Introduction

Utilization of fossil fuels to power heavy indus-

try and motorized transport results in increased

airborne pollutants. Biota (plants and animals)

come into direct contact with suspended atmos-

pheric aerosol particles by gas exchange or

via the dermis/cuticula. In humans, aerosol par-

ticles in the range of 1 nm to 10 µm such as

cigarette smoke and occupational as well as

environmental aerosols are inhalable and can

deposit in the respiratory system. Inhaled and

deposited particles may cause many diseases

in the human respiratory tract and the occur-

rence of such diseases depends not only on the

amount of mass deposited but also on the ad-

sorbed substances they carry into specific re-

gions of the lungs. Hence, the determination of

aerosol deposition in the lung, their uptake

(absorption into the blood), redistribution, stor-

age, and removal (clearance and excretion)

from the body are the most important proc-

esses involved to assess the extent of conse-

quent harmful effects.

It is worth to mention that knowledge of re-

gional deposition in the airways may have

important practical applications in the case of

therapeutic aerosols as well. Understanding

aerosol deposition in the lung is a complex

phenomenon, since it depends on the aerosol

properties, lung morphemetry, and respiratory

physiology.1 Aerosol deposition in the human

respiratory tract has been determined experi-

mentally, for certain aerosol systems either in

vivo or via in vitro studies. While in vivo studies

are ethically problematic, in vitro models also

have severe limitations in simulating realistic

physiological conditions. Therefore, several

mathematical models of the human respiratory

system have been developed to calculate the

deposition of inhaled aerosols. 2-6 Different

modeling and computation techniques such as,

a) semi-empirical models, 1, 7 b) deterministic,

symmetric generation or single-pathway mod-

els, 4, 8 c) one-dimensional cross section or

“trumpet” models, 9, 10, 11 d) deterministic

asymmetric multiple path modes, 12, 13 e) sto-

chastic, asymmetric generation models 6, 14 and

computational fluid dynamics (CFD) based

models 15-17 have been employed to estimate

the deposition efficiency within the lung. How-

ever, the calculated deposition fractions vary

primarily with respect to lung morphometry

and mathematical modeling technique. There-

fore, it is always on the user to select any of

the available techniques for deposition calcula-

tions depending on the purpose of its use and

Lung deposition predictions of airborne particles and the emergence of contemporary diseases Part-I

Inhaled particles can cause a variety of pulmonary illnesses such as asthma, bronchitis, chronic

obstructive pulmonary diseases (COPD) and even secondary organismic diseases. Thus,

predictions of inhaled aerosol deposition in the respiratory tract are essential not only to assess

their possible consequences but also to optimize drug delivery using pharmaceutical aerosols.

Deposition of inhaled aerosols is a complex phenomenon that depends on the physico-chemical

properties of the particles, lung anatomy, and respiratory patterns of the subject. Hence, the

prediction of particle deposition for an individual person poses real challenges. Different

conceptual particle deposition models are employed for the estimation of deposition fraction in

different region of the lung. However, these deposition fractions vary with the above mentioned

parameters in addition to the modeling and computational technique. Part-I of this review article

briefly describes the deposition behaviour of inhaled particulate matter and the currently

available approaches for the prediction of aerosol deposition in the respiratory tract. Part-II

continues this thread and provides a broad view of the health-related issues of particle exposure.

Hussain M1,2, Madl P1, Khan A1,2 1Division of Physics and Biophysics, Department of Materials Research and Physics, University of Salzburg, Austria, 2Higher Education Commission, Islamabad, Pakistan

theHealth 2011; 2(2):51-59

Correspondence

Majid Hussain

Division of Physics and

Biophysics, Department of

Materials Research and Physics,

University of Salzburg,

Hellbrunnerstrasse 34, A-5020,

Salzburg, Austria

E-mail:

majid.hussain@sbg.ac.at

Keywords:

Lung Deposition, Deposition

Mechanisms, Modeling,

Clearance

Funding

This work was funded in part

by European Union, Contract

No. 516483 (Alpha Risk) and

by the Higher Education

Commission of Pakistan under

the scholarship program

(Overseas Scholarship for

Pakistani Nationals)

Competing Interest

None declared.

