The paper presents a review of results obtained in the period 2014-2019 during complex studies performed in the frame of international projects by team of Dunarea de Jos University of Galati, regarding the investigation of indoor radon (222-Rn) and thoron (220-Th) concentrations and their temporal variations in selected dwellings and public spaces in Lower Danube and Danube Delta region (Galati and Tulcea counties – Galati, Sendreni, Sulina localities), SE Romania. For the measurements, RTM1688-2 and Thoron Scout calibrated instruments of SARAD type have been used. The data were processed with the aid of SARAD Radon Vision 6.3.4 software. A disscussion is made on the factors influencing the variability of radon/thoron levels, such as: type of building, construction material, location, geological features, ventilation of the space, existence of basement, floor level, measuring time, etc. For some locations, the ambient gamma-ray dose rate and immediate beta radioactivity of aerosols was also recorded. The obtained values are compared with the world average value reported by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) and other results published for Romania and other countries. The data collection was based on strong collaboration of research team with volunteers (inhabitants and students from master and doctoral programs), which participated at various training programs and courses, improving their knowledge, awareness and attitudes towards the ionizing radiation exposure and the radioactivity and indoor radon impact on human health. It is known the fact that the exposure of human beings to ionizing radiation from external and internal natural sources, is 2.4 mSv y-1 and from this 1.2 mSv y-1 is due to indoor radon. The obtained results show that the radon concentration in several houses, built from slag bricks or adobe, exceeds the legislated limit for the average annual reference level (300 Bq m-3 set by Romanian legislation, Law no. 63/2018). Utilization of the active detectors is very advantageous by offering a graphic with the hourly variations of radon/thoron concentration in air over a long period, based on which there could be identified the intervals in which the supplementary aeration of spaces would be necessary. Further research is focused on radon and thoron levels in soils and water from targeted areas of BSB27 MONITOX project. Acknowledgement: The authors would like to thank the funding of projects MIS ETC 1676 (INPOLDE), Joint Operational Programme Romania-UkraineRepublic of Moldova, and BSB27 (MONITOX), Joint Operational Programme Black Sea Basin 2014-2020, as well as the grants no. 24/2016, 21/2018 and 22/2019 of Joint Institute for Nuclear Research, Dubna, Russian Federation.