Thorium metabolism and bioassay of mineral sands workers

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.