Air pollution

Critical loads

Measuring the damage caused to ecosystems by acid deposition is a difficult undertaking. In order to monitor and quantify the effects of air pollutant emission reductions policies a quantitative measure of environmental sensitivity to acid deposition. Critical loads are a common measure of environmental sensitivity routinely used by the air pollution community.

The most commonly used definition of a critical load for acid deposition is that of Nilsson and Grennfelt (1988):

"A critical load for acid deposition is the highest deposition of acidifying compounds that will not cause chemical changes leading to long term harmful effects on ecosystem structure and function." Nilsson & Grennfelt (1988)

The critical loads concept can be illustrated in a simple diagram (below). The critical load for a site is exceeded at point a. Points b and c represent critical loads for individual species. As acid deposition decreases in the future a target load, T, can be chosen in relation to the wish to protect selected species, or ideally to enable full recovery (point a).

Critical and target loads diagram

If the critical load is less than the actual measured acidity loading at any site then the critical load is exceeded. Maps of exceedance for individual countries and for Europe as a whole have been drawn up and used for policy making and the fixing of target loads.

The first emission reduction protocols were seen as being inadequate because the reductions they specified were not specifically targeted at the largest emitters of pollutants, the 30% reductions in sulphur (S) were insufficient for recovery to take place, and because the reductions did not address the issue that some areas have been effected by acid deposition more than others.

For these reason critical loads have been adopted by the countries of the European Union, and the UNECE have replaced the 30% S reduction protocol by a protocol based on the critical loads approach.

There are various ways in which critical loads for sites can be calculated. Three methods in particular have been used in research under the Freshwater Umbrella contracts from Defra:

Follow the links above to find out more about the different critical loads models used to calculate freshwater critical loads.

The United Kingdom National Focal Centre (UK NFC) for critical loads modelling and mapping activities is based at CEH Monks Wood, Cambridgeshire, UK. It is responsible for co-ordinating the critical loads mapping activities in the UK and compiling national critical loads datasets and maps from data supplied by UK experts. The work of the UK NFC is funded by the Department for the Environment, Food and Rural Affairs (DEFRA) in England, and the devolved administrations of the National Assembly for Wales, the Northern Ireland Assembly and the Scottish Executive.


  • Nilsson, J. & Grennfelt, P. (Eds) (1988) Critical loads for sulphur and nitrogen. UNECE/Nordic Council workshop report, Skokloster, Sweden. March 1988. Nordic Council of Ministers: Copenhagen.