Building Materials as a Significant Contributor to Indoor Radon Concentrations in a Large Apartment Complex Mitigation: Case Study

The Florida Department of Health conducted an investigation to address reported difficulties in mitigating some units of a large apartment complex in Florida. The complex houses over 300 one and two bedroom apartments in over 40 three-story concrete structure buildings. Initial mitigation work successfully mitigated many of the first floor units to levels below 3.5 pCV1 using Active Soil Depressurization (ASD) systems. However, other units particularly those located at both ends of the buildings continued to exhibit elevated indoor radon concentrations. Since problematic units and successfully mitigated units can be found in the same building, radon entry contribution from building materials was not initially suspected as the cause of the problem. Modification and enhancement of ASD systems in problematic units demonstrated no effectiveness in reducing indoor radon levels. The investigation revealed that unexpected distribution of concrete mix materials inside the wall cavities can be responsible for the persistent elevated indoor radon concentrations. Wall cavities of the end apartment units were found to be filled with the concrete mix while no filling or inconsistent filling patterns were diagnosed in other wall cavities. It has been demonstrated that contributions of building materials can be accounted for the uncontrolled portion of the elevated average indoor radon concentration in the problematic units. Abnormality in structures such the one encountered in this investigation can be found occasionally. Therefore, it is recommended that radon mitigation work in large and complex structures should incorporate a practice of performing detailed building diagnosis prior to installation.