Received: March 29, 2011

Accepted: May 4, 2011

Abstract

ISSN (print): 2218-3299

ISSN (online): 2219-8083

Review

51 | theHealth | Volume 2 | Issue 2

on the available input information.

In this review, we shall briefly discuss the, 1) human lung

structure, 2) particle deposition mechanisms, 3) particle clear-

ance mechanisms in the lung, 4) introduction to basic compo-

nents of a computational model and of different modeling

techniques, 5) deposition patterns in human airway bifurca-

tions along with identification of high density deposition ar-

eas (hotspots), and 6) effects of particle size, flow rate, age,

and gender that affect deposition. Furthermore, possible con-

sequences as a result of the deposited aerosol fraction in

terms of possible health effects shall also be highlighted in

Part-II of this paper.

Human lung structure

Airflow, aerosol transport and aerosol deposition in airways

are strongly influenced by the dimensions and orientation of

flow passages in the respiratory tract. The lung volume ex-

pands during inhalation and contracts during exhalation, re-

sulting in a variable airways geometry. Lung airway structure

starts with oro-nasal passages, i.e. extrathoracic (ET) region

followed by the larynx, trachea, main broncholi, all the way

down to the gas-exchange tissue, the alveoli. Moving down to

the lower airways, the number of airways in humans multiply

mostly via diachotomous branching patterns. 18 With each

bifurcation also the airway dimensions become reduced

(Figure 1). At the same time and as one proceeds further

towards the alveolar regime, multiple bifurcation patterns

cause the overall lung volume to increase in non-linear fash-

ion. 4 The human respiratory tract can be divided into four

anatomical regions and 23 lung generations, which are

adopted for the calculation of deposition fractions of inhaled

aerosol particles. 1

Extrathoracic region (ET)

Under normal circumstances, air is inspired through the nose

and the pharynx into the larynx. In case of nasal obstruction

or when respiratory demand exceeds the nasal airflow ca-

pacity, air is inspired through the mouth. Nasal or oral path-

ways are known as ET regions (Figure 1). This region of the

human respiratory tract serves as an important first stage

filter for inhaled particles entering the lung. This region also

contains associated lymph vessels and lymph nodes called

bronchus-associated lymphoid tissue (BALT). Therefore, from

the point of view of toxicology and human health, under-

standing the nature of airflow, and particle transport in the

human nasal-oral passages is important.

Bronchial (BB) region

This region is a part of the air conducting system within the

thoracic region. BB consists of the trachea (assigned genera-

tion 0, Figure 1), the main bronchi, and the intrapulmonary

bronchi (usually grouped around generation 8). The region

also includes associated lymph vessels and lymph nodes. The

epithelium of the trachea primarily consists of ciliated cells

ending with mucus secreting goblet cells and specialized mu-

cus secreting glands. 1 The cilia reveal synchronized beating

patterns in order to propel mucus and deposited matter up to

the larynx where they are either expelled or ingested.

Bronchiolar (bb) region

This region is the second part of the air conducting system

within the thorax. It consists of the bronchioles comprising

generations 9 to 16 (Figure 1). The branches of the last gen-

eration are called terminal bronchioles; all airways beyond

the terminal bronchioles carry alveoli. The bronchioles are

airways without cartilage and glands. Basal cells are rarely

found in bronchioles. The surfaces of the bronchioles have

ciliated cells that clear secretory fluid towards the base of BB

region. 1

Alveolar-interstitial region (AI)

The AI compartment comprises the respiratory tract system up

to the terminal bronchioles that groups together generations

16 to 26 known as the respiratory bronchioles and the alveo-

lar ducts (Figure 1). It includes interstitial lymphatic tissues

and lymph vessels as well as bronchial lymph nodes. The gas-

exchange region is represented by the alveolar sacs, which

are closed at the peripheral end by a group of alveoli. The

target cells in the alveolar-interstitial region are the secretory

(Clara) cells of the respiratory bronchiole, type I and type II

epithelial cells covering the alveolar surface.

