The aim of this document is to outline and summarize the principles to be followed when the aim isto produce and to install a clean air handling system. A large Finnish research and developmentproject CLEAN VENTILATION has focused on to find the main reasons and phases where theodours and other harmful pollution from the ventilation system originate. In the project bettertechnical solutions have been developed to produce cleaner components with less harmfulemissions. The work in the project has resulted to many new products and improvements ofmanufacturing, designing and installation processes of ventilation systems.The Clean ventilation project has produced also practical guidelines for the design of cleanerventilation systems and developed and published as well as guidelines for the manufacturing andinstallation of cleaner ducts and other components for ventilation systems. New methods to measureand prove the cleanliness of ventilation systems has been developed and taken into use. Newimproved technology for the cleaning of the components and systems has also been developed andintroduced.An essential result has been the production of the background information for the CleanlinessClassification of Air-Handling Components as a part of the Finnish Classification of Indoor AirQuality. General requirements for the cleanliness of air-handling equipment and detailedrequirements for ventilation ducts, duct sections, air and fire dampers and filters has beendeveloped. Based on these principles a system for labelling the air handling components has beenestablished in Finland in 2001.