Different ground-coupled earth-to -air heat exchangers have been constructed in many residential and occupational buildings in Switzerland to precool or preheat the incoming air. Many technical and energetic facts favour such systems. The objective of this study was to determine if microbial growth occurs within these tubes and if adverse health effects must be considered. The results show large reductions in viable bacteria and spore concentrations along the tubes and very low concentrations in the supply air compared to the outdoor air. Fungal allergens however, could still be detected in the supply air even if no or only very small amounts of spores were determined with the viable sampling techniques. Based on the results from this study the operation of ground-coupled earth-to-air heat exchangers is acceptable. Regular controls and cleaning of the facilities should be undertaken.