The risk classes

Environment moisture is linked to biologic agents attacks distructors of the wood

As can be appreciated from the previous paragraphs, the agents causing degradation of wood are numerous. However, both as regards to fungi and insects, humidity is very often the limiting factor for their development.
Therefore there is a very important correlation between the environment the installed wood is in and the attacks of destructive biological agents.

For example it is easy to imagine how wood protected in a closed, dry environment is more long lasting than the same wood in contact with humid ground.

The ENC (European Normalisation Committee) through the European standard EN 335 parts 1, 2, and 3 have identified 5 risk classes that have been established on the basis of the humidity the wood id exposed to under different conditions of use. 
The higher the risk, the greater the need to increase the natural resistance level of the wood using impregnation treatment.

Definition of the risk classes

Risk class 1: situation where the wood or wood based product is shielded, completely protected from atmospheric agents and not exposed to humidity.

Risk class 2: situation where the wood or the wood based product is shielded, completely protected from atmospheric agents, but where high environment humidity may cause occasional but not persistent humidity.

Risk class 3: situation where the wood or the wood based product is not shielded and is not in contact with the ground. It is continuously exposed to the atmospheric agents or, even though protected from them, is subjected to frequent humidity.

Risk class 4: situation where the wood or the wood based product is in contact with the ground or fresh water and is therefore exposed to permanent humidity.

Risk class 5: situation where the wood or the wood based product is permanently exposed to salt water.