Particle deposition in the guinea pig respiratory tract

Equations relating particle size of aerosols to deposition by impaction, diffusion and sedimentation are applied to a previously established model of the guinea pig lung using a tidal volume of 4.44 cm³ and a respiratory rate of 60 breath min⁻¹. These calculated deposition values are combined with measured values of nasal deposition to give an estimate of the particle deposition characteristics of the guinea pig respiratory tract. The nasopharyngeal-tracheobronchial (NP-TB) region removes 99% of unit density spherical particle 10 μm or more in diameter. Deposition in this region reaches a minimum of 10% at a particle diameter of 0.8 μm. For particles less than 0.8 μm, deposition increases because of diffusion. Deposition in the pulmonary region is about 17% for particle diameters from 0.08 to 4 μm. For typical polydisperse aerosols with mass median diameters above 1 μm, a greater fraction of the mass than of the count is deposited in the NP-TB region, while a smaller fraction of the mass than of the count is deposited in the pulmonary region. Aerosol clouds with mass median diameters less than 0.1 μm deposit a greater fraction of the count than of the mass in the NP-TB region and a smaller fraction of the count than of the mass in the pulmonary region.     

Measurements of the total and regional deposition of inhaled particles in the human respiratory tract

The total and regional deposition of monodisperse aerosols in the human respiratory tract has been measured in 12 healthy subjects breathing through the mouth. Radioactively labelled polystyrene particles in the aerodynamic diameter range 3.5–10.0 μm were employed. The total deposition results are similar to those reported by Stahlhofen et al. (1980), showing only a slight progressive increase with particle size, from a mean fraction of 0.79 of the inhaled aerosol at 3.5 μm, to 0.88 for 10 μm particles. The extrathoracic airways show a very marked deposition at all sizes, predominantly in the throat. The throat values rise rapidly from a mean of 0.09 at 3.5 μm to 0.36 at 10 μm particle diameter. Two intrathoracic fractions were also obtained by the widely accepted method of measuring the relative amounts of activity cleared from the thorax as a function of time. Alveolar deposition was apparently still some 0.06 of the inhaled aerosol at 10 μm particle diameter. Tracheo-bronchial deposition showed little change at any particle size except at 3.5 μm, when it was 0.24 of the inhaled aerosol.     

Aerosol Deposition in the Extrathoracic Region

The extrathoracic region, including the nasal and oral passages, pharynx, and larynx, is the entrance to the human respiratory tract and the first line of defense against inhaled air pollutants. Estimates of regional deposition in the thoracic region are based on data obtained with human volunteers, and that data showed great variability in the magnitude of deposition under similar experimental conditions. In the past decade, studies with physical casts and computational fluid dynamic simulation have improved upon the understanding of deposition mechanisms and have shown some association of aerosol deposition with airway geometry. This information has been analyzed to improve deposition equations, which incorporate characteristic airway dimensions to address intersubject variability of deposition during nasal breathing. Deposition in the nasal and oral airways is dominated by the inertial mechanism for particles >0.5 w m and by the diffusion mechanism for particles <0.5 w m. Deposition data from adult and child nasal airway casts with detailed geometric data can be expressed as E n = 1 m exp( m 110 Stk), where the Stokes number is a function of the aerodynamic diameter ( d a ), flow rate ( Q ), and the characteristic nasal airway dimension, the minimum cross-sectional area ( A min ). In vivo data for each human volunteer follow the equation when the appropriate value of A min is used. For the diffusion deposition, in vivo deposition data for ultrafine particles and in vivo and cast data for radon progeny were used to derive the following deposition: E n = 1 m exp( m 0.355 S f 4.14 D 0.5 Q m 0.28 ), where S f is the normalized surface area in the turbinate region of the nasal airway, and D is the diffusion coefficient. The constant is not significantly different for inspiratory deposition than for expiratory deposition. By using the appropriate characteristic dimension, S f , one can predict the variability of in vivo nasal deposition fairly well. Similar equations for impaction and diffusion deposition were obtained for deposition during oral breathing. However, the equations did not include airway dimensions for intersubject variability, because the data set did not have airway dimension measurements. Further studies with characteristic airway dimensions for oral deposition are needed. These equations could be used in lung deposition models to improve estimates of extrathoracic deposition and intersubject variability.     

