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

Indoor radon (Rn-222) and thoron (Rn-220) are the most important natural sources of ionizing radiation to the public. Radiological studies that assess simultaneously Rn-222 and Rn-220, and their controlling factors are particularly scarce in African countries. Hence, we conducted a survey of indoor Rn-222 and Rn-220 in buildings located in the SW region of Angola. Bedrock samples were also collected, and a borehole was executed to assess Ra-226 and Ra-224 activity concentration, Rn-222 and Rn-220 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 Rn-220 to the TIRC averaged 35% but may reach 95%, demonstrating the relevance of discriminating radon and thoron in indoor radon surveys. Indoor Rn-222 and Rn-220 were not correlated, indicating both must be estimated to properly assess the AID. Indoor Rn-220 concentrations were statistically different according to the building materials and type of usage. Higher Rn-222 and Rn-220 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 Ra-226 activity concentration, and Rn-220 and Rn-222 exhalation and emanation potential. The borehole samples indicated a strong influence of weathering processes in the distribution of radioisotopes. The highest Ra-226 and Ra-224 activity concentration, and Rn-222 and Rn-220 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.