Indoor and Outdoor Exposure to Radon, Thoron and Thoron Decay Products in a NORM Area with Highly Elevated Bedrock Thorium and Legacy Mines

Radon (²²²Rn) and thoron (²²⁰Rn), and especially their short-lived decay products, are major contributors to dose received by the public from naturally occurring radioactive material (NORM), particularly in areas with elevated levels of naturally occurring radionuclides. Mining in such areas can involve ventilation of high amounts of these gases, which may influence outdoor levels. In this work, we assessed indoor and outdoor levels of ²²²Rn, ²²⁰Rn and ²²⁰Rn decay products (TnDP) in close proximity to an area with elevated bedrock levels of thorium (²³²Th) and a NORM legacy mining site with high natural ventilation. We assess municipal buildings at distances from a few hundred meters to 2 km from the NORM legacy mines. In some buildings, high indoor levels of ²²²Rn were observed in winter, as expected for temperate areas. In summer, high indoor levels of ²²²Rn and ²²⁰Rn were observed in some buildings, and very low associated levels of TnDP in actively ventilated buildings may suggest entry by ventilation and an outdoor source. Outdoor levels of TnDP increased with decreased distance from the legacy mines, suggesting dispersal from these during both summer and winter.