Particle Deposition in a Cast of Human Oral Airways

Oral and nasal airways are entryways to the respiratory tract. Most people breathe through the nasal airway during rest or light exercise, then switch to oral/nasal breathing during heavy exercise or work. Resistance through the oral airways is much lower than through the nasal airways, so fewer aerosol particles are deposited in the oral airways. Aerosol drugs are usually delivered by inhalation to the lung via the oral route for that reason. Oral deposition data from humans are limited, and those available show great intersubject variability. The purpose of this study was to investigate the effects of particle size and breathing rate on the deposition pattern in a human oral airway cast with a defined geometry. The airway replica included the oral cavity, pharynx, larynx, trachea, and 3 generations of bronchi. The oral portion of the cast was molded from a dental impression of the oral cavity in a human volunteer, while the other airway portions of the cast were made from a cadaver. Nine different sizes of polystyrene latex fluorescent particles in the size range of 0.93-30 mu m were used in the study. Regional deposition was measured at a constant inspiratory flow rate of 15, 30, and 60 L min^-1. Deposition in the oral airway appeared to increase with an increasing flow rate and particle diameter. Deposition at the highest flow rate of 60 L min^-1 was close to 90% for particles >20 mu m. Particles> about 10 mu m deposited mainly in the oral cavity. Deposition efficiency has been found to be a unique function of the Stokes number, suggesting that impaction is the dominant deposition mecha nism. Oral deposition can be approximated by a theoretical deposition model of inertial impaction in a 180 degrees curved tube, assuming perfect mixing in a turbulent flow. Our model suggests that the minimum dimension near the larynx and the average cross-sectional area are important parameters for oral airway deposition; however, additional data from the oral airway replica are needed to ascertain whether these are indeed the critical dimensions. Information from the present study will add to our knowledge of the deposition mechanism, the correlation of particle deposition with airway geometry, and eventually the best way to deliver aerosol drugs.     

Deposition of Ultrafine Aerosols in a Human Oral Cast

This paper reports experimental measurements of the total deposition of ultrafine aerosols in a human oral airway cast. A clear polyester resin cast of the upper airways of a normal human adult, including the nasal airways, oral cavity, tongue, nasopharynx, and larynx, was made from a postmortem solid cast. Measured pressure drop in the oral airway was slightly lower than in the nasal airway. The measured oral flow resistance was similar to the values reported for human volunteers breathing through the mouth at rest and for spontaneously opening of the mouth. Aerosol deposition data in the cast for monodisperse NaCl aerosols between 0.2 and 0.005 μm in diameter deposited in the cast were obtained for inspiratory and expiratory flow rates of 4, 20, and 40 L/min. Deposition efficiency increased with decreasing particle size and flow rate indicating that turbulent diffusion was the dominant mechanism for deposition. Higher deposition efficiency was observed for inspiratory flow in the oral airway than for expiratory flow. Oral deposition and nasal deposition for inspiratory flow were similar, but oral deposition was lower for expiratory flow. Deposition efficiency can be expressed as a function of the flow rate and diffusion coefficient of the particle.     

Physical factors determining particle deposition in the human respiratory tract

Total deposition in the human respiratory tract of uncharged, spherical aerosol particles depends on particle diameter, particle density, period of a breathing cycle and respiratory volumetric flow rate. Total deposition of particles larger than 0.5 μm in diameter increases at mouth-breathing with increasing values of these physical factors with the exception that total deposition of particles in the size range between 0.5 and 1 μm is independent of flow rate. These physical factors can be linked by a deposition parameter so that their complex effects on deposition can be described by an unique relationship.     

