Bioassay Monitoring Studies for Thorium

A mathematical model for thorium metabolism in man, based on ICRP 30 information, was used to calculate bioassay limits for personnel occupationally exposed to 232Th. Bioassay monitoring was performed in a group of employees from an industry that commercially explores monazite sand, which contains 1 to 6% thorium oxides. The results from thorium concentrations in excreta samples of a group of workers were above the derived limits. Nevertheless, personnel have been employed in the plant for many years, and sometimes three generations of the same family have been working there. Cytogenetic studies in a group of workers showed no correlation between chromosome aberration frequencies and time of employment. Our studies have shown the urgent need for further development of bioassay methodologies for thorium monitoring, and for better metabolic data for the nuclide.