A physiologically based pharmacokinetic (PBPK) model was developed to describe the fate of radon within systemic tissues. A schematic diagram of the model is shown in figure A-1. The model is based on the blood-flow model of Leggett and Williams (1995) (see also Williams and Leggett 1989; Leggett and Williams 1991). The blood of the body is partitioned into a number of compartments, representing various blood pools in the body (the compartment Large Veins represents the venous return from the systemic tissues, Right Heart and Left Heart the content of the heart chambers, Alveolar represents the region of gas exchange in the lung, and Large Arteries represent the arterial blood flow to the systemic tissues). The gastrointestinal tract is divided into four segments (St, SI, ULI, and LLI) denote stomach, small intestine, upper large intestine, and lower large intestine, respectively, and Cont and Wall refer to the content and wall of the segments; for example St Cont and St Wall denote the content and wall of the stomach. Ingested radon enters the St Cont compartment while inhaled radon would enter the Alveolar compartment. The walls of the gastrointestinal tract are perfused with arterial blood which, with that from the spleen and pancreas, enters the portal circulation of the liver as shown in figure A-1. Radon dissolved in blood entering the Alveolar compartment exchanges with the alveolar air and is exhaled from the body. Although the kinetics of blood circulation are complex, for the most practical purposes it can be viewed as a system of first-order transfer among the different blood pools.

The model, a system of compartments, depicts the manner in which radon is distributed among the tissues of the body and subsequently removed from the