Radon-Thoron Exhalation and Radiological Risk in Mineral-Enriched Beach Sand of the Chennai coast, India

The short-lived progenies of radon, thoron (²²⁰Rn), and radon (²²²Rn) are major global sources of ionizing radiation exposure and require continuous environmental monitoring because of their inhalation-related health risks. This study investigated the spatial distribution of soil-gas radon and thoron, their mass and surface exhalation rates, and the mineralogical controls on beach sands along the Chennai megacity coast in Southeast India. Twenty-four intertidal soil samples were analyzed using a RAD7 radon–thoron monitor (Durridge Co., USA), an electrostatic solid-state alpha detector optimized for thoron measurements. Radon mass exhalation rates ranged from 2 to 12 mBq/kg/h (mean: 2.42 mBq/kg/h), whereas thoron surface exhalation rates varied widely from 162 to 31,623 Bq/m²/h (mean: 3,688.08 Bq/m²/h). The highest soil-gas radon levels (4.5–12 Bq/m³) were observed in Kokkilamedu, a placer-rich zone. Exhalation rates were inversely related to grain size, with finer sediments releasing more radon and thoron than coarser sediments. However, higher heavy-mineral content, greater bulk density, and finer grain fractions also restricted exhalation, indicating strong mineralogical and textural control of radionuclide mobility. The sediments contained up to 12.5% heavy minerals, including ilmenite, zircon, and monazite, contributing to gamma radiation levels of up to 7.5 µR/h. The annual effective radon doses ranged from 0.12 to 0.45 mSv/y, remaining below the global safety limits. Elevated thoron exhalation in monazite-rich areas highlights the need for regular radiological surveillance, particularly among placer-deposit workers. These results provide baseline data on coastal sediment radiology and provenance to support environmental risk management and public health planning in the Chennai coastal zone.