THE DEPOSITION OF AEROSOLS IN THE RESPIRATORY TRACT. I. MATHEMATICAL ANALYSIS AND COMPARISON WITH EXPERIMENTAL DATA

Physicochemical Characteristics and Their Variation of Coal Dust Originating from Underground Mining Sites

The physicochemical properties of coal dust significantly affect its toxicity and dust suppression efficiency. Currently, lab-crushed coal dust is commonly used for characterization instead of the original coal dust (OCD) sampled from underground mining sites. This practice leads to an inaccurate understanding of the underground coal dust properties. To address this issue, the study directly collected 18 OCD samples from various underground mining sites and characterized their physicochemical properties, and the variation of these physicochemical parameters of OCD with various coal rank were analyzed. The results show: OCD has a small particle size (average 26.49 μm), and around 21% of particles are under 10 μm. OCD has a well-developed pore structure, with an average total pore volume of 8.24 × 10-3 cm3/g and an average specific surface area of 8.24 m2/g. OCD samples have a high oxidation degree, and the average relative content of total oxygen-containing functional groups is 45.71%. Between the 32 measured physicochemical parameters of OCD, 10 moderately correlates with R 0 and 6 highly correlates with R 0. These parameters mainly involve wettability, pore structure, moisture content, and elemental composition. The findings present valuable insights into accurately assessing the toxicology and health risks of coal dust in underground mining sites and for selecting efficient dust control technologies in different coal mines.

Modelling Drug Delivery to the Small Airways: Optimization Using Response Surface Methodology

AIM

The aim of this in silico study was to investigate the effect of particle size, flow rate, and tidal volume on drug targeting to small airways in patients with mild COPD.

METHOD

Design of Experiments (DoE) was used with an in silico whole lung particle deposition model for bolus administration to investigate whether controlling inhalation can improve drug delivery to the small conducting airways. The range of particle aerodynamic diameters studied was 0.4 - 10 µm for flow rates between 100 - 2000 mL/s (i.e., low to very high), and tidal volumes between 40 - 1500 mL.

RESULTS

The model accurately predicted the relationship between independent variables and lung deposition, as confirmed by comparison with published experimental data. It was found that large particles (~ 5 µm) require very low flow rate (~ 100 mL/s) and very small tidal volume (~ 110 mL) to target small conducting airways, whereas fine particles (~ 2 µm) achieve drug targeting in the region at a relatively higher flow rate (~ 500 mL/s) and similar tidal volume (~ 110 mL).

CONCLUSION

The simulation results indicated that controlling tidal volume and flow rate can achieve targeted delivery to the small airways (i.e., > 50% of emitted dose was predicted to deposit in the small airways), and the optimal parameters depend on the particle size. It is hoped that this finding could provide a means of improving drug targeting to the small conducting airways and improve prognosis in COPD management.

Aerosol Transport Modeling: The Key Link Between Lung Infections of Individuals and Populations

The recent COVID-19 pandemic has propelled the field of aerosol science to the forefront, particularly the central role of virus-laden respiratory droplets and aerosols. The pandemic has also highlighted the critical need, and value for, an information bridge between epidemiological models (that inform policymakers to develop public health responses) and within-host models (that inform the public and health care providers how individuals develop respiratory infections). Here, we review existing data and models of generation of respiratory droplets and aerosols, their exhalation and inhalation, and the fate of infectious droplet transport and deposition throughout the respiratory tract. We then articulate how aerosol transport modeling can serve as a bridge between and guide calibration of within-host and epidemiological models, forming a comprehensive tool to formulate and test hypotheses about respiratory tract exposure and infection within and between individuals.

A dynamical overview of droplets in the transmission of respiratory infectious diseases

The outbreak of the coronavirus disease has drawn public attention to the transmission of infectious pathogens, and as major carriers of those pathogens, respiratory droplets play an important role in the process of transmission. This Review describes respiratory droplets from a physical and mechanical perspective, especially their correlation with the transmission of infectious pathogens. It covers the important aspects of (i) the generation and expulsion of droplets during respiratory activities, (ii) the transport and evolution of respiratory droplets in the ambient environment, and (iii) the inhalation and deposition of droplets in the human respiratory tract. State-of-the-art experimental, computational, and theoretical models and results are presented, and the corresponding knowledge gaps are identified. This Review stresses the multidisciplinary nature of its subject and appeals for collaboration among different fields to fight the present pandemic.

