of annual mean chemistry data has been selected.  The quality of the chemistry data took precedence over the sampling date.
  • The site-specific and indicator species dose-response-specific nature of the acidity critical loads for freshwaters means their application to other sites or species is very limited.
  • Acidity critical loads for soils (except peat soils) are based on the mineralogy and weathering rate of the dominant soil type occurring in each 1x1 km grid square.  Other soil types within a grid square may be more or less sensitive to acidification than the dominant soil type and therefore may have critical load values that are the same, or higher or lower, than the critical load for the dominant soil type.
  • Acidity critical loads for peat soils are based on a critical pH threshold and a 1x1 km map of annual mean runoff (currently based on rainfall data for 1941-70).  These critical loads are applied to 1x1 km squares dominated by peat soils, with the exception of peat soils in areas dominated by arable land, which are deemed less sensitive and the critical load is set at 4 keq ha-1 year-1.
  • Slope is not taken into account in the national peat critical loads map, where higher runoff results in higher critical loads.  However, as runoff may vary across a 1x1 km square, depending on local topography, so the critical load may also vary.  Whilst ignored at the national-scale, slope could be very important in determining the appropriate critical load at the site-level, since valley-bottom peats may be enriched in base cations and have a higher pH (because valley bottom peats receive water from upslope) compared to hilltop peats. 
  • Acidity critical loads for terrestrial habitats are based on data relating to the dominant soil type occurring in each 1x1 km grid square.  The only exception to this is the bog habitat where the calculations assume the habitat occurs on peat soils.
  • The 1x1 km runoff data set mentioned above is also used in the calculation of acidity mass balance critical loads for woodlands - and again slope is not taken into consideration in the national calculations.
  • Empirical critical loads of nutrient nitrogen have been set on the basis of observed and published changes in structure or function of ecosystems using experimental (field) data, field observations and/or dynamic ecosystem models.  However, the critical load values will not necessarily protect all habitats/species within each ecosystem, since data are not available for all habitats/species.
  • Mass balance critical loads of nutrient nitrogen for managed (productive) coniferous and deciduous woodland are partly based on data relating to the dominant soil type occurring in each 1x1 km grid square.  Data values for other soil types may vary.
  • Critical loads are subject to further possible revisions in the future as knowledge improves and methods are modified.
  • The national critical loads maps and data are based on empirical or steady-state mass balance methods, used to define long-term critical loads for systems at steady-state.  Therefore, exceedance of these critical loads is an indication of the potential for harmful effects to systems at steady-state.
  • There are uncertainties associated with the calculation of critical loads and their exceedances; sensitivity and uncertainty analyses have been carried out to identify the sources of uncertainties and to quantify them where possible.  Further information is available on the Uncertainties pages of the web site.