Differential clearance of plutonium and americium oxides from the human lung

External X- and gamma-ray counting techniques have been used to study the retention of 238PuO2 and 241AmO2 in the lungs of a worker between 7 and 869 days after simultaneous exposure to aerosols of both oxides. From these data and measurements of early faecal excretion, it is concluded that approx. 50% of the amount of each nuclide initially deposited in the subject was removed during the first few days by ciliary clearance mechanisms. The lung clearance patterns of the other 50% differed for the two nuclides. The 238Pu was apparently cleared with a biological half-life of approximately 800 days, predominantly by systemic or lymphatic uptake, with at least part of the translocated material being subsequently deposited in bone. There was no evidence for the presence of a 30-day component of lung clearance, such as has been suggested elsewhere. In contrast, most of the residual 241Am was cleared relatively quickly, with a half-life of approximately 11 days, although a small proportion was apparently subject to long-term retention, with a half-life of approximately 900 days. The data for both oxides conflict in important respects with the patterns of lung clearance postulated by the ICRP.