An In-vivo Study on Clearance of Thorium from the Thorax

During routine monitoring of workers of a monazite processing plant, a few subjects were observed to have exceeded the normal levels of internal contamination. These subjects had chronic inhalation exposure at low levels for 25 to 30 years. Subsequently, they were removed from active work and subjected to whole-body counting at the thorax region, using a shadow-shield whole-body counter, over a period of time. Results of repeated counting, extended over a period of more than three years, indicated the clearance half-life for thorium to be 954 ± 97 days. As these subjects had chronic inhalation exposure for 25 to 30 years, the observed elimination may not be considered as lung clearance, but as thorax clearance. (author)