In more than 1500 schools and preschool facilities, long-term radon measurement was carried out in the last 3 y. The negative effect of thermal retrofitting on the resulting long-term radon averages is evident. In some of the facilities, low ventilation rates and correspondingly high radon levels were found, so it was recommended to change ventilation habits. However, some of the facilities had high radon levels due to its ingress from soil gas. Technical measures should be undertaken to reduce radon exposure in this case. The paper presents the long-term experiences with the two-stage measurement methodology for investigation of radon levels in school and preschool facilities and its possible improvements.

National Radiation Protection Institute Bartoškova 28 Prague 4 140 00 Czech Republic

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