TOXICOLOGICAL SCIENCES 71, 104 –111 (2003)
Copyright © 2003 by the Society of Toxicology
Respiratory Deposition and Inhalability of Monodisperse
Aerosols in Long-Evans Rats
Bahman Asgharian, 1 James T. Kelly, and Earl W. Tewksbury
CIIT Centers for Health Research, 6 Davis Drive, Research Triangle Park, North Carolina 27709-2137
Because of limitations on conducting exposure experiments using human subjects to evaluate adverse health effects, the deposition and fate of airborne particles in animals are often studied. The
results of such studies are extrapolated to humans to estimate
equivalent dose and subsequent response. In this article, particle
inhalability and respiratory deposition of micron-size particles are
determined for female Long-Evans rats. Monodisperse aerosols
were generated from a solution of radiolabeled iron chloride
( 59FeCl 3). Long-Evans rats were exposed to the radiolabeled particles in a Cannon nose-only exposure tower to determine head,
lung lobar, and total lung deposition fractions. Particle deposition
fractions in a hypothetical situation, when all particles are inhalable, were found from an experimentally validated deposition
model. Particle inhalability in a Cannon nose-only exposure scenario was obtained by comparing the measured deposition fractions with the predicted values for the case of 100% inhalability.
Particle deposition fraction and inhalability were compared with
data available in the literature. For large particles, the measured
deposition fraction was lower than the literature values. Consequently, our inhalability estimates were found to be lower than
previously published values. The findings here will directly affect
health risk assessments in humans from exposure to airborne
particles. The deposition results will improve the database on
particle deposition in the lung airways of rats, and inhalability
information will improve the accuracy of rat-to-human data extrapolation.
Key Words: aerosol; deposition; inhalability; Long-Evans rats;
mathematical modeling.
The adverse health effects of exposure to airborne particulate matter (PM), particularly among sensitive subpopulations,
have been recognized in recent years (Bascom et al., 1996;
Brunekreef et al., 1995; Dockery and Pope, 1994; Hruba et al.,
2001; Jedrychowski et al., 1999; McConnell et al., 1999;
Norris et al., 1999; Pope et al., 1995a,b; Roemer et al., 2000;
Schwartz, 1994; Schwartz and Neas, 2000). To assess potential
health risks of PM exposure, one must first determine the initial
This study was supported in part by the U.S. Environmental Protection
Agency. It has not been subjected to any review by the Agency. It does not
necessarily reflect the views of the U.S. EPA, and no official endorsement
should be inferred.
1
To whom correspondence should be addressed. Fax: (919) 558-1300.
E-mail: [email protected].
104
deposition and subsequent clearance of inhaled material. Studies have been conducted to measure the fate of PM in animals
and humans. In addition, morphologically based mathematical
models have been developed to predict and extrapolate dose
(deposition plus clearance) across species from animals to
humans. Predictive models are particularly useful when data
are not available and conducting experiments is not feasible.
Dosimetry models can be tied to biological end points to
construct comprehensive, biologically based dose-response
models for use in PM risk assessment.
To exert harmful effects, inhaled particles must deposit in
lung airways and translocate to various sites and organs in the
body to yield response. Thus the fate of inhaled particles in the
respiratory tract, as the initial port of entry in the body, is a
critical first step toward realistic risk assessment for exposure
to airborne particles. Due to ethical obligations or legal restrictions, studies in humans are limited. There is a wealth of
information in the literature on exposure studies using rodents
to study and model deposition, translocation, and response.
However, weak links exist in the process of extrapolation of
animal results to humans. For example, there are uncertainties
regarding the selection of a relevant dose-metric between humans and animals. The uncertainties arise from differences in
airway morphology, biological sensitivities, and breathing parameters. Equivalent rather than identical exposure scenarios
are required to yield an equal dose in animals and humans since
particles completely inhalable by humans may be only partially
inhalable by rodents. In addition, nasal filtration, which controls the number of particles that reach the deep lung and are
available for deposition, may vary considerably between humans and rodents. Thus, information on particle inhalability
and regional deposition in rodents is critical in studying the
adverse health effects posed to humans from exposure to
airborne particles and to performing appropriate risk analyses.