The impacts of coal dust on miners' health: A review

As one of the most important energy resources in the world, coal contributes a great deal to the world economy. Coal mining and processing involve multiple dust generation processes including coal cutting, transport, crushing and milling etc. Coal dust is one of the main sources of health hazard for the coal workers. Exposure of coal dusts can be prevented through administrative controls and engineering controls. Ineffective control of coal dust exposure can harm coal workers' health. Although many efforts have been made to eliminate these threats, recent years have seen an unexpected increase in coal workers' pneumoconiosis (CWP) in Appalachian basin in US. To explore the reasons for this phenomenon, in this review, we first reviewed the historical studies on coal mine dust including the regulation and engineering controls. Then, the effects of coal dust on human health was comprehensively reviewed. Next, the effects of nanoparticles on human health were reviewed, with an emphasis on toxicity of nanoparticles such as carbon nanotubes in other industries. From all this information, we hypothesize that nano-sized coal dust has contributed to the increase of CWP prevalence in recent years. As no research has been reported in this area, four directions which may need further investigation and future studies are recommended in this review. They include: 1) Systematic characterization of physicochemical properties of nano-size coal dust; 2) Toxicity and pathogenesis of nano-sized coal dust; 3) Development of real-time monitoring technology and equipment for nano-sized coal dust; 4) Development of exposure control technology and equipment. The intent of this review paper is to demonstrate the variation of coal dust properties and their impact on the mine worker's health. We suggest that the impact of nano-sized coal mine dust on miner's health has not yet been understood well and further improvements are necessary.

Anthropometry-based generation of personalized and population-specific human airway models

Understanding aerosol deposition in the human lung is of great significance in pulmonary toxicology and inhalation pharmacology. Adverse effects of inhaled environmental aerosols and pharmacological efficacy of inhaled therapeutics are dependent on aerosol properties as well as person-specific respiratory tract anatomy and physiology. Anatomical geometry and physiological function of human airways depend on age, gender, weight, fitness, health, and disease status. Tools for the generation of the population- and subject-specific virtual airway anatomical geometry based on anthropometric data and physiological vitals are invaluable in respiratory diagnostics, personalized pulmonary pharmacology, and model-based management of chronic respiratory diseases. Here we present a novel protocol and software framework for the generation of subject-specific airways based on anthropometric measurements of the subject's body, using the anatomical input, and the conventional spirometry, providing the functional (physiological) data. This model can be used for subject-specific simulations of respiration physiology, gas exchange, and aerosol inhalation and deposition.

Innovative preclinical models for pulmonary drug delivery research

Introduction: Pulmonary drug delivery is a complex field of research combining physics which drive aerosol transport and deposition and biology which underpins efficacy and toxicity of inhaled drugs. A myriad of preclinical methods, ranging from in-silico to in-vitro, ex-vivo and in-vivo, can be implemented.Areas covered: The present review covers in-silico mathematical and computational fluid dynamics modelization of aerosol deposition, cascade impactor technology to estimated drug delivery and deposition, advanced in-vitro cell culture methods and associated aerosol exposure, lung-on-chip technology, ex-vivo modeling, in-vivo inhaled drug delivery, lung imaging, and longitudinal pharmacokinetic analysis.Expert opinion: No single preclinical model can be advocated; all methods are fundamentally complementary and should be implemented based on benefits and drawbacks to answer specific scientific questions. The overall best scientific strategy depends, among others, on the product under investigations, inhalation device design, disease of interest, clinical patient population, previous knowledge. Preclinical testing is not to be separated from clinical evaluation, as small proof-of-concept clinical studies or conversely large-scale clinical big data may inform preclinical testing. The extend of expertise required for such translational research is unlikely to be found in one single laboratory calling for the setup of multinational large-scale research consortiums.

Implementation of a specific boundary condition for a simplified symmetric single-path CFD lung model with OpenFOAM

CFD modeling research about the lung airflow with a complete resolution and an adequate accuracy at all scales requires a great amount of computational resources due to the vast number of necessary grid elements. As a result, a common practice is to conduct simplifications that allows to manage it with ordinary computational power. In this study, the implementation of a special boundary condition in order to develop a simplified single conductive lung airway model, which exactly represents the effect of the removed airways, is presented. The boundary condition is programmed in the open-source software OpenFOAM®, and the developed source code is presented in the proper syntax. After this description, modeling accuracy is evaluated under different flow rate conditions typical of human breathing processes, including both inspiration and expiration movements. Afterward, a validation process is conducted using results of a Weibel's model (0-4 generations) simulation for a medium flow rate of 50 L/min. Finally, a comparison against the proposed boundary condition implemented in the commercial code ANSYS Fluent is made, which highlights the benefits of using the free code toolbox. The specific contribution of this paper will be to show that OpenFOAM® developed model can perform even better than other commercial codes due to a precise implementation and coupling of the default solver with the in-house functions by virtue of the open-source nature of the code.