Deposition of Fiber in the Human Nasal Airway

Inhalation is the main route for aerosol entering the human body. Many occupational lung diseases are associated with exposure to fiber aerosol in the workplace. However, very few studies to date have been conducted for investigating fiber deposition in the human airway. As a result, there is a notable lack of information on the nature of the fiber deposition pattern in the human respiratory tract. With this in mind, this research consisted of a large number of experimental works to investigate the effects of fiber dimension on the deposition pattern for a human nasal airway. Carbon fibers with uniform diameter (3.66 μm) and polydispersed length were adopted as the test material. Deposition studies were conducted by delivering aerosolized carbon fibers into a nasal airway replica (encompassing the nasal airway regions from vestibule to nasopharynx) at constant inspiratory flow rates of 7.5, 15, 30, and 43.5 l/min. Fibers deposited in each nasal airway region were washed out and the length distribution was determined by microscopic measurement. The results showed that impaction is the dominant deposition mechanism. Most of the fibers with high inertia deposited in the anterior region of the nasal airway (vestibule and nasal valve). In contrast, fibers with low inertia were found to pass through the entire nasal airway easily and collected on the filter at the outlet. Comparing the deposition results between fibers and spherical particles, our data showed that the deposition efficiencies of fibers are significantly lower than that of spherical particles, which implies that the inhaled fibers could pass through the entire nasal airway comparatively easier than spherical particles. Thus, relatively more fibers would be able to enter the lower respiratory tract.     

Particle Deposition in Ventilation Ducts: Connectors,Bends and Developing Turbulent Flow

In ventilation ducts the turbulent flow profile is commonly disturbed or not fully developed, and these conditions are likely to influence particle deposition to duct surfaces. Particle deposition rates at eight S-connectors, in two 90° duct bends and in two ducts where the turbulent flow profile was not fully developed were measured in a laboratory duct system with both bare steel and internally insulated ducts with hydraulic diameters of 15.2 cm. In the bare-steel duct system, experiments with nominal particle diameters of 1, 3, 5, 9, and 16 μm were conducted at each of three nominal air speeds: 2.2, 5.3, and 9.0 m/s. In the insulated duct system, deposition of particles with nominal diameters of 1, 3, 5, 8, and 13 μm was measured at nominal air speeds of 2.2, 5.3 and 8.8 m/s. Fluorescent techniques were used to measure directly the deposition velocities of monodisperse fluorescent particles to duct surfaces. Deposition at S-connectors, in bends, and in straight ducts with developing turbulence was often greater than deposition in straight ducts with fully developed turbulence for equal particle sizes, air speeds, and duct surface orientations. Deposition rates at all locations were found to increase with an increase in particle size or air speed. High deposition rates at S-connectors resulted from impaction, and these rates were nearly independent of the orientation of the S-connector. Deposition rates in the two 90° bends differed by more than an order of magnitude in some cases, probably because of the difference in turbulence conditions at the bend inlets. In straight sections of bare steel ducts where the turbulent flow profile was developing, the deposition enhancement relative to fully developed turbulence generally increased with air speed and decreased with downstream distance from the duct inlet. This enhancement was greater at the duct ceiling and wall than at the duct floor. In insulated ducts, deposition enhancement was less pronounced overall than in bare steel ducts. Trends that were observed in bare steel ducts were present, but weaker, in insulated ducts.     

Simulation of airflow and aerosol deposition in the nasal cavity of a 5-year-old child

As a human grows from birth to adulthood, both airway anatomy and breathing conditions vary that alter the deposition rate and pattern of inhaled aerosols. However, deposition studies have typically focused on adult subjects, results of which may not be readily extrapolated to children. Furthermore, because of greater ventilation rate per body weight, children receive a greater dose than adults and therefore are more susceptible to respiratory risks. This study is to evaluate the transport and deposition of respiratory aerosols in a nasal-laryngeal airway model based on MRI head images of a 5-year-old boy. Differences between this child and adults in nasal physiology and aerosol filtering efficiency will be emphasized. A validated low Reynolds number (LRN) k?ω turbulence model was employed to simulate laminar, transitional, and fully turbulent flow regimes within the nasal airways. Particle trajectories and deposition in the spectrum of 0.5–32 μm were evaluated using a well-tested Lagrangian tracking approach for inhalation flow rates ranging from sedentary (3 L/min) to heavily active (30 L/min) conditions. Simulation results of the inhalation pressure drop and particle deposition rate provided a reasonable match with existing experimental results in nasal airway casts of children. Much higher breathing resistance was observed in the 5-year-old child compared to adults. Furthermore, deposition patterns were sensitive to inhalation flow rate under low activity conditions. An empirical correlation of child nasal filtering efficiency was proposed for micrometer particles based on a wide range of test conditions. Results of this study demonstrate that significant child–adult difference exists in inhaled aerosol depositions, which should be taken into account for risk assessment of airborne toxicants on infants and children.     

