Soil and air radon/thoron exhalation rates and radon activity concentrations in the vicinity of Lake Monoun, West region of Cameroon

One crucial element to determine the radon amount in the environment is the rate at which radon is exhaled from the subsurface. To assess the radon and thoron potential exhalation in the vicinity of Monoun Lake in the West Region of Cameroon, 28 soil samples were collected around the lake. Radon activity concentration in soil gas and air and the related risk to radon inhalation were evaluated. Radon and thoron exhalation rates, mass exhalation rates, and radon soil gas activity concentrations were derived from a mathematical model as a function of radium content and soil parameters. Radium activity concentrations were estimated using a gamma-ray spectrometry technique with a High Purity Germanium detector (HPGe). The average radon exhalation rate, thoron exhalation rate, radon mass exhalation rate, and thoron mass exhalation rate were found to be 22.7 mBq.m-2.s-1, 5348 mBq.m-2.s-1, 0.018 mBq.kg-1.s-1, and 321 mBq.kg-1.s-1, respectively. The registered radon and thoron exhalation rates were above the world average values of 16 Bq.m-2.s-1 and 1000 Bq.m-2.s-1 published by UNSCEAR, respectively. The calculated thoron exhalation in the study area was about 75.3 times higher than that of radon, which suggests probably more exposure to thoron gas than the radon in the area, despite its shorter half-life. A good positive correlation was found between radon and thoron exhalation rates. The obtained average values of radon activity concentrations in air and annual effective dose were above the world average values of 10 Bq.m-3 and 0.095 mSv published by UNSCEAR, but still below the threshold values of 148 Bq.m-3 and 1.4 mSv set as action levels by USEPA, respectively.