G N Stradling, A Hodgson, T P Fell, A W Phipps and G Etherington
Publication date: May 2004
This report is concerned primarily with the optimisation of procedures for assessing intakes and doses to workers after acute and repeated exposure to thorium nitrate and dioxide of natural isotopic composition. The biokinetics of other thorium compounds and other isotopic mixtures are reviewed only briefly.
For the purpose of making such judgements, the biokinetics of thorium are predicted by combining material specific absorption parameter values (derived from animal or in-vitro data) with deposition and particle transport parameter values from the ICRP (International Commission on Radiological Protection) Human Respiratory Tract Model, and the ICRP systemic model for thorium.
For assessing the most appropriate monitoring procedure, the advantages and disadvantages of external monitoring of the chest, the assay of thorium in urine and faeces and thoron ( 220Rn) in breath assay are discussed in some detail. For interpreting excretion data, measurement of both mass and radioactivity are considered.
It is concluded that for thorium nitrate and dioxide of natural composition, lung monitoring and urine assay are inappropriate for demonstrating compliance with a dose limit of 20 mSv after repeated exposure; urine assay is useful for assessing doses at the mSv level after acute exposure to thorium nitrate. For both chemical forms, faecal monitoring can be used with advantage for assessing doses below 1 mSv after acute exposure and for demonstrating compliance with a dose limit of 20 mSv after repeated exposure. For thorium dioxide, thoron in breath measurements are capable of assessing doses of less than 6 mSv from acute and repeated exposure.
Last reviewed: 1 September 2009