Committed Effective Dose from Thoron Daughters Inhalation

INTRODUCTION Mankind has been constantly exposed to radiation by natural radioactivity, nuclear tests, medical applications and nuclear energy activities. The natural radioactivity is the main component to human exposure and it is recognized that the most important contributors to the committed effective dose received by population due to natural sources are the short-lived decay products of radon (Rn) and thoron (Rn). The Earth crust contains trace quantities of U and Th which decay to Rn and Rn, respectively. In most human radon exposure studies, the contribution of thoron is usually neglected. The reason for this are: the Rn concentration is generally much lower than Rn and Rn has a relatively short half-life (55.3 s) than Rn (3.83 d). For a long time, it is well known that residents of high background areas like Brazil, China and India, where the predominant geological material is decomposed granite which has a relatively high content of Th (1), receive elevated doses, so it is incorrect to neglected Rn in radon exposure studies. However, recent studies shown that besides the residents of those high background areas, workers in the mineral sand processing industries receive elevated doses up to 180 mSv/y. In addition, recent surveys indicate that for members of general public living in certain Japanese wooden and mud houses the contribution from thoron daughters can represent a significant fraction of the total lung exposure (2). The thoron direct determination is very difficult, due to its very short half-life (55.3 s). So, to obtain the thoron concentration in the air, we use the concept of equilibrium equivalent concentration (EEC), defined as the thoron concentration, in equilibrium with its short-lived daughters, that has the same potential alpha energy concentration per unit volume as exists in a sample mixture. The equilibrium equivalent concentration is calculated through the Pb and Bi concentrations (3). The committed effective dose from exposure to thoron progeny was obtained from the thoron equilibrium equivalent concentration and dosimetric models for inhalation and distribution of airbone radioactivity by human body. These models incorporate the data on respiratory tract deposition and clearance of particles, as well as the behaviour of inhaled radionuclide in the body. The primary pathway for health detriment associated with Rn arise from inhalation of the short-lived decay products (Pb e Bi) and the resultant alpha particle exposure to the respiratory tract, but, due to the relatively long half-life of Pb (10.6 h), a considerable fraction of the energy deposited in the lungs is transferred to other organs. For this reason, the committed effective dose from thoron daughters inhalation was calculated considering the activity incorporated as being only the activity of Pb. At the Instituto de Pesquisas Energéticas e Nucleares (IPEN) there is a thorium purification plant where materials containing thorium among others elements are manipulated and a nuclear materials storage site where safeguarded materials are stored; consequently, thorium exists in high concentration and the thoron contribution can not be neglected. In this study, we assess the committed effective dose from thoron for workers of those two places.