Regional deposition fractions can be found from direct measurements, and information on particle inhalability can be
obtained from particle deposition studies in control studies.
Particle inhalability is found by comparing particle deposition
fractions between scenarios where particles are all inhalable
and a fraction of particles are inhalable.
The lack of inhalability information on airborne particles
could increase the variability of the deposition data as well as
introduce errors in interspecies dosimetric comparisons. For
DEPOSITION AND INHALABILITY IN RATS
nasal breathing, particle inhalability is essentially the sampling
or aspiration efficiency of the nasal opening and generally
limits the number of particles entering the respiratory tract
(Vincent, 2002). Inhalability, therefore, depends in part on the
size and orientation of the nasal opening and is different
between humans and animals. Particles that are completely
inhalable by people may be only partially inhalable by animals.
Realistic extrapolation of deposition data from animals to
humans should include an inhalability adjustment. There is
considerable information on the inhalability (or aspiration efficiency) in humans (Aitken et al., 1999; Breyesse and Swift,
1990; Erdal and Esmen, 1995; Hsu and Swift, 1999; Kennedy
and Hinds, 2002; Ogden and Birkett, 1977; Vincent and Armbruster, 1981; Vincent et al., 1990). However, additional studies are necessary to correlate inhalability, since particle aerodynamic properties and subject breathing parameters are both
important in determining inhalability.
The database on particle inhalability in animals is inadequate. No experimental study has been designed specifically
for determining inhalability. The study by Menache et al.
(1995a) is the only study to date that offers inhalability adjustment curves, which were based on the particle deposition data
of Raabe et al. (1988) for various laboratory animal species.
Since particles larger than 3.5 ␮m showed a decrease in total
deposition, Menache et al. (1995a) assumed that these particles
deposited completely in the lung and that the ratio of the
internal dose to inhaled mass represented particle inhalability.
Due to the lack of sufficient data across particle size, the data
of various rodents were combined to calculate a single inhalability curve.
Due to experimental difficulties associated with measurement of particle deposition, few studies have addressed both
the deposition of fine and coarse inhalable particles in rats.
Chen et al. (1989) measured regional, lobar, and total deposition of cigarette smoke in Fischer-344 rats, exposed nose-only,
for 25 min a day for 5 days. Dahlbäck and Eirefelt (1994) and
Dahlbäck et al. (1989) exposed male Sprague-Dawley rats to
particles in a nose-only tower for 10 min and measured total
deposition fraction and distribution in the lung. Raabe et al.
(1977, 1988) studied deposition of various sized particles in
different regions of the respiratory tracts of common laboratory
species, including Long-Evans rats. These carefully conducted
studies did not measure minute ventilation of the test animals
(Raabe et al., 1988), which is essential for a precise assessment
of deposition. For large particles, deposition in the head region
is largely by impaction. This means that losses mainly depend
on the particle Stokes number, which, in turn, depends directly
on the animal’s ventilatory parameters. Raabe et al. (1977,
1988) used allometric equations to determine breathing rates
rather than measuring the animal’s ventilatory parameters. In
addition, since particle clearance in most nasal regions is rapid,
particle deposition studies have to be of short duration. This
requires a generation system that produces a high concentration
of monodisperse particles (10 6/cm 3) to give measurable depo-
105
sition during this period. In the aforementioned studies, some
of these points were not addressed fully, thereby increasing
variability of the results.