Particle deposition mechanisms in the lung

Deposition means the event of a particle to adhere or stick to

a surface. The most important mechanisms by which airborne

particles can deposit in the lung are diffusion, sedimentation

and impaction (Figure 2). Whereas some minor mechanism su-

Figure 1: ICRP1 anatomical regions and airway generation model;

Tracheobronchial region (generations 0-16) and pulmonary region

(generations 17-23). (modified 19)

theHealth | Volume 2 | Issue 2 | 52

Lung deposition predictions of airborne particles

ch as particle electrostatic charge and particle interception

(particles contact with the surface even if its trajectory does

not deviate from the streamline) also cause deposition. These

mechanisms are briefly described in this section.

Diffusion

Deposition by diffusion is caused by Brownian motion or a

larger diffusion coefficient. Diffusion is the deposition mecha-

nism for small particles (< 0.5 µm) due to collision with air

molecules (Figure 2). Diffusion may cause the particle to

move across the streamline and deposit upon contact with the

airway wall. Deposition by diffusion increases with decreas-

ing particle size and flow rate. Increased diffusion deposition

occurs in the alveoli region because of longer residence time

and smaller airways. The deposition probability by diffusion

in cylindrical airways is calculated as:

P(s) = ∆/R [1]

where R is the airway diameter, and is the parti-

cle mean displacement in the airway. ∆ is proportional to

the square root of the diffusion coefficient D defined by the

well known Stokes-Einstein equation:

D = kTCc/3πda [2]

where Cc is the correction factor for slip flow conditions, k the

Boltzmann constant, T the absolute temperature, the gas

viscosity, and da the particle diameter.

Sedimentation

Sedimentation is due to the dominating gravitational force

acting on the inhaled particles over the air resistance. Thus,

as a result of balancing between the gravitational force and

drag force of air, particle may deposit on lower surfaces of

the airway (Figure 2). This mechanism is important for particle

sizes greater than about 0.5 µm and becomes dominant in

the bb and Al where air flow decelerates. Sedimentation

deposition increases with an increase in particle size and a

decrease in flow rate. The deposition probability by sedi-

mentation in cylindrical airways is calculated as:

P(s) = Ut t/R [3]

where t is the particle residence time in the airway and Ut =

gd2a/18 is the terminal settling velocity of the particle

which depends on the particle density ρ and gravitational

constant g.

Impaction

Large-sized particles with higher momentum simply do not

follow the curvature of the air stream due to inertia and de-

posit on the airway wall. Because momentum is the product of

mass and velocity (mV), inertial impaction is important for

large particles, usually greater than 1 µm in size. Deposition

by impaction increases both with growing particle sizes an

flow rates (Figure 2). Impaction events take place predomi-

nantly in the naso-pharyngeal and TB region. The deposition

efficiencies of the nose or oral breathing patterns are plotted

as a function of the inertial impaction parameter, d2aQ,

where da is the aerodynamic particle diameter, and Q is the

volumetric flow rate. The deposition probability by impaction

in cylindrical airways is calculated as:

P(I) = hs/R [4]

Where hs is the particle stopping distance and it becomes

Stokes number ρda2QCC / (36μR) (µ is the air viscosity, ρ is

the particle density and Cc is the Cunningham slip correction

factor for the particle) if divided by airway radius R.

Particle clearance mechanism in the lung

Getting rid of deposited particles from the respiratory tract

by particle transport or by absorption into blood is called

clearance. Different clearance mechanisms operate in differ-

ent regions of the lungs to eliminate the trapped foreign ma-

terial.

Figure 2: Major mechanisms of particle deposition in the respiratory

tract. 20

Figure 3: Schematic drawing of airway showing different clearance

mechanism of the inhaled particles. 21

53 | theHealth | Volume 2 | Issue 2

Dt2

Lung deposition predictions of airborne particles

Mechanical clearance

Mechanical clearance is primarily associated with the ET re-

gion, while clearance takes place by sneezing, coughing or

swallowing of the deposited material. This phenomenon takes

place immediately after the deposition event of a particle

load still within the nasal or oral pathway. Coughing is a ma-

jor clearance mechanism and therefore critically important

for airway hygiene and for increased mucus production.

Typically, particles over 5.0 µm diameter are stopped and

deposited by nostril hair filters.

