The degree of defoliation is the extent to which trees and plants have lost their leaves or needles, often as a result of exposure to pollutants, in a visually-assessed measure of defoliation of trees, as developed by the United Nations International Cooperative Programme of the Executive Committee for the Convention on Long-range Transboundary Air Pollution in Europe (ICP Forests). This programme monitors the condition of forests in Europe.
Damage classes are from 0 to 4.
| Class | Needle/leaf loss | Degree of defoliation |
| 0 | up to and including 10 % | none |
| 1 | >10 to 25 % | slight (warning stage) |
| 2 | >25 to 60 % | moderate |
| 3 | >60 to < 100 % | severe |
| 4 | 100 % | dead |