Thoron (220Rn) is a gaseous decay product of232Th decay chain which becomes a potential source of internal contamination for individuals who handle significant amounts of ores and raw materials containing thorium. With the objective of establishing an occupational control of such workers, a methodology which permits, by means of in-vivo measuremens, the determination of the level of internally deposited220Rn daughters (212Pb,212Bi, and208TI), was developed at the Institute of Radiation Protection and Dosimetry/Brazilian Nuclear Energy Commission (IRD/CNEN) whole body counter. Previous calibration of an 8×4 NaI(Tl) detector was carried out using the lawrence Livermore National Laboratory (LLNL) thorax phantom with the lung cavity filled withi plastic bags containing28Th standard solution. After sealing, the bags are stored until radioactive equilibrium is reached. Patients are measured for I h in two steps, at an interval of approximately 20 h, Positioning the detector over the thorax. A comparison of the activities determined at each measurement allows the distrimination of the contribution of the internally deposited shoret-and longlived radionuclides. This technique was recently applied to a group of workers at a gas mantle industry in Rio de Janeiro. Results of these measurements will be shown.