Bronchial tree, deposition

The most significant information given in Table I is that the radon daughter ions do contribute to the total internal dose if only at about 3% in this study. The effect of free ions in contrast with daughters attached to aerosols is substantial when the mechanics of deposition in the bronchial tree and lungs are taken into account. The dose from "unattached" Po-218, which includes the Po-218 positive ions as a major component, can be from 3 to 40 times that of the attached Po-218 (NCRP, 1984). Current dosimetry models allow for the important role played by Po-218 in small-ion form. Hence, their effect is significantly greater than the 3% contribution to the PAEC (WL) shown in Table I. 

Figure 4. Deposition of submicrometer aerosols measured for cyclic flow through a hollow cast of the human bronchial tree, compared with calculated values.

Martin, D. and W. Jacobi, Diffusion Deposition of Small-sized Particles in the Bronchial Tree, Health Phys. 23 23-29 (1972). 

Tu and Knutson (1984) also measured the particle deposition of hydrophobic and hygroscopic particles in the human respiratory tract. They showed that the hygroscopic particles grow by a factor of 3.5 to 4.5 at the saturated humidity present in the lung. For the purpose of calculating bronchial deposition for a hygroscopic aerosol we assume an increase in size by a factor of 4 upon entry into the bronchial tree. 

It is well known that enhanced deposition in the first few airways occurs due to the turbulence produced. Turbulent diffusion is accounted for by using factors (ratio of observed deposition to calculated diffusion deposition) to correct the diffusion deposition. These had formerly been measured by Martin and Jacobi (1972) in a dichotomous plastic model of the upper airways. The data used here are from measurements performed by Cohen (1986) using hollow casts of the upper bronchial tree which included a larynx. This cast was tested using cyclic flow with deposition measured for 0.03, 0.15 and 0.20 urn diameter particles. Her turbulent diffusion factors are used in the calculation here (14 for generation 0, and 2 for generations 1 to 6). 

The size of the fibrous particles that appear to induce disease in the animal models is compatible with the measured respiratory range in humans (Lipp-man, 1977). Most particulate deposition takes place not in the upper or conducting portion of the airways but in the alveolar region of the pulmonary tree (the respiratory unit). Some surface deposition may occur at bifurcations in the bronchial tree, but the actual amount at each location is influenced by anatomy, specific to the species—probably to an individual—as well as the variety of fiber. A large proportion of airborne particulates are rejected as part of the normal clearance mechanisms in animals, but in humans clearance mechanisms may be compromised by smoking, for example. We are unaware of any experiments on fiber toxicity using smoking rats ... 

Martin D, Jacobi W. 1972. Diffusion deposition of small-sized particles in the bronchial tree. Health Phys 23 23-29. 

Yeates DB, Gerrity TR, Garrad CS. Particle deposition and elearanee in the bronchial tree. Ann Biomed Eng 1981 9 577 592. 

Phalen RF, Oldham MJ, Schum GM. Growth and ageing of the bronchial tree implications for particle deposition calculations. Radiat Prot Dosim 1991 38 15-21. 

Experimental studies in idealized bronchial structures (e.g., 97-103) or in replicates of casts (e.g., 104-112) allow inhalation of substantial amounts of particles into easily accessible structures without having to deal with secondary redistribution phenomena caused by continuous particle clearing or lung preparation processes. They are performed to obtain information on primary deposition patterns with finer spatial resolution than is possible in in vivo experiments. These types of measurements may also be considered as the experimental match, validating theoretical approaches on particle deposition (see later). Mean local deposition rates, but also the heterogeneity of particle deposition within bifurcating zones, were investigated. Many studies (98-103,105,111) specifically focused on localization of so-called hot-spots, which are sites of enhanced deposition within the larynx or the tracheobronchiolar tree and may be most important rela-... 

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