ATSDR Draft Toxicological Profile for Radon_September 2008.pdf

RADON 393. HEALTH EFFECTSof the toxicokinetics of lead. A further complication in relating radon toxicokinetics to adverse healtheffects associated with exposure to radon is that radon progeny are present with radon in the environmentand are inhaled or ingested along with radon. Progeny formed in the environment contribute substantiallyto radiation dose associated with environments that contain radon gas (Kendall and Smith 2002).3.4.1 Absorption3.4.1.1 Inhalation ExposureInhalation exposures to radon gas deliver the gas into the respiratory tract along with aerosols of radonprogeny (e.g., 214 Bi, 214 Pb, 218 Po) that form as a result of the progeny reacting with natural aerosols in theair (Marsh and Birchall 2000). Longer-lived radon progeny (e.g., 210 Pb and 210 Po) contribute little to theradiation dose to lung tissue because they have a greater likelihood of being physically cleared from thelung by mucociliary or cellular transport mechanisms before they can decay and deliver a significantradiation dose.Progeny aerosol formation involves distinct physical-chemical processes (Butterweck et al. 2002;El-Hussein et al. 1998; Ishikawa et al. 2003b): (1) immediately after formation, progeny react with gasesand vapors and form clusters, referred to as unattached particles, having diameters of approximately 0.5–3 nm or (2) unattached particles form complexes with other aerosols in air to form attached particles,which can undergo hygroscopic growth to achieve diameters ranging from approximately 50 to 1,500 nm.The magnitude of the unattached fraction in inhaled air depends on the concentration and size distributionof aerosols in the ambient environment, and will vary with the exposure conditions (e.g., indoor, outdoor)and activities of the individual (e.g., sleeping, activities that release particulates into the air) (Marsh andBirchall 2000). The unattached fraction for typical indoor environments has been estimated to be 5–20%of the total airborne potential alpha energy concentration (PAEC) (Porstendörfer 1994, 2001). The PAECgives a measure for the potential energy originating from the alpha decays of radon progeny in air.Smoking and other aerosol-generating activities (e.g., vacuum cleaning, cooking, fireplace and circulatingfan usage) will decrease the unattached fraction and dose (Sun 2008).Deposition and the subsequent absorption of inhaled radon and radioactive decay progeny are influencedby physiological factors as well as chemical and physical characteristics of the radionuclides and carrieraerosols. Radon is a relatively nonreactive gas, and deposition and absorption will be determined largelyby its solubility in tissues and blood flow to the lungs. The blood:air partition coefficient for radon hasbeen estimated to be approximately 0.4 (Nussbaum and Hursh 1957; Sharma et al. 1997); therefore, at***DRAFT FOR PUBLIC COMMENT***