Within the Belgian Radon Programme the Belgian Building Research Institute is responsible for theinvestigation of techniques for prevention and mitigation of radon problems in dwellings. One of themost regular actions is the installation of a sub-slab depressurisation (SSD) system. As there are noready-to-use systems available on the Belgian market, a system has to be designed for every specificsituation. In this case, one of the most difficult points is the choice of an appropriate fan.Important characteristics are the relation between air flow and pressure difference, the fan power, theacoustical performances, the durability, the electrical protection, etc. Some of these characteristicshave an influence on the effectiveness of the radon reduction (e.g. air flow vs. pressure difference),while other parameters are important from the point of view of the occupant (e.g. acoustics,consumption).Within the Belgian Radon Programme some of these performances (air flow vs. pressure difference,fan power vs. pressure difference, acoustics) were measured in the laboratory for a number of fans(10) available on the Belgian Market. Moreover a comparison was made with two ready-to-use SSDsystemssold abroad.The paper will discuss the different characteristics in detail and use the measurement results of the 12fans as illustrations.The study revealed that only a limited number of fans is appropriate to be used for a sub-slabdepressurisation. Some remarkable findings:- Some fans with a high power (and as a consequence a high consumption) seemed to operate atabout the same working point as fans with a much lower power. This means that the fan power iscertainly not a good point of reference to evaluate the efficiency from the radon point of view.- There seemed to be a very important difference from the acoustical point of view. It is important tokeep in mind that a silent fan will be necessary to assure that the occupier will not stop theextraction due to irritating noise. The laboratory tests showed that it is possible to reduce the soundlevel importantly by the installation of a simple silencer.