The Deposition of Unattached Radon Progeny in a Tracheobronchial Cast as Measured with Iodine Vapor

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Abstract
The deposition of the unattached radon progeny in hollow cast models of the human tracheobronchial region was studied using iodine vapor. The experiments were conducted in a replicate cast whose inner surface was coated with NaOH impregnated charcoal powder. This coating can trap iodine molecules by converting iodine into iodide and iodate, so that the iodine gas molecules behave like particles and stick to the surface upon contact. The iodine vapor is selected as a surrogate of radon progeny because the effective diffusion coefficient of iodine vapor, 0.08 cm2 s−1, is close to the diffusivities of unattached radon progeny (0.03–0.07 cm2 s−1). Deposition experiments have been conducted under constant and cyclic inspiratory flow between 5 and 30 LPM. It was found that the deposition of iodine vapor under constant flow can be described by diffusion in laminar flow. The cyclic inspiratory flow pattern does not significantly change the total deposition in the tracheobronchial cast. This observation, combined with the enhanced particle deposition due to charge (Cohen et al., 1996) suggest that particle charge plays an important role in the deposition of submicron particles in human airways.