1. Vanja Radolić, Odjel za fiziku Sveučilišta u Osijeku, Croatia
2. Marina Poje Sovilj, Odjel za fiziku Sveučilišta u Osijeku, Croatia
3. Denis Stanić, Odjel za fiziku Sveučilišta u Osijeku, Croatia
4. Igor Miklavčić, Odjel za fiziku Sveučilišta u Osijeku, Croatia
Radon concentrations in educational institutions (schools and kindergartens)are systematically measured by track etched detectors from 2012 and it is still ongoing. Detectors are exposed for a year in every playroom in kindergartens and selected classrooms and offices (such as: libraries, teaching staff offices, administrative offices etc.) evenly distributed across areas and floors in schools. In total, around 6000 radon detectors in 874schools and 341 kindergartens were exposed. The obtained average radon concentrations were higher than the reference value (300 Bq m-3) in 14.5% of kindergartens and 16.7% of schools.At fifteen selected locations where radon concentrations were elevated,continuous radon measurements are conducted by TSR3 radon detectors with 1-hour time resolution.The observed daily variations of radon concentrations in these objects (schools and kindergartens) showed strong dependence on increased natural ventilation during working hours. It means that radon concentrations significantly decrease in the morning and early afternoon and consequently increase at night and during weekends. Despite these variations, due to very high maximum values during nights and short period of working time in some schools, radon levels are during working hours, in average15% less than during the entire measurement period. Consequently, estimation of effective doses received from radon and its short-lived progenyare also decreased by 15% using radon concentrations corrected on working hours. Daily variations of radon concentrations and their influence on corrected radon concentrations during working hours show the need for revision of the protocol for radon measurements in schools and kindergartens introducing two step measurements in those objects with elevated radon levels. This will be proposed as a part of activity within existing national Radon Action Plan.