Human biokinetics of inhaled terbium oxide.

Four healthy men inhaled a monodisperse aerosol of 160Tb-labelled terbium oxide particles. The behaviour of the tracer was studied through measurements of body radioactivity and of its urinary and faecal excretion. Estimated early faecal losses in the four subjects ranged from 3% to 31% of the initial respiratory-tract deposit; most of the residue had become systemic within a year, with the principal deposit apparently in bone. Interference from this systemic deposit prevented accurate determination of the long-term pulmonary clearance kinetics, but the pattern was broadly what would be expected for Type M materials in the ICRP's Human Respiratory Tract Model. Averaged trends in the whole-body residue after approximately 1 year suggest a clearance half-life of approximately to 5 y.