Use of iodine vapor to measure airway deposition of unattached

Measurements of deposition patterns and efficiencies of aerosol particles within realistic, physical models of the tracheobronchial airways of humans and experimental animals are used to develop quantitative predictive models for the deposition efficiency of the short lived decay products of radon. The fraction of 218Po that is not attached to ambient aerosol particles typically represents about 8% of airborne alpha activity but can be much higher. This unattached fraction is highly mobile and the portion that is inhaled, and penetrates the nasal region, will deposit efficiently in the central airways and contribute significantly to the tracheobronchial dose. To measure deposition of the unattached fraction of 218Po we have developed a system that uses gas molecules with the same diffusivity as those of subnanometer particles. This is based on the hypothesis that diffusion is the major mechanism for deposition in this size range, thus the diffusion coefficient will control deposition efficiency. We have tested methods that will allow us to utilize iodine gas as a surrogate for the unattached 218Po. The unattached particles are reported to have diameters of 1.1 nm but estimates range from about 0.5 to 5 nm. The diffusion coeffient of iodine gas is approximately the same as that of 0.8 nm particles. We found NaOH-impregnated charcoal powder (100-200 mesh) is not only an efficient absorbent for iodine gas, but also a good material for coating the interior surface of hollow airways casts. The absorption efficiency of NaOH-impregnated charcoal for iodine gas was tested in a glass tube (28.2 cm in length and 0.40 cm inner diameter) by passing a known quantity of iodine gas through the tube. The removal fraction can be theoretically predicted by diffusion theory. Excellent agreement was obtained between the results of the experimental measurements and theoretical calculation for flowrates ranging from 1 to 4 liters/minute