What is a Critical Load?

A critical load can be defined as "a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge" (Nilsson & Grennfelt, 1988).

 A critical load refers to deposition of pollutants, while a critical level refers to pollutant concentrations in the atmosphere (which usually have direct effects on vegetation or human health).

When pollutant loads (or concentrations) exceed the critical load (or critical level) it is considered that there is risk of harmful effects. The excess over the critical load or level has been termed the exceedance. A larger exceedance is often considered to pose a greater risk of damage.

Maps of critical loads and their exceedances have been used to show the potential extent of pollution damage and as an aid to developing strategies for reducing pollution. Decreasing pollutant deposition below the critical load is seen as the means for preventing the risk of damage. Even a decrease in the exceedance might infer that less damage will result.