Radon is the main pollutant of indoor air, being considered a primary cause of lung cancer, especially in non-smokers. In 2013, European legislation required the standardization of regulations on radon exposure in member states using EC Directive 59/2013 EURATOM. The directive establishes basic safety rules for protection against the dangers represented by the exposure to ionizing radiation. Directive 59/2013 mandates each European country to establish a national action plan to reduce the risks of lung cancer attributed to radon in the general population and exposed workers. Romania is a European country with almost 20 million citizens, with over half of population living in urban locations. Most urban buildings in Romania are high-rise structures (4–10 floors), having many residential units. In this paper, we present results of season-dependent measurements of radon concentration and ventilation rates in high-rise apartment buildings and show the influence of ventilation on indoor radon levels. Indoor radon concentrations varied between 70 and 168 Bq m−3 in winter and 17–30 Bq m−3 in summer, being higher than the respective outdoor concentrations by 2–5 times in winter and 1–4 times in summer. Ventilation rates varied with season and were influenced by meteorological conditions at the time of measurements. Findings indicate that levels of indoor radon concentration depend on ventilation rates of the unit, but also on building materials, wind speed around building, time of day (higher around noon) and season of the year (higher in the cold season when ventilation rate is lower).

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