A number of studies have been published on modeling
particle deposition in the rat lung. These models are semiempirical (Menache et al., 1995b), stochastic (Koblinger et al.,
1995), or deterministic (Schum and Yeh, 1980; Yu and Xu,
1986). Deterministic models are most often typical-path, and
are therefore based on symmetric lung structures. Typical-path
deposition models cannot address variation of dose among
different airways at a given generation. Anjilvel and Asgharian
(1995) introduced a new generation of the mathematical model
of particle deposition in the lung. They calculated particle
deposition using a multiple-path analogy operative for any lung
geometry structure. This model gave a more realistic prediction
of deposition, since variations of deposition among different
airways of the lung were innately calculated. The deposition
model calculated particle deposition in every airway of the
lower respiratory tract (LRT) from the information on breathing rate, lung parameters (e.g., airway length and diameter,
branching angle), and deposition efficiencies per airway. Sitespecific, lobar, regional, and total deposition fractions were
calculated by adding deposition fractions of the individual
airways.
Realistic PM deposition and clearance models for animals
that incorporate physical phenomena require accurate lung
geometries as input. Although one of the most commonly used
laboratory animals is the rat, a set of structural data comprising
the entire respiratory tract for any strain of rat is currently not
available. Different strains of rats have been used for measurements of structure in different regions of the respiratory tract.
For example, a detailed reconstruction of nasal geometry was
developed for the Fischer 344 rat (Kimbell et al., 1993), while
numerous anatomical measurements of the conducting airways
of a Long-Evans rat were made by Raabe et al. (1976). In
addition, the acinar region, for which structural data are extremely difficult to obtain, has been reconstructed for SpragueDawley and CD-1 rats (Mercer and Crapo, 1988).
Despite progress on particle deposition in the lungs of rats,
a significant data gap remains to be filled. A unified dataset on
lung structure in a given strain of rat is desirable. Particle
deposition in various regions of the lung, as well as inhalability
fraction, must be established. This article details a study conducted to measure deposition and inhalability of particles in the
respiratory tract of the Long-Evans rat, which was selected for
this study because of the availability of anatomical data for its
conducting airways and because this data set was used in a
mathematical dosimetry model to calculate particle deposition
in the rat lung (Anjilvel and Asgharian, 1995). Particle deposition fraction in the nasal passages and in various lobes and
regions of the Long-Evans rat lung was measured following a
nose-only exposure to radiolabeled monodisperse particles. Rat
inhalability fraction was obtained by comparing lung deposition measurements with predictions.
106
ASGHARIAN, KELLY, AND TEWKSBURY
MATERIALS AND METHODS
Animals. Fifty-five female Long-Evans rats were obtained from Charles
River Breeding Laboratories (Raleigh, NC). The rats were certified to be free
of respiratory disease and had a mean body weight of 298 g (⫾ 24 g). Animals
were housed, two per cage, in 48 ⫻ 27 ⫻ 20-cm polycarbonate cages with
Alpha-dri bedding (Shepherd Specialties Paper, Kalamazoo, MI). The temperature in the animal rooms was maintained at 20 ⫾ 5°C, with a relative humidity
of 50 ⫾ 15%. Animals were kept on a 12-h light cycle (0700 –1900 h). Food
(NIH-07 pelleted diet, Zeigler Bros., Gardener’s Point, PA) and reverseosmosis water were provided ad libitum.
Aerosol generation and characterization. A condensation monodisperse
aerosol generator (CMAG) (Model 3475; TSI Inc., St. Paul, MN) was used that
produced monodisperse, radiolabeled aerosols with aerodynamic diameters
ranging from 0.9 to 4.2 ␮m (0.9, 1.3, 1.9, 2, 2.3, 2.6, 2.7, 3.1, 3.4, 3.6, and 4.2
␮m). Aerosol diameters had an average geometric standard deviation (␴ g) of
1.12 (all values of ␴ g were less than 1.18), reflecting monodisperse aerosols.
