Thorium ore dust research applicable to mineral sands industry workers

Historically, radiation exposure to mineral sands workers arose primarily from intake of thorium associated with monazite dust generated in mineral separation plants. Research investigations in the 1990s provided greater insight into the characteristics of inhaled thorium ore dust and bioassay studies inferred that some workers had accumulated significant lung burdens of thorium. Recent changes to biokinetic models have increased the radiation dose assessed to arise from thorium intake, raising questions on the appropriateness of current assumptions used in exposure assessment and feasibility of further bioassay research. Past radiation research undertaken in the Western Australian mineral sands industry is summarised and findings from contemporary research relevant to thorium ore dust exposure, thorium health effects and the associated assessment of internal radiation dose are reviewed and analysed. Radiation exposures in the industry have reduced substantially in the last two decades, however current workplace exposure measurement regimes may not reflect the actual intake of monazite-bearing dusts on an individual basis. Past research indicated that thorium associated with monazite dust is relatively insoluble and avidly retained in the lung. There is a paucity of published research on thorium retention and excretion by mine workers over the last 20 years, however significant advances have been made in the detection of thorium in biospecimens. Improvements in measurement technology should make periodic bioassay measurements feasible for selected long-term workers involved in the mining and processing of naturally occurring radioactive materials. Past worker dose estimates require re-evaluation following recent updates to biokinetic models and long-term follow up of the health of workers chronically exposed to thorium ore dusts is recommended.