Insulation fiber deposition in the airways of men and rats. A review of experimental and computational studies

The typical insulation rock, slag and glass wool fibers are high volume materials. Current exposure levels in industry (generally ≤ 1 fiber/cm³ with a median diameter ∼1 μm and length ≥10 μm) are not considered carcinogenic or causing other types of severe lung effects. However, epidemiological studies are not informative on effects in humans at fiber levels >1 fiber/cm³. Effects may be inferred from valid rat studies, conducted with rat respirable fibers (diameter ≤ 1.5 μm). Therefore, we estimate delivery and deposition in human and rat airways of the industrial fibers. The deposition fractions in humans head regions by nasal (∼0.20) and by mouth breathing (≤0.08) are lower than in rats (0.50). The delivered dose into the lungs per unit lung surface area during a 1-day exposure at a similar air concentration is estimated to be about two times higher in humans than in rats. The deposition fractions in human lungs by nasal (∼0.20) and by mouth breathing (∼0.40) are higher than in rats (∼0.04). The human lung deposition may be up to three times by nasal breathing and up to six times higher by oral breathing than in rats, qualifying assessment factor setting for deposition.

Aerosol transport throughout inspiration and expiration in the pulmonary airways

Little is known about transport throughout the respiration cycle in the conducting airways. It is challenging to appropriately describe the time-dependent number of particles entering back into the model during exhalation. Modeling the entire lung is not feasible; therefore, multidomain methods must be used. Here, we present a new framework that is designed to simulate particles throughout the respiration cycle, incorporating realistic airway geometry and respiration. This framework is applied for a healthy rat lung exposed to  ∼ 1μm diameter particles, chosen to facilitate parameterization and validation. The flow field is calculated in the conducting airways (3D domain) by solving the incompressible Navier-Stokes equations with experimentally derived boundary conditions. Particles are tracked throughout inspiration by solving a modified Maxey-Riley equation. Next, we pass the time-dependent particle concentrations exiting the 3D model to the 1D volume conservation and advection-diffusion models (1D domain). Once the 1D models are solved, we prescribe the time-dependent number of particles entering back into the 3D airways to again solve for 3D transport. The coupled simulations highlight that about twice as many particles deposit during inhalation compared to exhalation for the entire lung. In contrast to inhalation, where most particles deposit at the bifurcation zones, particles deposit relatively uniformly on the gravitationally dependent side of the 3D airways during exhalation. Strong agreement to previously collected regional experimental data is shown, as the 1D models account for lobe-dependent morphology. This framework may be applied to investigate dosimetry in other species and pathological lungs.

A Lagrangian Approach for Calculating Microsphere Deposition in a One-Dimensional Lung-Airway Model

Using the open-source software openfoam as the solver, a novel approach to calculate microsphere transport and deposition in a 1D human lung-equivalent trumpet model (TM) is presented. Specifically, for particle deposition in a nonlinear trumpetlike configuration a new radial force has been developed which, along with the regular drag force, generates particle trajectories toward the wall. The new semi-empirical force is a function of any given inlet volumetric flow rate, micron-particle diameter, and lung volume. Particle-deposition fractions (DFs) in the size range from 2 μm to 10 μm are in agreement with experimental datasets for different laminar and turbulent inhalation flow rates as well as total volumes. Typical run times on a single processor workstation to obtain actual total deposition results at comparable accuracy are 200 times less than that for an idealized whole-lung geometry (i.e., a 3D–1D model with airways up to 23rd generation in single-path only).

Lung dosimetry of inhaled radon progeny in mice

Biological response of exposure to radon progeny has long been investigated, but there are only few studies in which absorbed doses in lungs of laboratory animals were estimated. The present study is the first attempt to calculate the doses of inhaled radon progeny for mice. For reference, the doses for rats and humans were also computed with the corresponding models. Lung deposition of particles, their clearance, and energy deposition of alpha particles to sensitive tissues were systematically simulated. Absorbed doses to trachea and bronchi, bronchioles and terminal bronchioles, alveolar-interstitial regions, and whole lung were first provided as a function of monodisperse radon progeny particles with an equilibrium equivalent radon concentration of 1 Bq m(-3) (equilibrium factor, 0.4 and unattached fraction, 0.01). Based on the results, absorbed doses were then calculated for (1) a reference mine condition and (2) a condition previously used for animal experiments. It was found that the whole lung doses for mice, rats, and humans were 34.8, 20.7, and 10.7 nGy (Bq m(-3))(-1) h(-1) for the mine condition, respectively, while they were 16.9, 9.9, and 6.5 nGy (Bq m(-3))(-1) h(-1) for the animal experimental condition. In both cases, the values for mice are about 2 times higher than those for rats, and about 3 times higher than those for humans. Comparison of our data on rats and humans with those published in the literature shows an acceptable agreement, suggesting the validity of the present modeling for mice. In the future, a more sophisticated dosimetric study of inhaled radon progeny in mice would be desirable to demonstrate how anatomical, physiological, and environmental parameters can influence absorbed doses.

