NORM dust intake: comparison of personal air sampling and bioassays

Intake of naturally occurring radioactive materials (NORM), particularly the inhalation of thorium-bearing dust in mineral sand operations, has traditionally been assessed via personal air sampling (PAS). However, data from bioassay studies suggest that the actual intake determined via PAS may be underestimated. This review integrates thorium intake estimates derived from historical bioassay datasets, such as urine, faeces, lung counting, and thoron-in-breath tests, with historical PAS data. The findings indicated that bioassay-based predictions of intake were consistently greater, with mean bioassay-to-PAS intake ratios ranging from 2.6 to 4.2. The contributing factors to this discrepancy include biokinetic model uncertainties, potential dust sampler bias, the PAS strategy, high variability of exposure within similar exposure groups (SEGs), self-absorption of alpha particles during the radiometric analysis of filtered dust samples, and nonaccounting for respiratory protection. This review suggests improvements to current PAS-based dose assessments, including sampling and analysis protocols. We recommend that PAS be supplemented by targeted bioassay monitoring, where feasible, to improve dose estimation and worker protection in NORM industries.