Radon and thoron concentrations in the southwest region of Angola: dose assessment and implications for risk mapping

Indoor radon ( 222 Rn) and thoron ( 220 Rn) are the most important natural sources of ionizing radiation to the public. Radiological studies that assess simultaneously 222 Rn and 220 Rn, and their controlling factors are particularly scarce in African countries. Hence, we conducted a survey of indoor 222 Rn and 220 Rn in buildings located in the SW region of Angola. Bedrock samples were also collected, and a borehole was executed to assess 226 Ra and 224 Ra activity concentration, 222 Rn and 220 Rn exhalation and emanation potential in the surface and at depth. The aim of this study was to determine the factors (geological and anthropogenic) that may influence the annual inhalation dose (AID) received by the population. Overall, the sum of indoor radon and indoor thoron concentrations, labelled the total indoor radon concentration (TIRC), was higher than 300 Bq/m 3 in only 5% of the buildings studied. The contribution of 220 Rn to the TIRC averaged 35% but may reach 95%, demonstrating the relevance of discriminating radon and thoron in indoor radon surveys. Indoor 222 Rn and 220 Rn were not correlated, indicating both must be estimated to properly assess the AID. Indoor 220 Rn concentrations were statistically different according to the building materials and type of usage. Higher 222 Rn and 220 Rn concentrations were observed in dwellings compared to workplaces. The median AID estimated for dwellings was 1.50 mSv/y compared to 0.26 mSv/y for workplaces, which are lower than the estimated average radiation exposure due to natural sources of 2.4 mSv/y. AID values higher than 1 mSv/y effective dose threshold established in the Council Directive 2013/59/EURATOM for the purpose of radiation protection in workplaces were observed in 12% of the workplaces studied suggesting the need for mitigation measures in those buildings. The analysis of bedrock samples revealed statistically significant correlations between 224 and 226 Ra activity concentration, and 220 Rn and 222 Rn exhalation and emanation potential. The borehole samples indicated a strong influence of weathering processes in the distribution of radioisotopes. The highest 226 Ra and 224 Ra activity concentration, and 222 Rn and 220 Rn exhaled per unit mass, TIRC and AID were observed in association with A-type red granites and porphyries. We conclude that both geological and anthropic factors, such as the type of building usage and building materials, must be considered in dose assessment studies and for the development of risk maps.