Deposition of inhaled particles in the lungs

Inhaled medication is the first-line treatment of diseases such as asthma or chronic obstructive pulmonary disease. Its effectiveness is related to the amount of drug deposited beyond the oropharyngeal region, the place where the deposit occurs and its distribution (uniform or not). It is also important to consider the size of the inhaled particles, the breathing conditions, the geometry of the airways and the mucociliary clearance mechanisms. Currently, mathematical models are being applied to describe the deposition of inhaled drugs based on the size of the particles, the inspiratory flow and the anatomical distribution of the bronchial tree. The deposition of particles in the small airways gets maximum attention from pharmaceutical companies and is of great interest as it is related with a better control in patients receiving these drugs.

Pulmonary targeting of nanoparticle drug matrices

BACKGROUND

Nanoparticle drug matrices using lipids or liposomes, with diameters of 40-300 nm, have recently been developed to encapsulate drugs like Insulin, Budesonide, and Rifampicin for pulmonary delivery raising interest in their regional lung deposition.

METHODS

Lung deposition has so far been modeled using a one-dimensional transport equation, with or without moving airway boundaries, and a lumped deposition term for particle diffusion, sedimentation, and impaction. Here, a two-dimensional transport model has been developed with an explicit treatment of radial diffusion, the primary mechanism for nanoparticle deposition. Regional lung deposition was calculated using Weibel's whole lung model geometry during normal breathing and medical inhalation cycles.

CONCLUSIONS

Model predictions agree well with measurements of total and pulmonary lung deposition for particles of 10 nm to 10 μm, with earlier models incorporating moving boundaries and aerosol dynamics, and with the reported regional lung deposition of inhaled dry powder insulin. To simulate medical inhalation, the model was run with inhalation times from 2-6 sec and breath hold from 0-10 sec. A high and relatively invariant pulmonary deposition fraction between 70 and 95% was predicted for a broad nanoparticle size range (50-200 nm) for inhalation cycles with breathing rate between 500 and 2000 cm(3) sec(-1) and breath hold of 5-10 sec. Thus, nanoparticles may be able to deliver consistent lung doses, over modest breath hold periods, even with intrapatient variability in breathing rate. A linearized nomogram was provided as a heuristic for design of nanoparticle drug matrices to target the pulmonary lung.

Particle deposition in human respiratory system: deposition of concentrated hygroscopic aerosols

In the nearly saturated human respiratory tract, the presence of water-soluble substances in the inhaled aerosols can cause change in the size distribution of the particles. This consequently alters the lung deposition profiles of the inhaled airborne particles. Similarly, the presence of high concentration of hygroscopic aerosols also affects the water vapor and temperature profiles in the respiratory tract. A model is presented to analyze these effects in human respiratory system. The model solves simultaneously the heat and mass transfer equations to determine the size evolution of respirable particles and gas-phase properties within human respiratory tract. First, the model predictions for nonhygroscopic aerosols are compared with experimental results. The model results are compared with experimental results of sodium chloride particles. The model reproduces the major features of the experimental data. The water vapor profile is significantly modified only when a high concentration of particles is present. The model is used to study the effect of equilibrium assumptions on particle deposition. Simulations show that an infinite dilution solution assumption to calculate the saturation equilibrium over droplet could induce errors in estimating particle growth. This error is significant in the case of particles of size greater than 1 mum and at number concentrations higher than 10(5)/cm(3).

Alveolar duct expansion greatly enhances aerosol deposition: a three-dimensional computational fluid dynamics study

Obtaining in vivo data of particle transport in the human lung is often difficult, if not impossible. Computational fluid dynamics (CFD) can provide detailed information on aerosol transport in realistic airway geometries. This paper provides a review of the key CFD studies of aerosol transport in the acinar region of the human lung. It also describes the first ever three-dimensional model of a single fully alveolated duct with moving boundaries allowing for the cyclic expansion and contraction that occurs during breathing. Studies of intra-acinar aerosol transport performed in models with stationary walls (SWs) showed that flow patterns were influenced by the geometric characteristics of the alveolar aperture, the presence of the alveolar septa contributed to the penetration of the particles into the lung periphery and there were large inhomogeneities in deposition patterns within the acinar structure. Recent studies have now used acinar models with moving walls. In these cases, particles penetrate the alveolar cavities not only as a result of sedimentation and diffusion but also as a result of convective transport, resulting in a much higher deposition prediction than that in SW models. Thus, models that fail to incorporate alveolar wall motions probably underestimate aerosol deposition in the acinar region of the lung.

