Background
In the late 1970s, a deterioration in the condition of tree crowns has been observed in several forest areas in Europe. The associated decrease in tree vitality was attributed to the very high level of air pollution at that time. Pollutants such as nitrogen oxides (NOx) or sulfur dioxide (SO2) cause smog and acid rain and do not stay within national borders, thus becoming a supra-regional problem that cannot be solved by one single country alone.
The Convention on Long-range Transboundary Air Pollution (CLRTAP or Air Convention) was adopted in 1979 within the framework of the United Nations Economic Commission for Europe (UNECE). It is the first international treaty to limit, reduce and prevent air pollution and its effect on ecosystems, human health, and materials.
The International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests) was established under the Air Convention in 1985.