Mucociliary clearance

Mucociliary clearance is the primary clearance mechanism to

remove insoluble deposited particles in the TB region. The

mucociliary escalator operates in the TB region to transport

the aerosol loaded mucus towards the larynx where the mu-

cus is eventually swallowed or removed as sputum (Figure 3).

It has been estimated that in the absence of any disease the

majority of the deposited particles are cleared within 24 h

from the trachea and bronchi. 1 However, mucociliary clear-

ance progressively decreases from the larger airways (BB) to

the smaller airways (bb).

Macrophage-mediated clearance

Macrophages are white blood cells produced by the differ-

entiation of monocytes in tissues. Macrophages operate in the

alveolar region and help to engulf and relocate the depos-

ited foreign material towards bronchiolar airways. If upward

translocation is no longer possible, the material is routed to

the circulatory or lymphatic system. Since only a restricted

number of macrophages can deal with the deposited parti-

cles, the macrophage-mediated clearance is considered a

slow clearance mechanism and it may take years to clear the

lung from the deposited material.

Translocation

The transfer of material absorbed from the respiratory tract

to other tissues in the body is called translocation. Ultrafine

(UF) particles (< 0.1 µm) and relatively insoluble particles

can be translocated from the TB or alveolar region to other

lung compartments and body tissues as explained in figure 4.

In the TB region, translocation usually occurs via the peribron-

chial region. Whereas in the Al region, this process takes

place via the Al epithelium straight into the interstitum (Figure

4).

A small fraction of the UF particles can be translocated to the

circulatory system and hence can reach extrapulmonary or-

gans via the blood stream. 22 Inhalation of nano sized parti-

cles refers to the translocation from the nasal epithelium to

the brain. Elder et al, 23 showed that UF manganese oxide

particles translocate to the olfactory bulb and other regions

of the central nervous system. However, according to current

knowledge, particle translocation and accumulation in extra-

pulmonary organs is a minor clearance mechanism as com-

pared to macrophage mediated clearance and still requires

further study to investigate its effect on human health. 22

Absorption into circulation system (blood absorption)

Movement of the soluble substances and material dissociated

into blood stream regardless of the mechanism is called ab-

sorption. Absorption into blood is a two stage process, a) the

dissociation of the particles into material that can be ab-

sorbed into the blood (dissolution) and b) the uptake of ma-

terial dissolved from particles or of material deposited in a

soluble form. Each stage can be time-dependent. Absorption

mechanism is a material specific phenomenon. 24 Soluble in-

haled particles can slowly absorb into the blood depending

on their absorption properties and their history that is, spe-

cific activity (radioactive particles), temperature treatment,

and surface area. The particle dissolution rate is a major

determinant controlling the rate of absorption for the mate-

rial retained in the respiratory tract. Absorption clearance

takes place in entire of the respiratory tract with different

absorption rates in different regions. Materials have been

categorized in fast, medium, and slow materials depending

upon their specific dissolution rate. 24 Accordingly, toxicity is

also tightly related to their velocity of dissolution.

Aerosol deposition models

Semi-empirical models

The International Commission on Radiological Protection

(ICRP) has proposed a semi-empirical model for regional

deposition and clearance from the human respiratory tract. 1

The purpose of this model was to calculate radiation doses

to the respiratory tract of workers resulting from the intake

airborne radionuclides. The models consider human respira-

tory tract as a series of anatomical compartments through

which aerosol pass during inhalation and exhalation. Each

compartment is treated as a filter in which deposition is calcu-

Figure 4: Schematic drawing of the alveolar tissue showing different

clearance mechanism of deposited particles. Pulmonary alveolar

macrophage (PAM) mediated removal from the lungs away towards

other excretory organs. Inlet: PAM loaded with 80 nm particles.

Approximately 40 nm-sized caveolar openings disappear and

reappear, forming vesicles that constitute pathways through the cells

for encapsulated macromolecules. 25

theHealth | Volume 2 | Issue 2 | 54

Lung deposition predictions of airborne particles

lated by semi-empirical equations 26 derived from fitting

experimental data as a function of particle size and flow

rate. The advantages of such semi-empirical models are that

they are easy to use with less computational work. Yet, they

can neither be used for deposition calculations at the airway

generation level, nor for flow rates and particles sizes be-

yond the limits of the experimental data sets on which the

semi-empirical model is based upon.

