In the last decade he has caused himself an increase of the education of the interior ambient conditions of the buildings caused by the appearing of numerous scientific reports that they relate them to serious diseases. Between these, the education of concentration of radon branded like carcinogen wilfully publicise one. The present article parses the concentrations of this gas that have taken place in a single-family housing and as the ventilation system in work has influence upon her. For it, two samples in three outbuildings of the house have been taken, (car park, dining room and bedroom). The first unaccomplished sample the ventilation of the spaces and the second with ventilation. The obtained peak concentrations when the ventilation system does not find itself in operation evidence values from among 391 and 94 Bq/ m3 coming right across over the doorsteps established in the standard 2013/59 EURATOM. On the contrary, the peak concentrations of radon decrease to values from among 71 and 29 Bq/ m3 interveningventilation. In this sense, it is confirmed that the ventilation system in work is determining to decrease the concentration of the gas in the analysed sojourns getting an average efficacy from the 62 %.