Abstract: A radon measuring device having a housing formed by at least two mating portions. The housing includes a plurality of apertures along at least one of its surfaces to permit entry of ambient atmosphere into said housing, a membrane or filter positioned proximate an interior surface of the housing covering the apertures for filtering the ambient atmosphere which enters the housing, and a radon detector within the housing positioned in a plane substantially normal to the surface of the housing containing the apertures. The housing can also include a filter holder within its interior for positioning both the filter and the detector in their respective positions as well as alignment tracks for alignment of the filter holder with respect to the housing.