Deterministic or single-path models

Deterministic models 4, 8 are simple and can be applied to an

average path without having detailed knowledge about the

branching structure of the lung. In a deterministic or single

path model, airway generations have identical linear dimen-

sions in which each parent airway branches into two identical

daughter airways. 2 Thus all pathways of an inhaled particle

from the trachea to the alveolar sacs are identical and thus

can be represented by a single path. Because of symmetric

branching and equal diameters of daughter airways, the

inhaled air flow and hence deposition fraction is equally dis-

tributed among all airways in a given generation. Deposition

fractions in each airway are computed by applying analyti-

cal deposition equations for a defined flow rate. The deposi-

tion fraction for a given generation is obtained by summing

up deposition fractions of each airway of that particular

generation.

Trumpet models (one dimensional)

The trumpet lung model was proposed by Yu 9, which uses

Weibel’s 2 morphometry of the human lung and treats air-

ways as a one-dimensional airway system in which the flow

cross-sectional area varies along the airway depth. In this

approach, the cross-sectional area of each generation is

plotted as one parameter, thereby yielding a single enve-

lope-function that is shaped like a trumpet (Figure 5). The

breathing process is viewed as the movement of air into and

out of this trumpet since airways and alveoli expand and

contract uniformly. The model was developed for stable and

monodisperse particles. The transport of inhaled particles

occurs via convective and diffusive processes while their ulti-

mate deposition along the axis of the trumpet takes place by

mixing between tidal air and reserve air volume as de-

scribed mathematically by mass balance equations using

various deposition mechanisms. The aerosol concentration

during breathing is found to be a function of airways depth

and time. Once the concentration is known, the deposition in

each region can be calculated.

Deterministic, asymmetric multiple-path models (MPPD)

Multiple-path lung deposition models were developed by the

Center for Health Research (CIIT, currently the Hamner Insti-

tutes for Health Sciences, Research Traingle Park, NC, USA).

The models are more realistic than single-path models be-

cause they are based on actual airway measurements rather

than on average values and thus consider the asymmetric

branching pattern of the lung. The theoretical approaches for

the MPPD model were described by Anjilvel and Asgharian. 28 Later, the multiple-path bronchial airway models have

been derived on the basis of the stochastic lung model. 12, 13

For each airway of the lung, particle concentrations are de-

termined as a function of time for the proximal and distal

ends. Knowing the concentration of particles at the proximal

end of an airway, the concentration at its distal end is calcu-

lated by considering the deposition efficiencies for the vari-

ous deposition mechanisms.

Stochastic, asymmetric generation models

The stochastic nature of the lung structure that is, the variabil-

ity in morphometric, physiologic, and histologic parameters

used for lung dosimetry may cause significant variations in

dose calculations at the cellular level. Such parameter vari-

ability is dealt mathematically by stochastic models, in which

these parameters are selected randomly from their probabil-

ity distributions using Monte Carlo methods. Inhaled particles

are transported through this stochastic airway structure by

randomly selecting a sequence of airways for each particle

until deposition occurs. A fully stochastic deposition model

(IDEAL), simulating the trajectories of single particles was

presented by Koblinger and Hofmann 6 and Hofmann and

Koblinger, 14 and further developed by Hofmann et al. 13, 29,

30, 31 By simulating the random paths of many particles, typi-

cally of the order of tens of thousand trajectories, statistical

means can be calculated for total, regional and generation-

by generation deposition.

Computation fluid dynamics (CFD)-based model

Contrary to the whole lung model, CFD models utilize de-

tailed three-dimensional fluid flow and particle transport

equations. The airways are divided into small volumetric ele-

ments allowing the calculation of local deposition for each

airway surface element. Detailed airway geometry including

axial and lateral airway surface morphological variations

can be incorporated in a CFD model. A CFD model utilizes

transport equations and solves them simultaneously assuming

major deposition mechanisms and symmetric flow conditions.

55 | theHealth | Volume 2 | Issue 2

Lung deposition predictions of airborne particles

Figure 5: Schematic illustration of a trumpet shaped lung model 27

Most CFD based aerosol deposition models address the oro-

nasal and upper respiratory tracts (to generations 5-6) only,

and some focus on the alveolar regions only. These models

still needed validation based on fundamental principles.

