Aerosol deposition in models of a human lung bifurcation

Citation

Bell, Karl A. (1974) Aerosol deposition in models of a human lung bifurcation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9165-2r63. https://resolver.caltech.edu/CaltechETD:etd-09282005-162018

Abstract

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The bifurcations of the human upper respiratory tract are regions in which enhanced local deposition of airborne particles occurs. Existing mathematical models do not characterize these local "hot spots", which may be important in the development of certain respiratory diseases, the setting of air quality standards according to particle size, and the diagnosis and therapy of respiratory diseases by aerosols.

Idealized two-dimensional ("2-D") and three-dimensional ("3-D") models of the first bifurcation of the human lung with parent and daughter branches were used in theoretical and experimental studies designed to characterize local deposition patterns (Bell and Friedlander, 1973). Mono-disperse latex aerosols, having diameters from 0.088 to 7.6[...], were passed through the experimental models at unsteady flow rates simulating conditions of rest and moderate exercise in humans. Deposited particles were counted by optical and electron microscopy, and the aerosol concentration was measured either by gravimetry or by light scattering photometry. Deposition patterns of the "3-D" model data are depicted by computer plotted maps having contours of constant transfer coefficients. Steady potential flow around a wedge was employed to model local deposition by impaction, sedimentation and interception. Steady and quasi-steady laminar boundary layer flows along a wedge were used to model deposition by convective diffusion.

In both models data and theory from 0.088 to 7.6[...] for transfer coefficients were similar in trend to curves of deposition efficiency for the entire human lung or the collection efficiency for fibrous filters. Although similar average transfer coefficients are observed in the diffusion and impaction sub-ranges, the deposition patterns are strikingly different. Data < 0.5[...] agreed well with theories of convective diffusion and data > 2[...] agreed fairly well with theories of impaction, sedimentation and interception. The 0.5 - 2.0[...] data fall between the two theories on account of unsteady boundary layer effects not included in the theories. Effects of secondary flows on particle deposition must be taken into account in estimating local nonuniformities and "hot spots", however, they can be neglected when calculating the deposition efficiency for each branch. "3-D" model theories more accurately approximate the experimental efficiencies than the models of either Landahl or the ICRP Task Group on Lung Dynamics, and all "3-D" model theories and deposition patterns can be used to model deposition in the tracheobronchial trees of humans and animals. Measurements indicate that the 150,000 epithelial cells lining the lung wall near the carina receive 25.4 times more 5.7[...] particles than the average over the branch. The corresponding figure falls to 3.8 as particle size is reduced to the diffusion range. The relative intensity of "hot spots" calculated for cigarette smoke in the first three generations of the human lung roughly agreed with the frequency that bronchial carcinomas originate in each of these generations. In addition to local deposition rates, the relative rates of dissolution of irritant aerosols are crucial for modeling acute dose-response.

To assure a fairly uniform coating of therapeutic or diagnostic aerosol over > 75% of the surface of human airways, unit density particles with diameters < 1[...] can be inhaled at any flow rate. For maximum nonuniformity, least surface coverage, and maximum collection at bifurcations, unit density particles with diameters >= 5[...] should be inhaled rapidly.

The 1.1[...] deposition patterns and efficiencies were significantly different for unsteady and steady inhalations at the same time-averaged flow rates, and similar conclusions should be valid for all respirable particles. Therefore, steady flow can not be used in lung models to accurately estimate deposition in the real lung.

Results from the diffusion subrange can be used to estimate local transfer rates of pollutant gases in lung airways, gases and fat molecules at bifurcations in blood vessels, and heat and mass in other flow systems.

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