Mechanisms for Long-term Retention of Plutonium in the Respiratory Tract: Inferences from Animal and Human Studies

The International Commission on Radiological Protection (ICRP) attributes the long-term retention of plutonium (Pu) in the respiratory tract partly to chemical binding, represented by the "bound fraction" within the human respiratory tract model (HRTM). Determining whether such binding occurs, and to what extent, is important for dosimetry and, consequently, for epidemiological and dose response modeling. Our analyses of data and review of literature show that the bound-fraction model fails to explain observed regional retention of Pu in the respiratory tract and contradicts many observations. To account for the observed retention of Pu in the respiratory tract of four individuals who inhaled Pu, we incorporated two mechanisms not included in current ICRP models but supported by substantial empirical evidence: (1) systemic uptake of Pu by the respiratory tract, and (2) physical encapsulation of Pu within scar tissues. Evidence of Pu in the lungs of an individual with a wound intake and in animals injected with Pu confirms and enables quantification of systemic uptake. Similarly, histological and autoradiographic observations demonstrate sequestration of Pu in fibrotic scar tissues. We therefore modified the ICRP-130 HRTM to include scar-tissue compartments, with transfer rates derived using Markov Chain Monte Carlo analyses of bioassay and post-mortem data. The model successfully described observed retention across materials ranging from soluble Pu nitrates to highly insoluble Pu oxides, predicting that scar-tissue retention accounts for 30-90% of total Pu activity in the respiratory tract, consistent with literature observations.