Radiolabeled particles were formed in the CMAG by heterogeneous condensation of triphenyl phosphate (TPP) with a density of 0.95 g/cm 3 (Tomaides et
al., 1971) onto radioactive iron chloride seed particles. Iron chloride solution
with gamma-emitting 59Fe of 44.6 days half-life and a specific activity of about
20 –30 mCi/mg was purchased from Perkin Elmer Life Sciences, Inc. (Boston,
MA) as iron chloride in 0.5 M hydrochloric acid.
The 59Fe solution was transferred to an atomizer inside the CMAG and
diluted with distilled water to a concentration of 12 to 16 mg/l. Compressed air
was delivered to the atomizer using a setting of 60 psi. The resulting aerosol
was diffusion-dried and then bubbled through a temperature-controlled saturator containing heated TPP. The mixture of dry 59Fe seed aerosol and TPP
vapor leaving the saturator entered a condensation chimney where TPP condensed into particles with a radioactive 59Fe core. Monodisperse aerosol was
sampled from the condensation chimney and delivered to the exposure system.
Desired particle sizes were achieved by controlling the concentration of TPP
vapor and 59Fe seed aerosol in the condensation chimney. Particle size was
monitored during animal exposures with an aerodynamic particle sizer (APS)
(Model 3320; TSI Inc., St. Paul, MN).
Exposure system. A schematic diagram of the exposure apparatus is
shown in Figure 1. Aerosol, pumped from the CMAG by a peristaltic pump
(Masterflex威 Model No. 7565, Cole Parmer Instrument Co, Chicago, IL), was
combined with filtered air generated with a house air source and delivered at
a constant flow rate using a mass flow controller (MKS Instruments, Model
246B, Andover, MA). A fraction of this flow was diverted to an APS; the
remaining flow, approximately 4.5 l/min, was delivered to a 52-port, Cannon
nose-only tower (Lab Products, Maywood, NJ). Seven ports of the tower were
used for experimental purposes (5 for animal exposure, one for a filter sample,
and one for a pressure gauge); the unused ports were plugged. Therefore
approximately 640 ml/min of aerosol flow was delivered to each port.
Aerosol from one port of the nose-only tower was collected on a 47-mm
glass filter (0.5 ␮m Prefilter, Osmonics Inc., Minnetonka, MN) during exposure. A constant flow rate through the filter was maintained using a mass flow
controller and house vacuum source. A slightly negative pressure (⬃2.5 mm of
water) was maintained in the nose-only tower by controlling the vacuum pull
on the tower with a rotameter and monitoring pressure in one port (Fig. 1) with
a differential pressure gauge (Magnehelic威, Dwyer Instrument Co., Michigan
City, IN).
Five female Long-Evans rats were loaded into nose-only tubes (Skornik
Plethysmograph, CH Technologies Inc., Westwood, NJ) 10 –15 min prior to an
exposure event. An exposure event involved exposing 5 rats to a single size of
monodisperse, radiolabeled aerosols for a given period of time. A total of 11
exposure events were conducted. The length of exposure ranged from 10 to
17.5 min, with an average of 12 min. Exposures times were chosen to enable
depositing sufficient radioactivity for detection in airway tissues while minimizing the time for nasal and TB clearance. Immediately following cessation
of exposure, animals were killed by CO 2 asphyxiation, and relevant tissues
(i.e., nasal passage, larynx, esophagus, trachea, lung lobes, stomach, and
FIG. 1. Schematic diagram of the generation and exposure system. Particles were generated by a CMAG and delivered to rats via a nose-only
exposure apparatus at a flow rate of approximately 1.5 times the per/min
ventilation of the rats. Airflow conditions were kept constant in all exposure
events.
duodenum) were dissected. The dissection involved isolating an entire lobe or
region of the respiratory or digestive tract, not just a representative sample;
total activity in a lobe or region was counted and used to estimate deposition.
Breathing measurements. Animal respiratory parameters were monitored
for a 5-min period prior to aerosol exposure and continuously during exposure.
