Radioprotection, vol. 46,n ◦ 6 (2011) S91–S95 C  EDP Sciences, 2011 DOI: 10.1051 / radiopro / 20116768s Measurement of indoor radon concentration and actual effective dose estimation of schools at high radon area in Korea B.U. Chang 1,2 , Y.J. Kim 1,2 , M.H. Song 1 , G.H. Kim 2 , S.Y. Jeong 1,2 and K.W. Cho 1,2 1 Korea Institute of Nuclear Safety, 34 Gwahak-ro, Yuseong-gu, 305-338, Daejeon, Republic of Korea 2 University of Science and Technology, 113 Gwahak-ro, Yuseong-gu, 305-333, Daejeon, Republic of Korea Abstract. On the basis of previous survey results of indoor radon for schools, detailed radon survey was conducted in one hundred three selected schools in Korea during the second semester in 2010. According to the results of effective dose assessment due to inhaled radon of during occupancy time, fourteen schools for requiring urgent mitigation action were selected, and recommended to the government. 1. INTRODUCTION In 2008–2009, a new radon survey in public buildings (local governmental offices and elementary schools) was conducted in Korea [1]. This survey was the fourth national radon survey in Korea and the average indoor radon concentration in some elementary schools were found to be over 300 Bq · m -3 , which is the new recommended reference level in dwellings of the Statement of ICRP on radon in November 2009 [2]. This result was released into the public through the mass media, and became a pubic concern as a possibility of high exposure of young students in schools was raised. However, occupancy time of the students is relatively short compare with that of dwellings. They generally stay during day time and are exposure to relatively low indoor radon concentration, which means that actual dose level by exposure to radon may be lower than the estimated dose calculated from the average radon concentration. The passive alpha-track detectors, commonly used for indoor radon survey to provide only average concentration during the exposure period, is not adequate to evaluate the actual effective dose for students or daytime workers in schools. In addition, due to the over estimation by measurement of only average radon concentrations obtained from passive detectors, it is possible that the unnecessary or excessive action to determine radon mitigation can be conducted. Therefore, in order to determine the real necessity of mitigation (including installing active radon removal system) for schools, this study was carried out by the request of the Ministry of Education, Science and Technology (MEST). 2. SELECTION OF SCHOOLS AND SURVEY METHODS One hundred three schools found with the high indoor radon concentration were selected on the basis of previous survey results (fourth national radon survey [1] and unpublished preliminary survey results accomplished by the Local Education Authority). The locations of schools for sampling were shown in Figure 1. To estimate actual effective dose due to radon inhalation during working time, the radon concentration was continuously measured and recorded with the average of every 30 minute’s concentration using twelve different active radon detectors (9 RAD7s, Durridge co., USA; 2 RTM2200s, SARAD co., Germany; and, 1 Alpha Guard, Genitron Instruments GmbH., Germany) during one