Mathematical analysis of particle deposition in human lungs: an improved single path transport model

A dynamic single-path mathematical model was developed that is capable of analyzing detailed deposition patterns of inhaled particles in human lungs. Weibel's symmetric lung morphology was adopted as the basic lung structure, and detailed transport processes were evaluated numerically using the fully implicit procedure. Deposition efficiencies by specific mechanisms were individually examined for accuracy and new empirical formulas were incorporated whenever appropriate. Deposition in the alveolar region was divided into deposition fractions in the alveolar duct and alveoli, considering active transport processes between the two regions. The deposition fractions were obtained for each airway generation, serial lung volumetric compartments, and conventional three-compartment anatomic lung regions. In addition, the surface dose and cumulative deposition with time were analyzed. The results showed excellent agreement with available experimental data. The present model provides an improvement from the previously reported models and can be used as a tool in assessing internal dose of inhaled particles under various inhalation conditions.

The Lung in Space

The lung is exquisitely sensitive to gravity, which induces gradients in ventilation, blood flow, and gas exchange. Studies of lungs in microgravity provide a means of elucidating the effects of gravity. They suggest a mechanism by which gravity serves to match ventilation to perfusion, making for a more efficient lung than anticipated. Despite predictions, lungs do not become edematous, and there is no disruption to, gas exchange in microgravity. Sleep disturbances in microgravity are not a result of respiratory-related events; obstructive sleep apnea is caused principally by the gravitational effects on the upper airways. In microgravity, lungs may be at greater risk to the effects of inhaled aerosols.

Anatomically based three-dimensional model of airways to simulate flow and particle transport using computational fluid dynamics

We have studied gas flow and particle deposition in a realistic three-dimensional (3D) model of the bronchial tree, extending from the trachea to the segmental bronchi (7th airway generation for the most distal ones) using computational fluid dynamics. The model is based on the morphometrical data of Horsfield et al. (Horsfield K, Dart G, Olson DE, Filley GF, and Cumming G. J Appl Physiol 31: 207-217, 1971) and on bronchoscopic and computerized tomography images, which give the spatial 3D orientation of the curved ducts. It incorporates realistic angles of successive branching planes. Steady inspiratory flow varying between 50 and 500 cm(3)/s was simulated, as well as deposition of spherical aerosol particles (1-7 microm diameter, 1 g/cm(3) density). Flow simulations indicated nonfully developed flows in the branches due to their relative short lengths. Velocity flow profiles in the segmental bronchi, taken one diameter downstream of the bifurcation, were distorted compared with the flow in a simple curved tube, and wide patterns of secondary flow fields were observed. Both were due to the asymmetrical 3D configuration of the bifurcating network. Viscous pressure drop in the model was compared with results obtained by Pedley et al. (Pedley TJ, Schroter RC, and Sudlow MF. Respir Physiol 9: 387-405, 1970), which are shown to be a good first approximation. Particle deposition increased with particle size and was minimal for approximately 200 cm(3)/s inspiratory flow, but it was highly heterogeneous for branches of the same generation.

Risk assessment dosimetry model for inhaled particulate matter: I. Human subjects

Pollutant particulate matter (PM) is a serious global problem, presenting a threat to the health and well being of human subjects. Inhalation exposures tests with surrogate animals can be performed to estimate the threat. However, it is difficult to extrapolate the findings of animal tests to human conditions. In this two-part series, interspecies dosimetry models especially designed for implementation with risk assessment protocols are presented. In Part I, the mathematical integrity of the source model per se was tested with data from human subjects, and theoretical predictions agreed well with experimental measurements. In Part II, for surrogate (rat) simulations, appropriate algorithms for morphologies and ventilatory parameters were used as subroutines in the validated model. We conducted a comprehensive series of computer simulations describing the behavior of a representative air pollutant, secondary cigarette smoke. For risk assessment interests, a range of states from rest to exercise was considered. PM hygroscopicity had a pronounced effect on deposition in a complex but systematic manner, in humans and rats: deposition was increased for particles larger than about 1 microm, but was decreased for particles smaller than about 0.1 microm. The results clearly indicate that dosimetry models can be effectively used to a priori determine the laboratory conditions necessary for animals tests to accurately mimic human conditions. Moreover, the use of interspecies models is very cost effective. We propose, therefore, that mathematical models be used in a complementary manner with inhalation exposure experiments and be actively integrated into PM risk assessment protocols.