Experimental filtration efficiencies for large pore nuclepore filters

Experimental filtration data were collected in an effort to validate an impaction model previously developed and presented. Using a sampler with a 9.5 μm pore diameter Nuclepore filter, collection efficiencies were measured for both liquid and solid aerosols over a size range of 2–9 μm. Data for the liquid aerosol showed good agreement with the impaction model; however, data for the solid aerosol indicated an appreciably lower collection efficiency than predicted by the model. The liquid aerosol data validate the impaction model. The solid aerosol data indicate particle bounce or reintrainment subsequent to impact and underscore particle capture as a problem to be dealt with if the Nuclepore surface is to be used as a size selective filter.     

Deposition of Ultrafine Aerosols and Thoron Progeny in Replicas of Nasal Airways of Young Children

The deposition efficiencies of ultrafine aerosols and thoron progeny were measured in youth nasal replicas. Clear polyester-resin casts of the upper airways of 1.5-yr-old (Cast G), 2.5-yr-old (Cast H), and 4-yr-old (Cast I) children were used. These casts were constructed from series of coronal magnetic resonance images of healthy children. The casts extended from the nostril tip to the junction of the nasopharynx and pharynx. These casts were similar in construction to those used in previous studies (Swift et al. 1992; Cheng et al. 1993). Total deposition was measured for monodisperse NaCl or Ag aerosols between 0.0046 and 0.20 (Jim in diameter at inspiratory and expiratory flow rates of 3, 7, and 16 L min^?1 (covering a near-normal range of breathing rates for children of different ages). Deposition efficiency decreased with increasing particle size and flow rate, indicating that diffusion was the main deposition mechanism. Deposition efficiency also decreased with increasing age at a given flow rate and particle size. At 16 L min^?1, the inspiratory deposition efficiencies in Cast G were 33% and 6% for 0.008- and 0.03-μm particles, respectively. Nasal deposition of thoron progeny with a mean diameter of 0.0013 μm was substantially higher (80%-93%) than those of the ultrafine aerosol particles, but still had a similar flow dependence. Both the aerosol and thoron progeny data were used to establish a theoretical equation relating deposition efficiency to the diffusion coefficient (D in cm² s^?1) and flow rate (Q in L min^?1) based on a turbulent diffusion process. Data from all casts can be expressed in a single equation previously developed from an adult nasal cast: E = 1 - exp(-aD ^0.5 Q ^?0.125). We further demonstrated that the effect of age, including changes to nasal airway size and breathing flow rate, on nasal deposition can be expressed in the parameter “a” of the fitted equation. Based on this information and information on minute volumes for different age groups, we predicted nasal deposition in age groups ranging from 1.5- to 20-yr-old at resting breathing rates. Our results showed that the nasal deposition increases with decreasing age for a given particle size between 0.001 to 0.2 μm. This information will be useful in deriving future population-wide models of respiratory tract dosimetry.     

The dependence of particle deposition velocity on particle inertia in turbulent pipe flow

Recent measurements of particle deposition velocities on the walls of a pipe in turbulent flow (Liu and Agarwal, 1974) show a decline with increasing particle size beyond a critical particle size. A stochastic model of particle deposition is presented which explains this result. As in other models, the deposition process is composed of turbulent diffusion, together with inertial projection through the boundary layer; in this model, both processes are particle inertia dependent, in opposing ways. The observed decline is due to the increased fractional penetration of the boundary layer with increasing particle size being insufficient to compensate for the reduced rate of transport to that region.

A simple expression is given for the particle deposition velocity in terms of the r.m.s. velocity at that point and the fractional penetration of the boundary layer. The inertial dependence of the particle velocity is expressed in terms of the particle's response to the turbulent velocity fluctuations of its neighbouring fluid by relating the velocity spectral densities of the particle and fluid using a linear dimensionless form of the equation of motion of the particle. The fractional penetration of the boundary layer is based on Stokes' drag with a quiescent fluid.

The deposition profile shows good agreement with the experiments of Liu and Agarwal.     

