Mineral sands processing involves exposure to external and internal radiation sources. The level of exposure is associated with the production of monazite, which contains approximately 6% thorium by weight. External radiation levels may range from less than 1 µGy h-1 to 10 µGy h-1 in the general plant environment, to greater than 150 µGy h-1 in monazite storage areas. Internal radiation may also be significant since airborne gross alpha activity levels from less than 0.05 Bq m-3 to about 5 Bq m-3 are found. Recent estimates of radiation exposure indicate that approximately 15% of workers in the industry exceed 15 mSv y-1. A small percentage of workers are estimated to be receiving radiation doses approaching or exceeding the statutory 50 mSv y-1 limit. Whilst the majority of the radiation dose is contributed by inhaled radioactivity, recent reviews of ICRP models and data reveal that the assessment protocols and default values used in internal dose estimation may be overly conservative, possibly by more than an order of magnitude. The basis for present assessment procedures is briefly reviewed, as are the major areas of uncertainty in internal dose estimation protocols. Priorities for radiation research are suggested and the practicability of obtaining better estimates of internal radiation dose is discussed.
The concentration of thorium in the blood serum and urine of Western Australian mineral sands workers was studied to complement estimates of radiation dose derived from air sampling measurements. The concentration of thorium in urine samples from occupationally unexposed persons and pooled serum samples was also investigated. The concentration of thorium in the urine of the workers varied from 3-210 ng L-1 (geometric mean = 31 ng L-1, n = 34) while the concentration of thorium in the serum varied from 170-2,000 ng L-1 (geometric mean = 480 ng L-1, n = 25). No correlation was found between the bioassay results and cumulative airborne thorium exposure. The geometric mean ratio of daily excretion of thorium in urine to total thorium in the serum pool was 2.5%, considerably lower than the value of 10% proposed by the ICRP. These data indicate that more information is required to clarify the biokinetic models for thorium and that doses assessed from air sampling data must be interpreted with caution.
Thorium lung burdens have been measured in workers in the dry separation plants operated by the mineral sands industry in Western Australia. The data have been compared with historical employment records of the worker's exposure to thorium-bearing airborne dusts in order to assess the reliability of personal air sampling and with the predictions of the new Task Group lung model. The thoron exhaled in the breath of 62 workers was measured using a double filter tube. Six of the workers also underwent in-vivo gamma counting to determine their thorium lung burden. A thoron exhalation rate of 4.7% was obtained from a comparison of the two data sets. The estimated thorium lung burdens from the thoron-in-breath measurements had a geometric mean value of 10 Bq. The workers had a geometric mean employment period in the industry of 9.2 y and a geometric mean total inhaled alpha activity of 9,000 Bq, estimated from contemporary personal air sampling data and a retrospective assessment of previous workplace conditions. This exposure corresponds to a mean daily intake of 232Th of 0.45 Bq. Predictions from the new Task Group lung model indicate that, for the 45 workers with a thorium lung burden in excess of the minimum detectable level (6 Bq), the daily intake of 232Th is a factor of 1.6 higher than expected. This result suggests that previous intake of radioactive dust was higher than generally assumed for some workers. The application of the new Task Group lung models to the bioassay data results in an estimated mean annual committed effective dose for the workers of 8 mSv. Two workers (3%) were found to have been exposed for many years in excess of the 50 mSv y-1 annual limit for occupational exposure, while eight workers (13%) exceeded the ICRP's proposed new occupational standard of an average of 20 mSv y-1. All eight had been employed for more than 6 y and the majority of their exposure was attributed to early employment years, prior to extensive workplace improvements in dust control.
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