Many European standards refer to 'hazard' and 'service' classes when detailing the use of various panel products. These two classes are related but different and are easily confused.   Both define environmental conditions which will cause different equilibrium moisture contents in panels. A 'hazard' class describes the risk of biological attack in a given condition, while 'service' class is used to modify the mechanical properties of a 'dry' panel so structural designs have adequate safety margins.   The table shows there are three service classes, defined in the first part of Eurocode 5: Design of timber structures; and there are five hazard classes defined in EN 335 Durability of wood and wood based products. It's clear there is a link between the two class types, but that they are not exactly the same. The most striking difference is that the limits of the conditions of hazard class 2 would be classed as service class 3; here lies the risk of confusion.   For structural purposes, all panel types can be used in service classes 1 and 2 (for certain grades of a panel type). But for service class 3, only veneer based products can be used - with precautions to prevent bio-deterioration.   Densified panels - OSB, MDF and particleboard - should not be used in service class 3 conditions as they will swell, causing irreversible loss of mechanical performance. However, solid wood and many veneer based products will recover their mechanical properties when re-dried.   Under EN335, cement-bonded particleboard can be used in any of the hazard classes without preservative treatment. Plywood would have to be treated or made with naturally durable veneers.