Each animal was placed in a nose-only tube with an opening through a screen
pneumotachograph in the back end of the tube. The animal’s nose was led
through a latex collar placed near the nose-only tube entrance, creating an
airtight seal (Fig. 2). During breathing, contraction and expansion of the
animal’s thorax forced air into and out of the chamber through the pneumotachograph, thereby causing periodic changes in chamber pressure. Measurements of respiration were made using a conventional data acquisition system
(Buxco Electronics, Sharon, CT) by monitoring fluctuations in chamber pressure and calibrating pressure fluctuations to volume displacements.
Deposition fraction. Following exposure and necropsy, deposited gamma
radioactivity was measured in dissected tissues by scintillation counting. A
7.62-cm NaI(T1) detector was used in a computer-controlled gamma counting
system (Cobra Model 5003, Packard Instrument Company, Downers Grove,
IL). Energy emissions between 940 and 1400 keV were measured in each tube
for 2 min. Energy emitted outside this range was not related to 59Fe, whose
principal gamma photons are 1099 (56%) and 1292 (44%) keV. Background
radiation was accounted for by subtracting the activity measured in an empty
sample tube from that measured in each tissue sample.
Each dissected tissue was placed in a single sample tube except for the
nasopharynx and stomach. Since these tissues were each too large to fit into a
single sample tube, the nasopharynx and stomach tissues were split into two
tubes. Tools used to divide these tissues were wiped onto the respective tissue
to minimize the potential loss of deposited activity. Cross-contamination of
tissue samples was avoided by sanitizing the tools between tissue dissections.
Deposition fraction of inhaled material per breath of animal is a unique
quantity as long as breathing parameters and lung geometry remain the same.
DEPOSITION AND INHALABILITY IN RATS
107
where DF i is the deposition fraction in the i th airway, A i the activity in the i th
airway, A fs the activity on the filter sample, Q fs the filter sample flow rate, MV
the animal minute volume, t fs the filter sampling time, and t exp the exposure
time.
The exposure time, filter sampling time, and flow rate were set during each
exposure event. These values plus the measurements of A i, A fs, and animal
breathing parameters were used in Equation (1) to determine the deposition
fraction in each animal per exposure event.
RESULTS AND DISCUSSION
FIG. 2. Animal breathing measurement apparatus. (a) Skornik Plethysmograph tube housing the rat with a pneumotach. (b) Rats in a Cannon nose-only
tower during an exposure episode. Breathing rates were measured during the
exposure.
Deposition fraction is defined as the amount of material deposited in the tissue
divided by the amount of the material inhaled. Deposition fraction depends on
particle aerodynamic properties, lung geometry, and breathing parameters of
each animal and varies among different regions of the lung. Deposition fraction
is determined by dividing the activity counted in each tissue sample by the
activity counted on the filter sample that collected aerosol from the exposure
atmosphere. The following formula, which accounts for differences in sampling rate of the animals and filter sample, was used to calculate the deposition
fraction:
DF i ⫽
A i Q fs t fs
A fs MV t exp
(1)
Since both impaction and sedimentation mechanisms contribute to the deposition of fine and coarse particles in the lung,
particle deposition in the lung depends on several parameters
such as particle diameter, breathing parameters, and particle
inhalability. Particle deposition in the lung of a single animal
can be justifiably plotted as a function of particle diameter
when other variables just mentioned remain constant. The
deposition fractions in the left lung and caudal, medial, cranial,
and accessory lobes of the right lung were plotted against
particle aerodynamic diameter in Figures 3A–3E. Deposition
patterns were similar among the lobes of the lung. Consistent
with model predictions (Anjilvel and Asgharian, 1995), particle deposition is expected to increase with particle diameter,
reach a maximum, and decline thereafter due to the filtering
effects of the upper respiratory tract. This trend was also
observed in the deposition measurements of Raabe et al.
(1988) in Fischer 344 rats (Menache et al., 1995a). A general
trend was observed (Figs. 3A–3E) despite intersubject variability, differences in breathing parameters, and particle inhalability in particular, which somewhat smeared the expected pattern
of deposition fraction with particle diameter.