Deposition patterns in human airways

The shape of deposition patterns strongly depends on the

inhaled particle sizes, breathing pattern and the lung airway

geometry. Generally speaking, lung deposition is a superpo-

sition of two separate deposition patterns - sedimentation as

well as impaction for submicron particles and diffusion for

nanometer-sized particles. As deposition decreases with

smaller submicron particle diameter, it reaches a minimal

deposition efficiency at the intersection between the two pat-

terns. With further decrease in particle size into the nanome-

ter scale, deposition picks up again as one proceeds towards

the alveolar domain (Figure 6). Due to consideration of real-

istic airway geometry that is, with a rounded carinal ridge

and smooth transitions between the parent and daughter

branches, deposition preferentially takes place at the bifur-

cation sites. Characteristic inspiratory deposition patterns as

a result of sitting breathing conditions are shown in Figure 7

for 0.2 and 5 µm diameter unit density particles near the

vicinity of the carinal ridge. The submicron, 0.2 µm particle

size is characteristic of cigarette smoke and radon progeny

attached to indoor aerosols, whereas the large, 5 µm size is

commonly observed in urban environments or produced by

therapeutic inhalation devices. The deposition patterns for

both particle sizes result in the formation of hot spots at the

top and bottom parts of the central bifurcation zone where

the cross sectional area is decreasing downstream into the

daughter branches.

Identification of high density area (hot spots)

Deposition fractions are analyzed in a way that enhancement

factors are used to account for such high density areas. The

enhancement factor is defined as the ratio of the maximum

number of deposited particles per unit area to the number of

deposited particles in the whole bifurcation surface area.

Thus, enhancement in the carinal ridge of central airways

attain values that range from 50 to 400. 33 These values

indicate that there are areas within the bifurcation where the

local dose caused by aerosolized air pollutants can be more

than two orders of magnitude higher than the average dose.

The probability of occurrence of such high density areas in-

creases with growing particle size where inertial impaction

becomes the more dominant deposition mechanism. 33

Factors affecting deposition

Effect of particle size

The preferential deposition sites in terms of deposition frac-

tion in different regions of the lung as a function of particle

diameter are shown in Figure 6. Total Deposition Fraction

(TDF) increases for large particle (> 0.4 µm) and for small

particle sizes (< 0.2 µm) for a given breathing pattern. The

most common one found in publications related to deposition

is that particles in the range of 1-5 µm are deposited in the

deep lungs (BB and bb airways); while UF and coarse (>5

µm) particles generally deposited in the ET region. 34-37

theHealth | Volume 2 | Issue 2 | 56

Figure 6: Average predicted total and regional lung deposition

based on ICRP 1 deposition model for nose breathing for light

exercise breathing condition. Highest deposition (ET region for 0.001

and 10 µm particles, bronchi region for 0.005 to 0.007 µm particles

and alveolar region for 0.01 to 0.05 µm particles).

Figure 7:

a) Illustrations of carinal

ridges in vivo taken during

broncho-fiberscope

examinations 32 b) Projected

inspiratory spatial deposition

patterns and related

deposition efficiencies for

particle diameter 0.2 and 5

µm in a physiologically

realistic model bifurcation on

the basis of 100, 000

randomly selected particles.

Where A and B are two

daughter airways. 33

Lung deposition predictions of airborne particles

Studies by Newman and Chan suggest that a particle size of

3 µm as an upper limit for related deposition in the deep

lungs. 38 Whereas, the deposition of the smaller particles

take place in the smaller airways (peripheral lung region).

Effect of flow rate

Flow rate is an index for representing the breathing intensity

of human lung ventilation that is, the volume of air inspired or

exhaled per unit of time. Deposition fractions of inhaled par-

ticulates are significantly influenced by this flow rate, which

varies with activity level and age of a person. The ICRP has

defined four different breathing conditions and relevant flow

rates according to the human activity condition, i.e. sleeping,

sitting, light exercise and heavy exercise breathing conditions

with flow rates of 15, 18, 50, and 100 L/min respectively.

