(listed above it), including 222Rn. The kinetic energy of the emitted alpha particles for the 222Rn series (24.5 MeV) accounts for 89% of the total emitted energy (27.6 MeV). A substantial fraction (78%) of the alpha energy is associated with the short-lived radon decay products (19.2 of 24.5 MeV). If an atom of 222Rn entered the body, in the absence of any biologic removal mechanisms for it or its decay products, the energies listed in table 4.2 would be available for deposition within the tissues of the body. However, ingested and inhaled radon is known to be promptly removed from the body by exhalation. Biologic removal processes are also applicable to the decay products formed within the body, but the short half-life of some decay products limits the importance of these removal processes. The decay products formed within the body may enter their own metabolic pathways and routes of excretion from the body. Ingested radon is removed from the body through exhalation while the longer-lived decay products are eliminated by urinary and fecal excretion.
The extent to which radon is absorbed from the gastrointestinal (GI) tract and retained in the body is determined, in part, by its solubility in blood and in the tissues. The solubility of the various noble gases in water (CRC 1996) at body temperature is shown graphically as a function of atomic mass in figure 4.1. Radon is considerably more soluble in water than the lighter noble gases—about 15 times as soluble as helium and neon. Data on solubilities of the noble gases in body tissues exhibit a similar relationship although the data are more variable.