Each lobe of the lung received a different amount of deposition since deposition correlates with lobar volume. This is
evident by comparing mean deposition fractions in different
lobes of the lung (Fig. 3). Basically, the left lung received
higher deposition than each lobe of the right lung. In the right
lung, the caudal and accessory lobes had the highest and lowest
deposition, respectively. Depositions in the right medial and
right cranial lobes were similar. This information is useful in
FIG. 3. Mean (⫾ standard deviation) of lung lobar deposition fraction of particles in Long-Evans rats for all exposure events: (a) left lung, (b) right caudal
lobe, (c) right medial lobe, (d) right cranial lobe, and (e) right accessory lobe. Each event included exposure of 5 animals to monodisperse particles with 59Fe
seeds at a prescribed particle size that varied between events. An entire lobe was used for deposition measurements.
108
ASGHARIAN, KELLY, AND TEWKSBURY
FIG. 4. Regional head and lung deposition fraction of particles in LongEvans rats as a function of particle diameter. Each measurement indicates the
mean particle deposition fraction (⫾ standard deviation) for five animals, each
having different breathing rates.
the lobe selection for dissection to study physical and biological end points.
The mean and standard deviation of the head and lung
deposition fractions in Long-Evans rats were plotted against
particle diameter (Fig. 4). Head deposition fractions showed an
increase followed by a decrease with particle size. Variability
in the data prevented identifying the particle size at which
maximum head deposition fraction was achieved. Most deposition occurred in the nasal passages. Due to strong filtering in
the head and inertial losses in the upper airways of the LRT,
particle deposition in the lung appeared to remain relatively
constant. The lung deposition fraction was below 10% for most
particle sizes. Considerable variation in deposition results was
observed, since each animal had different breathing parameters
and head orientation in the nose-only tube, which would alter
particle inhalability.
Total deposition fraction results in the respiratory tract were
compared with the results of Raabe et al. (1977, 1988). The
initial data of Raabe et al. (1988) showed a decrease in deposition with particle size for particles larger than about 3 or 4
␮m but was adjusted to 100% deposition. Menache et al.
(1995a) used the original data of Raabe et al. (1988) and
recalculated deposition fractions. The values given by Menache et al. (1995a) are shown here in Figure 5, along with the
results of the present study. There was a good agreement
between the results for particles of submicron and micron size.
For particles of 3 to 10 ␮m, however, the deposition values of
Menache et al. (1995a) were significantly greater than those of
the current study. An uncertainty in the recalculated values of
Raabe et al. (1988) by Menache et al. (1995a) arose from the
lack of breathing-parameter measurements that were required
to calculate the amount of inhaled material. Instead, the authors
used the expression by Guyton (1947), which was for a resting
FIG. 5. Comparison of total deposition fraction with available relevant
data in the literature. Values provided by Menache et al. (1995a) are recalculations of the measurements made by Raabe et al. (1988).
animal, to estimate breathing parameters. This equation underestimates an animal’s minute ventilation during a nose-only
exposure. The minute ventilation in the study of Raabe et al.
(1988) was calculated to vary between 85 and 140 cm 3/min,
which is clearly lower than our measurements (Table 1) despite
animals in the two studies having similar weights. The measured breathing parameters in Table 1 are consistent with the
measurements of Mauderly (1986). The deposition data of
Menache et al. (1995a) are probably high due to underestimation of animal breathing parameters, which resulted in underprediction of inhaled dose and over-prediction of deposition
fraction.
Our findings as well as others (Dahlbäck and Eirefelt, 1994;
Raabe et al., 1988) showed a decrease in total particle deposition, with increasing particle size for micron-size particles.
