This study aimed at providing fundamental information for development of governmental policy on radon management, investigated the radon levels of residential homes nationwide. It also suggested the necessity for policy development which focuses on management of the degree of harm through the installation of radon alarm devices and radon reduction consulting for homes with radon readings in excess of recommended threshold. Results showed that the radon level of the subjects of this study, 1,167 houses, was 97.3 ± 65.8 Bq/m3. Regionally, Seoul had the highest level, while Jeju had the lowest. In the first round of the investigation, the number of houses, with radon level which exceeded the recommended threshold, 148 Bq/m3, was 171. However, as a result of the radon alarm installation and radon reduction consultation, the indoor radon level of 137 households decreased to less than the recommended threshold. In the second round of the investigation, 80% of the households, the radon concentration of which exceeded the current recommended threshold in the first round, appeared to maintain their radon concentration below the recommended threshold. As a result of the communication about radon's harmfulness and the installation of the radon alarm device for recognition of harmful environments. It could be deduced from this result that the communication about harm contributes to the reduction of radon.