Thermophoretic particle deposition efficiency in turbulent tube flow

This study investigated the thermophoretic particle deposition efficiency numerically. The critical trajectory was used to calculate thermophoretic particle deposition in turbulent tube flow. The numerical results obtained in turbulent flow regime in this study were validated by particle deposition efficiency measurements with monodisperse particles (particle diameter ranges from 0.038 to 0.498 μm) in a tube (1.18 m long, 0.43 cm i.d., stainless-steel tube). The theoretical predictions are found to fit the experimental data of Tsai et al. [Tsai, C. J., J. S. Lin, S. G. Aggarwal, and D. R. Chen, “Thermophoretic Deposition of Particles in Laminar and Turbulent Tube Flows,” Aerosol Sci. Technol., 38, 131 (2004)] very well in turbulent flows. In addition, an empirical expression has been developed to predict the thermophoretic deposition efficiency in turbulent tube flow.     

System design and test method for measuring respirator filter efficiency using mycobacterium aerosols

Recently, the protection of health care workers from tuberculosis-containing aerosols has been the subject of considerable debate. An experimental apparatus and test protocol were developed to measure the collection efficiency of surgical mask and respirator filter media using a microbial aerosol challenge. Mycobacterium chelonae (M. chelonae), used as a surrogate for Mycobacterium tuberculosis, was generated from liquid suspension using a Collison nebulizer. Upstream and downstream concentrations of viable aerosol particles were measured using Andersen cascade impactors, while total particle concentrations were measured with an aerodynamic particle sizer (APS). A monodisperse polystyrene latex (PSL) sphere aerosol (0.804 μm) was used in separate experiments to measure filter efficiency; concentrations were determined with the APS. The mycobacterial aerosol ranged in size from 0.65 to 2.2 μm when measured with the cascade impactor. A similar size range was found with the APS, yielding a count median diameter of about 0.8 μm. Samples of the mycobacterial aerosol were collected on glass slides, stained M. chelonae, as determined by environmental scanning electron microscope, were found to be rod shaped with an average length of 2 μm and average width of 0.3 μm. To evaluate the apparatus over a range of filter efficiencies (10–100%), different layers of fiberglass filter paper were tested for penetration using a 0.12 μm dioctyl phthalate (DOP) aerosol measured with a light scattering photometer, in addition to the mycobacterial and PSL aerosols. For the range of efficiencies tested it was shown that filter collection of DOP was linearly related to that of both mycobacterial and PSL sphere aerosols (r² = 0.99), demonstrating that an inert aerosol may be used to predict the collection of biological aerosols by such filter media.     

Deposition of Man-Made Fibers in a Human Nasal Airway

Many occupational lung diseases are associated with exposure to aerosolized fibers in the workplace. The nasal airway is a critical route for fiber aerosol to enter the human respiratory tract. The fiber deposition efficiency in the nasal airway could be used as an index to indicate the fraction of the inhaled fibers potentially transported to the lower airways. In this research, experiments of fiber deposition in the human nasal airway were conducted. Man-made carbon, glass, and titanium dioxide fibers in the inertia regime were used as the test fiber materials. The deposition studies were carried out by delivering aerosolized fibers into a human nasal airway replica at constant human inspiratory flow rates ranging from 15 l/min to 43.5 l/min. The deposition results were compared in detail between these fiber materials to study how the fiber characteristics affected the nasal airway deposition. The results showed that the deposition efficiency of the carbon fiber increases as the fiber impaction parameter increases. Many carbon fibers deposited in the anterior region of the nasal airway. In contrast, very few glass or titanium dioxide fibers deposited in the nasal airway, but relatively more of these two fibers deposited in the turbinate region. This result implies that, if a fiber in the inertia regime is inhaled during normal human breathing, the smaller the fiber, the more easily it could enter the human lower respiratory tract, possibly causing harm to the human respiratory tract.     

Wind Tunnel Studies of Particle Transport and Deposition in Turbulent Boundary Flows

In this study, particle transport and deposition were studied in a wind tunnel by measuring both the airflow turbulence characteristics and deposition of monodisperse uranine particles of 2.0-4.5 w m diameter on smooth, horizontal surfaces. Turbulence characteristics behind a 2.54 cm high rectangular bar were investigated for free stream velocities ranging from 3.3 m/s to 15.3 m/s. The well-developed boundary layer thickness was approximately four times the height of the rectangular bar at a distance of about 55 cm from the bar. Results of the wind tunnel experiments show the complex nature of deposition in turbulent flows due to the interactions between particles and turbulence. In general, the particle deposition flux is larger in the near wake region than in the far wake region. The particle deposition flux is roughly independent of the dimensionless particle relaxation time when the relaxation time is less than one, but increases rapidly as the relaxation time increases above one.     

