ATSDR Draft Toxicological Profile for Radon_September 2008.pdf

RADON 563. HEALTH EFFECTSrespiratory tract (see Figure 3-3). The ICRP (1994b) model defines three categories of solubility andreactivity: SR-0, SR-1, and SR-2:• Type SR-0 compounds include insoluble and nonreactive gases (e.g., inert gases such as H 2 , He).These compounds do not significantly interact with the respiratory tract tissues, and essentially allcompound inhaled is exhaled. Radiation doses from inhalation exposure of SR-0 compounds areassumed to result from the irradiation of the respiratory tract from the air spaces.• Type SR-1 compounds include soluble or reactive gases and vapors which are expected to betaken up by the respiratory tract tissues and may deposit in any or all of the regions of therespiratory tract, depending on the dynamics of the airways and properties of the surface mucousand airway tissues, as well as the solubility and reactivity of the compound.• Type SR-2 compounds include soluble and reactive gases and vapors which are completelyretained in the extrathoracic regions of the respiratory tract. SR-2 compounds include sulfurdioxide (SO 2 ) and hydrogen fluoride (HF).Radon gas is categorized by ICRP (1994b) as SR-1, because, even though it has a low reactivity, it issufficiently soluble to be taken up in the alveolar region where it can be absorbed into blood. ICRP(1994b) recommended default values for regional distribution of inhaled gases (except for those havinglow solubility) as follows: 10% ET 1 , 20% ET 2 , 10% BB, 20% bb, and 40% AI. Radon progeny, such as218 Po, 214 Pb, and 214 Bi are sufficiently reactive to attach to aerosols in the respiratory tract (and externalair) and deposit in the respiratory tract according to factors that determine particulate deposition (e.g.,sedimentation, inertial impaction, diffusion, and interception). Radon progeny are represented in theICRP (1994b) model and in extensions of the model (e.g., RADEP) as a mixed distribution of unattachedparticles (i.e., products of the initial reactions between progeny with gases and vapors) and attachedparticles (i.e., products of hygroscopic growth of complexes between unattached particles and aerosols inair). AMADs for the two fractions are typically represented in the ICRP model as 1 nm for unattachedparticles and 200 nm for attached particles (Butterweck et al. 2002; Ishikawa et al. 2003b), although theuse of more complex mixed distributions for attached particles has also been used (Marsh and Birchall2000; Porstendörfer 1994, 2001).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 such assmoking) (Marsh and Birchall 2000). The unattached fraction for typical indoor environments has beenestimated to be 5–20% (Porstendörfer 1994, 2001). NRC (1991) recommended a default value of 3% formodeling exposures in homes where smoking occurs and 5% for exposures during cooking or vacuum***DRAFT FOR PUBLIC COMMENT***