Deposition bifurcations

This apparent contradiction is best explained by the ensemble behavior of dense mainstream smoke, described by Fuchs (1964). When undiluted mainstream tobacco smoke is passed horizontally into a chamber, the dense smoke column settles as an ensemble, with a gravitational settlement rate equivalent to that for individual particles of 47 /mi diameter. Such behavior can explain the remarkable retention of undiluted smoke (Hinds et al., 1983) as well as the highly nonuniform pattern of deposition for inhaled smoke tars observed by Ermala and Holsti (1955). The heaviest tar deposits occur where the dense smoke column strikes directly on projecting surfaces of the pharynx and the larynx, and at the tracheal and bronchial bifurcations (Er-... 

The deposition of ultrafine particles has been measured in replicate hollow casts of the human tracheobronchial tree. The deposition pattern and efficiency are critical determinants of the radiation dose from the short lived decay products of Rn-222. The experimental deposition efficiency for the six airway generations just beyond the trachea was about twice the value calculated if uniform deposition from laminar flow is assumed. The measured deposition was greater at bifurcations than along the airway lengths for 0.2 and 0.15 ym diameter particles ... 

After exposure, the outside surface of the cast was cleansed until the activity of the washes was less than 10X the background of a gamma well scintillation counter. The cast was cut into separate bifurcations and airway sections and each section was counted to determine the amount of aerosol deposited. Sane samples contained both airway and bifurcation sections because of the complex configuration of the cast. For combination samples, the total activity deposited was equally apportioned between each of the airways and bifurcations. End airways were included for determination or total deposition but not in any of the analyses because flow disturbances at open ends may have affected deposition. The surface area of each sample was measured separately. The surface density for each cast segment was calculated by dividing the activity measured in the sample by the interior surface area of that sample. 

The mean measured activity per unit surface area are shown for airways and bifurcations separately in Table II. These data are for those segments which contained only airway lengths bifurcations. The results are given as the number of particles which deposit per cm2 for 10 particles which enter the trachea. This assumes that the particle and activity distributions are equivalent. For the 0.2 and 0.15 ym particles the surface density at the bifurcations is greater than that along the airway lengths at p <. 01 when the paired data are compared by a one tailed t-test. 

For regions in which the flow is not quasisteady, a transient-flow solution may be possible. For example, Lakin and Lakin and Fox developed a two-dimensional transient-flow solution for an idealized symmetric bifurcation during the period at the end of inspiration and before expiration. Their finding that vortidty decreases at the carina or bifurcation apex suggests that particle- and gas-deposition rates may be increased at these sites in the respiratory tract. It also suggests that reactive-gas deposition rates during normal oscUlatory breathing differ... 

Ka can be defined as a gas-phase transfer coefficient, independent of the liquid layer, when the boundary concentration of the gas is fixed and independent of the average gas-phase concentration. In this case, the average and local gas-phase mass-transfer coefficients for such gases as sulfur dioxide, nitrogen dioxide, and ozone can be estimated from theoretical and experimental data for deposition of diffusion-range particles. This is done by extending the theory of particle diffusion in a boundary layer to the case in which the dimensionless Schmidt number, v/D, approaches 1 v is the kinematic viscosity of the gas, and D is the molecular diffusivity of the pollutant). Bell s results in a tubular bifurcation model predict that the transfer coefficient depends directly on the... 

Values for the average vapor-transfer coefficient from the gas phase to the airway epithelium can also be estimated from heat-transfer data in straight, curved, or bifurcating cylindrical tubes by using the analogy between heat transfer and mass transfer. Such an approach has been used by Yeh to predict the diffusional deposition of small particles in the conducting airways. 

FIGURE 7-6 Transfer-coefficient contours during inhalation for 0.088-Mm-diameter particle.deposition in the daughter branch of the three-dimensional bifurcation model shown in Figure 7-1. The time-averaged velocity in the parent branch, U, is 100 cm/sec. Total surface area 18.6 cm X distance (in centimeters) downstream from the Carina Y - deposition locations around the branch circumference with F - 0 at the Carina. Each unit of the ordidnate corresponds to a distance of 0.215 cm. Adapted from Bell. ... 

Bell, K. A. Aerosol Deposition in Models of a Human Lung Bifurcation. Ph.D. Thesis. Pasadena California Institute of Technology, 1974. 426 pp. 

Bell, K. A., and S. K. Friedlander. Aerosol deposition in models of a human lung bifurcation. Staub-Reinhalt Luft. 33 178-182, 1973. 

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 ... 

Third, the line can become unstable during laser writing, and instead of a single line, a periodic pattern of discrete deposits is obtained (233-235). This pattern is analogous to bifurcations in other spatially distributed systems, such as catalytic fixed-bed reactors, and can be analyzed in the same manner (235). 

Deposition by diffusion is the main mechanism for particles smaller than 0.5 pm, and is important in bronchioles, alveoli, and bronchial bifurcations. Aerosol particles are displaced by a random collision of gas molecules this results in particle collision with the airway walls [24]. Deposition by diffusion increases with the decrease in particle size, and breath-holding following inhalation was also found to increase this deposition [25]. 

This is the dominant deposition mechanism for particles >1 pm in the upper tracheobronchial regions. A particle with a large momentum may be unable to follow the changing direction of the inspired air as it passes the bifurcations and as a result will collide with the airway walls as it continues on its original course. 

Yao et al. [26] combined spectro-electrochemistry at grazing incidence angle with EQCM to study copper deposition from ammonial solutions. They also reported the use of a bifurcated fibre optic reflection spectro-electrochemical cell combined with an EQCM for the study of poly-naph-thylamine film growth on platinum from acetonitrile solutions. 

Figure 27.19. Illustration of early asbestos-induced fibroproliferative lesions in a rat inhalation model. (1) Inhaled fibers deposit at alveolar duct bifurcations. (2) Within 24 hr, macrophages accumulate at sites of fiber deposition and become activated by fibers to secrete growth factors. (3) Within 72hr fibroblasts proliferate.

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