Microgravity and the lung

Although environmental physiologists are readily able to alter many aspects of the environment, it is not possible to remove the effects of gravity on Earth. During the past decade, a series of space flights were conducted in which comprehensive studies of the lung in microgravity (weightlessness) were performed. Stroke volume increases on initial exposure to microgravity and then decreases as circulating blood volume is reduced. Diffusing capacity increases markedly, due to increases in both pulmonary capillary blood volume and membrane diffusing capacity, likely due to more uniform pulmonary perfusion. Both ventilation and perfusion become more uniform throughout the lung, although much residual inhomogeneity remains. Despite the improvement in the distribution of both ventilation and perfusion, the range of the ventilation-to-perfusion ratio seen during a normal breath remains unaltered, possibly because of a spatial mismatch between ventilation and perfusion on a small scale. There are unexpected changes in the mixing of gas in the periphery of the lung, and evidence suggests that the intrinsic inhomogeneity of the lung exists at a scale of an acinus or a few acini. In addition, aerosol deposition in the alveolar region is unexpectedly high compared with existing models.

Aerosol deposition modeling using ACSL

An aerosol deposition model has been written for inclusion into physiologically based pharmacokinetic (PBPK) models, allowing PBPK model based risk assessments to be performed for aerosolized materials. Previously, PBPK models could only treat inhaled gases and vapors. The deposition model employs a semi-empirical equation to describe extrathoracic deposition and employs data concerning the geometry of the thoracic conducting airways as well as that of the gas exchange regions of the lung to compute the deposited aerosol mass based on aerosol diffusion, sedimentation, and impaction. Provisions are made to allow calculations for polydisperse aerosols whose size distribution and mass vary with time. Variations in the model subject's respiration can be accommodated through selection of respiratory parameters at model startup as well as through consideration of carbon dioxide stimulation of respiration. The model is compared with other similar calculations and experimental data to validate the calculations. An example model application is presented in the form of a comparison of two inhalation atmospheres, one from an inhalation toxicity study and one from a similar atmosphere produced for fire extinguishing agent testing.

Effect of microgravity and hypergravity on deposition of 0.5- to 3-micron-diameter aerosol in the human lung

We measured intrapulmonary deposition of 0. 5-, 1-, 2-, and 3-micron-diameter particles in four subjects on the ground (1 G) and during parabolic flights both in microgravity (microG) and at approximately 1.6 G. Subjects breathed aerosols at a constant flow rate (0.4 l/s) and tidal volume (0.75 liter). At 1 G and approximately 1.6 G, deposition increased with increasing particle size. In microG, differences in deposition as a function of particle size were almost abolished. Deposition was a nearly linear function of the G level for 2- and 3-micron-diameter particles, whereas for 0.5- and 1.0-micron-diameter particles, deposition increased less between microG and 1 G than between 1 G and approximately 1.6 G. Comparison with numerical predictions showed good agreement for 1-, 2-, and 3-micron-diameter particles at 1 and approximately 1.6 G, whereas the model consistently underestimated deposition in microG. The higher deposition observed in microG compared with model predictions might be explained by a larger deposition by diffusion because of a higher alveolar concentration of aerosol in microG and to the nonreversibility of the flow, causing additional mixing of the aerosols.

Effect of lung airway branching pattern and gas composition on particle deposition. I. Background and literature review

This paper reviews (1) the influence of airflow and particle motion on particle deposition within human lung airways, (2) the effects of carrier gas properties on mass transport in lung airways, and (3) the differences in particle deposition patterns and efficiencies between humans and experimental animals. The primary purpose of this review of general principles and available literature is to identify critical factors affecting the dosimetry of inhaled toxicants. Special attention is paid to studies utilizing gases of varying composition, which illuminate the role of flow profiles and turbulence in gas and particle penetration, and to studies of the effects of interspecies differences in airway branching patterns on particle deposition patterns within the airways.

