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Reference: Sarangapani, R. and Wexler, A.S. 2000. The role of dispersion in particle deposition in human airways. Toxicol. Sci. 54:229-36 Dispersion and Particle Deposition Does dispersion affect the total deposition of inhaled particles? Can aerosol dispersion be included in models to assess particulate deposition in human airways? Several processes are responsible for particle deposition in the lung. These include impaction, diffusion, and sedimentation. Several lung models are available that incorporate these processes in computing deposition rates and efficiency. However, dispersion is a process that occurs concurrently with deposition in the human airways, but is not currently incorporated into deposition models. Dispersion arises due to a convective mixing process between the inhaled bolus and the residual air in the lung. It is the result of both axial streaming during inhalation and radial mixing during exhalation. This is inherent given the bifurcating geometry and diameter changes along the daughter airways. The relative contribution to dispersion of mixing processes and streaming processes during inhalation determines the distance that a bolus penetrates in the pulmonary bifurcating airways as well as the location of deposition. Two existing models, the compartment and trumpet models account for axial streaming during inhalation, but neither one accounts for factors leading to dispersion. Recently, Sarangapani and Wexler published the results of a model that they developed that incorporates dispersion into a deposition model. This model allowed them to study total lung deposition as well as the influence of dispersion in regional particle dosimetry in the lung. The authors compared the results of modeled derived total lung deposition fraction (with and without dispersion) to experimentally measured total lung dose in humans for a wide range of particle sizes. The authors observed that their model predictions compare well with experimental results for the coarse particles (> 3um) and the particles of ultrafine size (<50 nm), as well as to the results of other published dosimetry models. They noted that with respect to fine mode (intermediate) size particles(0.1-1.0 um), their model under-predicted total lung deposition. Since the total deposited fraction is the sum of the fraction deposited during the breathing maneuver (including impaction, diffusion, & sedimentation) and the fraction retained at the end of exhalation, incorporating the entrained fraction into total deposition, the model correlates well with the experimental data for intermediate sized or fine-mode particles. Over a series of breathing cycles, the entrained particles will move distally (axial streaming) and finally, flood the lung with particles. With respect to regional particle dosimetry, their results showed that dispersion results in a small decrease in the conducting airway deposition fraction, but results in a much larger increase in the pulmonary region. Their reasoning follows that axial streaming causes particles to spend less time in the conducting region and allows them to penetrate deeper into the lung. Since daughter airways are smaller, the particle deposition efficiency due to diffusion and sedimentation increases, thereby enhancing pulmonary deposition. It appears that properly counting for dispersion in deposition models will aid in the interpretation of clinical and epidemiological results associated with particle deposition in the lung. Reference: Sarangapani, R. and Wexler, A.S. 2000. The role of dispersion in particle deposition in human airways. Toxicol. Sci. 54:229-36 By Arlene L. Weiss, MS, DABT |