Vertical distribution of aerosol particles near the air-sea interface in coastal zone

Measurements of vertical aerosol concentration profiles in the Mediterranean coastal zone are presented in the size band 0.1 to 20 μm, with focus on aerosol particles smaller than 10 μm. The results show that the profile shape depends mainly on turbulent processes induced by the wind rather thanthe whitecap production of freshly generated aerosols from the wind-wave interaction. Therefore, the data recorded during weak and moderate wind speeds show the prevalence of turbulent mixing processes induced by the wind for profiles in the size range 0.1–1 μm. For the size range 1–10 μm, in addition to the effect of wind velocity and direction (i.e. the coastal influence), we also noted the importance of the relative humidity gradient (very close to the interface). Good agreement was found between vertical concentration profiles recorded for winds of marine origin and the marine mixed layer model (Davidson and Shutz, 1983). However, we show that the model cannot be used for winds of continental origin. For wind speeds above >9 ms⁻¹, a maximum occurs in the concentration profiles (near 2–3 m height) for all particles larger than 1 μm, which confirms the predominant influence of an eddy which appears in the lee of the crest, referred to as the “wave rotor” model (De Leeuw, 1986).     

Experimental Studies of the Effect of Rough Surfaces and Air Speed on Aerosol Deposition in a Test Chamber

Understanding the fate of particles indoors is important for human health assessment because deposited particles, unless resuspended, cannot be inhaled. To complement studies in real buildings, where control of variables is often difficult, an experimental test chamber facility (8 m 3 ) was designed to study particle deposition under well-stirred conditions using monodisperse tracer aerosol particles in the range of 0.7 to 5.4 w m. The use of neutron-activatable tracers facilitated simultaneous surface sampling and aerosol concentration decay measurements. Aerosol deposition on both smooth surfaces and regular arrays of three-dimensional roughness elements under 3 different airflow speeds was investigated in the test chamber.It was expected that the texture of the chamber surface would significantly influence particle deposition, but some counterintuitive results were observed: under the lowest airflow condition and for the smallest particle size, particle deposition onto rough samples was found to be less than on the corresponding smooth surfaces. The ratio of particle deposition on rough surfaces relative to smooth surfaces increased with particle size and magnitude of airflow. For the largest particle size and airflow speed, particle deposition on the rough surfaces exceeded that on the smooth surfaces by a factor of 3.     

A comprehensive approach in modeling Lagrangian particle deposition in turbulent boundary layers

Modeling of particle deposition on adjacent walls is a key issue in various applications like separation or transport processes. The present paper focuses on the modeling of turbophoretic deposition of particles in the micron size range. The first step is to evaluate the important range where turbophoresis plays an important role in comparison to other mechanisms e.g. gravity or electrostatic separation. The disadvantages of commonly used models will be analyzed and overcome by implementing a more sophisticated approach considering damping of turbulent fluctuations in the wall-boundary layer. In contrast to previous work, commonly used turbulence models are applied to solve the mean flow field of the examples under consideration. The results will show a good prediction of particle deposition in comparison to experimental values [B.Y.H. Liu, J.K. Agarwal, Experimental observation of aerosol deposition in turbulent flow, Aerosol. Sci. 5 (1974) 145-155.] by using the advanced model.     

A Charge Coupled Device System to Image Local Particle Deposition Patterns in a Model of a Human Nasal Airway

A new method was developed to measure local particle deposition patterns in a full-scale multisectional replica of a human nasal airway. Monodisperse dioctyl sebacate aerosol particles, labeled with the fluorescent dye Nile Red, were deposited in a human nasal airway model by drawing the aerosol through the replica at a constant airflow rate. Particle deposition patterns were then measured for each section of the model by using a charge coupled device to record particle fluorescence patterns. Customized imaging analysis software was used to extract the position and intensity of fluorescent sites. A resolution of 0.3 mm on fluorescent particle position was achieved. This method can be used to develop more efficient techniques for delivering medicinal drugs to the human body via inhalation and to understand better the mechanisms that control particle deposition in the geometrically complex nasal airways.