The effect of surgery with cardiopulmonary bypass on alveolar-capillary barrier function in human beings

We measured the rate of clearance of technetium 99m-labeled diethylenetriamine pentaacetate (99mTcDTPA) (molecular weight, 492 daltons) from the lung into the blood (T1/2LB) in 9 patients before and after operation with cardiopulmonary bypass (CPB). Two hours postoperatively, T1/2LB fell from 49.3 +/- 13.6 minutes (mean +/- standard deviation) to 24.0 +/- 12.8 minutes (p less than 0.001). In addition, alveolar-arterial oxygen tension difference P(A-a)O2 had increased from 73 +/- 28 mm Hg to 164 +/- 37 mm Hg (p less than 0.001). The rates of clearance of 99mTcDTPA had returned to preoperative times by 7 days after operation, although there was still a significant (p less than 0.05) elevation in P(A-a)O2. Postoperative respiratory failure developed in 1 patient. The only abnormality of lung function detected preoperatively was an increased clearance rate for 99mTcDTPA (T1/2LB, 18 minutes). This study has shown an increased clearance from the lung of a low-molecular-weight molecule following operation with CPB. This finding should allow a more rational approach to elucidating the mechanisms of injury to the gas-blood interface in the lung following this type of operation.

Morphometric variability of the human upper bronchial tree

Available morphometric models of the human tracheobronchial tree are based on measurements of a small number of individuals from a few laboratories. In order to determine the degree of intersubject variability, and its relationship to intrasubject variability, analysis of airway lengths, diameters and branching angles was performed using solid casts of the upper bronchial trees from a series of human lungs. The results indicate that there are significant differences between subjects. Inter- and intrasubject variabilities should be considered in occupational and environmental hazard evaluation, and when extrapolating and modeling inhalation toxicologic data.

Lung response to particulates with emphasis on asbestos and other fibrous dusts

Many theories have been proposed to explain asbestosis and asbestos-related pulmonary disease. However, none of the theories give a completely plausible explanation for the pathogenesis. Recently, attention has been drawn to a theory that the fibrogenicity or carcinogenicity of fibrous dust particles is related to fiber diameter and length rather than to chemical properties. This theory may help partially elucidate the disease process but is still far from solving the enigma of pulmonary fibrosis or carcinogenesis. The theory cannot explain the absence of these pathological effects among fiberglass workers or experimental animals exposed by inhalation (even though mesotheliomas are induced by intrapleural implantation and fiber dimension-related fibrogenicity is demonstrated by intratracheal injection). Little information regarding the pulmonary response to manmade fibrous particles is available in animals following inhalation exposure. Attempts should be made to confirm the absence of adverse effects using animal inhalation experiments even though to this point there is no conclusive evidence that either lung cancer or pulmonary diseases can be produced among employees in manmade fiber industries. A new research trend seems concentrated on testing the durability of asbestos or manmade fibers. This is based on the concept that biological effects of fibrous particles are the result of relative durability and that particles which can be fragmented or shortened may be less pathogenic. In the last two decades, considerable understanding about pulmonary fibrosis and carcinogenesis of asbestos has been achieved by clinical and animal experiments. In vitro tests including cytotoxicity, hemolysis, immunology, and enzyme biochemistry have provided important information on the interrelationships among these various biological effects of asbestos.

Deposition and clearance of inhaled particles

Theoretical models of respiratory tract deposition of inhaled particles are compared to experimental studies of deposition patterns in humans and animals, as governed principally by particle size, density, respiratory rate and flow parameters. Various models of inhaled particle deposition make use of approximations of the respiratory tract to predict fractional deposition caused by fundamental physical processes of particle impaction, sedimentation, and diffusion. These models for both total deposition and regional (nasopharyngeal, tracheobronchial, and pulmonary) deposition are compared with early and recent experimental studies. Reasonable correlation has been obtained between theoretical and experimental studies, but the behavior in the respiratory tract of very fine (less than 0.1 micron) particles requires further investigation. Properties of particle shape, charge and hygroscopicity as well as the degree of respiratory tract pathology also influence deposition patterns; definitive experimental work is needed in these areas. The influence upon deposition patterns of dynamic alterations in inspiratory flow profiles caused by a variety of breathing patterns also requires further study, and the use of differing ventilation techniques with selected inhaled particle sizes holds promise in diagnosis of respiratory tract diseases. Mechanisms of conducting airway and alveolar clearance processes involving the pulmonary macrophage, mucociliary clearance, dissolution, transport to systemic circulation, and translocation via regional lymphatic vessels are discussed.

