Rn-222 exists in nature in the form of a rare radioactive gas. In terms of environmental radiation, issues regarding Rn-222 have persisted because of its radiological hazardousness. Ulju County is one of the regions of Ulsan metropolitan city, with a population of 227,699. Ulju County has the highest density of industrial complexes in Korea. In this study, Rn-222 radioactivity concentration was measured and analyzed in 57 schools in Ulju County using 114 passive LR-115 type detectors to secure radiological safety and confirm basic information for reduction of resident exposure to Rn-222. The effective dose of Rn-222 was assessed to find the actual risk of the concentration surveyed in schools to human beings. The dose depended on four factors: subjects, Rn-222 concentration, dose coefficient, and time. The individuals subjected to dose estimation were classified into three types: students, teachers, and office workers. The subjects had different dwelling locations and times. The findings demonstrate that the radiological hazard to students and workers at schools in Ulju County owing to Rn-222 is negligible in terms of Rn-222 activity recommendation level. (C) 2018 Korean Nuclear Society, Published by Elsevier Korea LLC.