Thoron in the living environments of Japan

High thoron concentrations have been investigated in various living environments with radon-thoron discriminative measurements; however, behavior of the thoron and its decay products and their effects on human health have not been clearly elucidated yet. In Japan, some kinds of building materials rich in natural radionuclides of thorium series, such as clay and soil plaster, are commonly used for construction and/or surface finishing of wall in traditional Japanese style houses. Measurements with different types of monitor for thoron and its decay products were carried out to investigate general characteristics of thoron in Japanese living environments. From the result of the measurements, thoron concentrations ranged from almost 0 to more than thousands Bq m−3. Extremely high concentrations can be found in the places near wall surface, and it decreases drastically as distance from the surface of the wall increases. Thoron concentration was found to be changed drastically with time. The equilibrium equivalent thoron concentrations (EETC) ranged from nearly 0 to about 6 Bq m−3. Correlation between the thoron concentration and EETC was not good. This report summarized the results of an investigation of thoron in living environments of Japan.