In order to study the air quality in houses, a large collection of data was carried out in new or renovated energy-efficient houses from Western Switzerland. The statistics consisted of analyzing questionnaires completed by the inhabitants as well as the measures of radon, volatile organic compounds and occurrence of molds. We had also to find which parameters (structure of the house, type of ventilation, maintenance,…) have an influence on a high level of radon, of volatile organic compound and mold.