A simple separator to generate half micron aqueous particles for lung imaging

An aerosol of a radionuclide may be used for ventilation imaging as an alternative to radioactive rare gases. For good-quality images as few particles as possible must be deposited in the trachea and bronchi, which means that no, or very few, particles should have aerodynamic diameter of more than 2 micron. We have developed a separator to modify the output of an Acorn jet nebuliser. It reduced the proportion of particles in the aerosol with aerodynamic diameters of more than 2 micron from 60% for the unmodified output of the nebuliser to only 6%; it consisted of a cylinder of perspex with two compartments containing inert spheres, either stainless steel or glass, of diameter 3 mm (the first compartment contained two layers of spheres, and the main section was packed with spheres). To examine the efficiency of the separator system for routine clinical use we compared the images produced and the deposition of the particles in the modified aerosol (using 99TcmDTPA) with the distribution of ventilation of 81Krm in eight subjects. Despite the theoretical differences between the behaviour of a gas molecule and of a particle in the respiratory tree, the images produced using the modified aerosol delivery system were as good as those produced with 81Krm in normal subjects. In abnormal subjects the images produced were only distinguished following the calculation of a penetration index.

Analysis of aerosol deposition in the healthy human lung

Wide variation in the pattern of deposition of inhaled aerosols has previously been described in both healthy and diseased humans. To investigate the factors responsible for such variation, the authors studied a group of 13 healthy nonsmoking subjects. One two occasions each subject inhaled a monodisperse 8.1 mm (mass median aerodynamic diameter) Fe2O3 aerosol labelled with 99mTc using a standardized breathing pattern. Pulmonary function was defined by tests of forced expiratory airflow. Total activity in the right lung at 0 hr and at 24 hr (24-hr percent retention) was measured using a gamma camera. Numerical indices of deposition pattern were derived in several ways from the initial gamma camera image of the right lung by comparing the ratio of activity within a mid- and peripheral lung region of interest, by analyzing the profile of radioactivity within a horizontal band across the right lung from the midline to the lung edge, and by analysis of a distribution histogram of activity within the whole lung (skew and kurtosis). The 24-hr percent retention of aerosol showed considerable intrasubject variability unlike the deposition indices. The various deposition indices were found to correlate with the 24-hr percent retention, FEV1.0, FEV1.0/FVC%, and MMFR at varying levels of significance. Results indicate that the pattern of aerosol deposition in healthy humans is influenced by mild degrees of obstruction to airflow, as reflected by tests of forced expiratory airflow, increasing airways obstruction being associated with more central deposition of the inhaled aerosol. Deposition indices derived from the initial pattern of aerosol distribution within the lung may prove to be more reliable and sensitive than measurements of 24-hr percent retention in defining aerosol deposition pattern.

Deposition, retention, and clearance of inhaled particles

The relation between the concentrations and characteristics of air contaminants in the work place and the resultant toxic doses and potential hazards after their inhalation depends greatly on their patterns of deposition and the rates and pathways for their clearance from the deposition sites. The distribution of the deposition sites of inhaled particles is strongly dependent on their aerodynamic diameters. For normal man, inhaled non-hygroscopic particles greater than or equal to 2 micrometers that deposit in the conducting airways by impaction are concentrated on to a small fraction of the surface. Cigarette smoking and bronchitis produce a proximal shift in the deposition pattern. The major factor affecting the deposition of smaller particles is their transfer from tidal to reserve air. For particles soluble in respiratory tract fluid, systemic uptake may be relatively complete for all deposition patterns, and there may be local toxic or irritant effects or both. On the other hand, slowly soluble particles depositing in the conducting airways are carried on the surface to the glottis and are swallowed within one day. Mucociliary transport rates are highly variable, both along the ciliated airways of a given individual and between individuals. The changes in clearance rates produced by drugs, cigarette smoke, and other environmental pollutants can greatly increase or decrease these rates. Particles deposited in non-ciliated airways have large surface-to-volume ratios, and clearance by dissolution can occur for materials generally considered insoluble. They may also be cleared as free particles either by passive transport along surface liquids or, after phagocytosis, by transport within alveolar macrophages. If the particles penetrate the epithelium, either bare or within macrophages, they may be sequestered within cells or enter the lymphatic circulation and be carried to pleural, hilar, and more distant lymph nodes. Non-toxic insoluble particles are cleared from the alveolar region in a series of temporal phases. The earliest, lasting several weeks, appears to include the clearance of phagocytosed particles via the bronchial tree. The terminal phases appear to be related to solubility at interstitial sites. While the mechanisms and dynamics of particle deposition and clearance are reasonably well established in broad outline, reliable quantitative data are lacking in many specific areas. More information is needed on: (1) normal behaviour, (2) the extent of the reserve capacity of the system to cope with occupational exposures, and (3) the role of compensatory changes in airway sizes and in secretory and transport rates in providing protection against occupational exposures, and in relation to the development and progression of dysfunction and disease.