This Recommendation sets out the principles of good practice for corrosion protection of air handling units. It explains how to correctly match the corrosion resistance of materials and components of air handling units to the corrosivity of the operating environment. The first two chapters establish the fundamentals of corrosion protection, corrosivity categories, and testing methods for corrosivity resistance of various materials. The third chapter gives recommendations on properties of materials and components depending on the environmental corrosivity category classified according to EN ISO 9223. The last chapter provides guidance for the design of air handling units for specific applications, with detailed guidelines for swimming pool units. The Recommendation provides a tool to planners, contractors, and investors to verify that an air handling unit is properly adapted to their needs in terms of corrosion protection.