The flow rate itself exerts different effects on the deposition

of fine (< 0.2 µm) and coarse (> 0.4 µm) particle sizes. The

deposition fraction decreases with an increase of flow rate

for fine particle and increases for coarse particles for both

nasal and oral breathing conditions (Figure 8).

Effect of age and gender

As a person ages, anatomical changes of the respiratory

tract also affect deposition fraction. 39 When ventilatory

conditions are held constant, age dependent changes in mor-

phology of the lung results in a decrease of the deposition

fraction with age (Figure 9 and 10). For any person, de-

creasing lung volume not only increases the deposition frac-

tion but also causes the major site of particle deposition

within the airways to shift from the lung periphery to more

central airways. 40 At low lung volumes, airways have

smaller cross-sectional areas, higher linear velocities, and thus

enhanced deposition by impaction in central airways for a

given flow rate. Children under 8 years old who are breath-

ing at rest have higher total, ET, and TB deposition fractions

and lower alveolar deposition fraction than adults. 41-44 The

studies conducted by Kim et al. 45 for young and elderly

suggest that healthy elderly subjects are not subjected to

greater lung deposition of ultrafine particulate matter than

younger ones.

Gender differences in laryngeal and airway geometries may

cause women to have greater upper airway deposition as

compared with men. 47 However, comparable to or smaller

deposition in woman than those of men in the deep lung is

found in studies by Kim et al. 48 Total lung deposition was

comparable between men and women for fine (0.1 to 2.5

µm) but was consistently greater in women than men for

coarse particles (>2.5 µm) regardless of the flow rates used:

it ranges from 9 to 31 % and are probably related to the

smaller dimensions of their upper airways and subsequent

enhancement of inertial impaction (Figure 10).

Ultrafine aerosols characteristics

Ultrafine (UF) aerosol constitutes the major concentration of

ambient aerosols. The major contributors of these aerosols

are the combustion aerosols emitting from motor vehicles. The

combustion aerosols contain heavy metals, including lead and

mercury. UF can produce toxicity even at low concentrations

due to their small size and surface characteristics. 49 The non-

soluble UF particles deposited in the pulmonary region (with

slow clearance mechanism) have sufficient time to penetrate

into interstitial region and ultimately accumulate there for

years. 50 The UF particles translocated to other organs can

interact with organs at the cellular level. The scientific commu-

nity is working hard to determine the mechanisms of these

interactions and their impact at the sub-cellular and molecu-

lar levels.

Conclusion

Presently, available deposition models predict almost similar

total, regional and to some extent, also local deposition. In g-

Figure 8: Nasal, oral and thoracic deposition fractions as function of

particle diameter during sleeping, sitting, light exercise and heavy

exercise breathing conditions calculated using stochastic lung

depositing code IDEAL.

Figure 9: Deposition fraction as a function of lung airway generation

for children of ages 5, 10, and 15 years and for adults for 3 µm

particle size under sitting breathing conditions. (modified 46)

57 | theHealth | Volume 2 | Issue 2

Lung deposition predictions of airborne particles

eneral, empirical and semi-empirical models are more reli-

able predictive tools than the theoretical models. However,

the stochastic lung deposition model takes care of the sto-

chastic nature of the lung structure and particle transport

within the lung thereby providing information about the sta-

tistical distribution of deposition pattern, reflecting intra and

inter subject variability in particle deposition. The results ob-

tained with different modeling approaches reveal that the

structure of the deposition patterns strongly depends on the

flow rates and the inhaled particle sizes. The deposition pat-

terns reveal enhanced deposition at the top and bottom parts

of a central bifurcation unit for all particle sizes, even for

nanometer-sized ultrafine particles and flow rates. TDF in-

creases for both large particle sizes (> 0.4 µm) and for small

particle sizes (< 0.2 µm) for a given breathing pattern. Chil-

dren exhibit higher deposition efficiency than adults for all

inhalable particle size ranges and states of activity. Similarly,

females are consistently found to have greater deposition

than men for coarse particles regardless of flow rates.

Acknowledgements

The authors wish to thank Prof. Werner Hofman and Dr.

Renate Winkler-Heil for their support in modifying and using

the IDEAL code.

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