Thorium lung burdens of NORM workers revisited

Historical thorium bioassay data collected from Australian mine workers in the 1990s, including in vivo lung counting and thoron-in-breath (TIB) measurements, were re-evaluated using current dosimetric and biokinetic models. Revised daily thorium intakes from bioassay were compared with previous estimates to assess changes in the dose profile and potential implications for contemporary operations. Reanalysis revealed lower estimates of annual dose, including the number of workers assessed as exceeding 20 mSv. However, it was found that intakes derived from industry personal air sampling (PAS) underestimated intake by up to three-fold. This study determined that sensitive bioassay techniques, such as TIB using an electrostatic collection chamber, are feasible for detecting low thorium lung burdens in longer-term workers at current mineral processing operations involving naturally occurring radioactive materials (NORM). This study improves the accuracy of historical exposures and highlights that PAS protocols require improvement to enhance radiation protection practices in industries handling NORM.