Further Thorium Lung Burden Data on Mineral Sands Workers

Abstract

The thorium lung burdens of 19 workers in the mineral sands industry of Western Australia were measured using in vivo gamma counting. The thorium lung burdens ranged from 11 Bq to 181 Bq with a geometric mean of 42 Bq (GSD = 2.2). Measurements of the amount of exhaled thoron in breath were conducted on 145 workers using a double filter tube method. The activity of 224Ra freely emanating 220Rn at the mouth ranged from the minimum detection limit of 0.4 Bq to 3.6 Bq. A comparison of the data for workers who had undertaken both in vivo lung counting and breath measurements yielded a mean thoron exhalation rate of 3.7 ± 1.2%. The estimated geometric mean thorium lung burden of the workers from the thoron-in-breath tests was 10 Bq (GSD=2.2), corresponding to a geometric mean employment period of 9.0 y (GSD=2.0). The lung burdens from the exhaled thoron data were compared with the predicted lung burdens for each worker, as estimated from the total inhaled activity from personal air sampling records and a retrospective assessment. The average ratio of measured to predicted values was about a factor of two higher than predicted which suggests that a review of parameters used in the internal dose assessment protocol may be necessary. The discrepancy could be due to the accuracy of the inhaled activity, the validity of the metabolic data (e.g. pulmonary deposition fraction, biological clearance half-life), the particle size of dust, or the mode and rate of breathing. A dose assessment of the 64 workers above the minimum detection limit using the ICRP 66 lung model predicts a geometric mean value of 16 mSv.y-1 for the annual committed effective dose from intake of thorium ore dust over an employment period of 11.5 y. Seventeen per cent of the workers were estimated to have an average annual internal dose in excess of 20 mSv over the duration of their employment in the industry. Most of the inhaled radioactivity was attributed to early years of employment since extensive dust control measures have been implemented in recent years.