TABLE 1
Average of the Animal Breathing Parameters for Each
Exposure Event
Particle
diameter (␮m)
Breathing
frequency (/min)
Tidal
volume (cm 3)
Ventilation
(cm 3/min)
1.3
0.9
2.0
2.6
3.1
3.6
2.8
3.4
2.3
1.9
4.2
125 ⫾ 26
190 ⫾ 46
149 ⫾ 13
143 ⫾ 22
137 ⫾ 12
127 ⫾ 13
129 ⫾ 15
120 ⫾ 9
169 ⫾ 38
152 ⫾ 13
134 ⫾ 27
1.66 ⫾ 0.72
1.57 ⫾ 0.56
1.73 ⫾ 0.48
1.62 ⫾ 0.54
1.55 ⫾ 0.30
1.48 ⫾ 0.30
1.45 ⫾ 0.45
1.45 ⫾ 0.24
1.50 ⫾ 0.37
1.49 ⫾ 0.45
1.81 ⫾ 0.54
192 ⫾ 51
287 ⫾ 101
255 ⫾ 74
236 ⫾ 101
211 ⫾ 44
189 ⫾ 64
189 ⫾ 70
175 ⫾ 24
244 ⫾ 68
209 ⫾ 52
249 ⫾ 105
Note. Values are average ⫾ standard deviation.
109
DEPOSITION AND INHALABILITY IN RATS
FIG. 6. Computed inhalability fraction of aerosols from deposition fraction measurements. The deposition fraction of 100% inhalable particles (⌬)
(Asgharian and Anjilvel, 1995) and those for partially inhalable particles (DF)
presented in Figure 3 are used in Equation 2 to calculate inhalability.
This trend can be readily explained in terms of particle inhalability (Menache et al., 1995a). When the particle exceeds a
certain size in a given breathing scenario, the inertia reduces
the probability of a given-sized particle entering the respiratory
tract. The inhalability (or aspiration efficiency) can be calculated from comparison of the deposition measurements with
theoretical prediction of deposition at different sizes. Menache
et al. (1995a) assumed that particles larger than 3.5 ␮m had
100% deposition in the respiratory tract, and thus inhalability
was computed as the ratio of deposited to inhaled material for
particles larger than 3.5 ␮m. However, particle size transition
from 100% inhalable to partially inhalable does not necessarily
start at 100% deposition fraction. The analyses neglected the
group of particles that have deposition fractions of less than
100% but were partially inhalable. Deposition measurements
for the entire inhalable size range are necessary to accurately
assess inhalability. The study of Raabe et al. (1988) used few
particle sizes and excluded particle sizes in the interval from 1
to 3 ␮m.
Inhaled particle concentration may be less than that in the
exposure air for fine and coarse particles. Particle inhalability
depends on particle size as well as on breathing parameters and
is calculated using the following equation,
IF ⫽
DF
⌬
model of particle deposition in the lungs of rats based on the
morphometric lung measurements of Long-Evans rats (Raabe
et al., 1976). The model was used here to calculate ⌬ based on
the measured breathing parameters of the animals in the study.
Inhalability was calculated from Equation 2 and plotted against
the inertial parameter ␳d 2Q in Figure 6, where ␳ is particle
density, d is particle diameter, and Q is inhalation flow rate.
The parameter ␳d 2Q combines particle diameter and breathing
parameters and is a measure of particle inertia. Due to experimental variability, a number of calculated inhalability values
were higher than 100%. These values were not included here.
The calculated inhalability fractions showed variability with
␳d 2Q, but there was a decline in inhalability with increasing
␳d 2Q. A predictive model of inhalability in Long-Evans rats
was found by fitting a function of the form similar to that given
by Menache et al. (1995a) to the data.
IF ⫽ 1 ⫺
1
1 ⫹ ␣ 共 ␳ d 2 Q兲 ␤
(3)
Equation 3 has the same functional form as that of Menache et
al. (1995a), with the difference of dependence on ␳d 2Q in place
of particle diameter. Fitting Equation 3 to the measurements
yielded ␣ ⫽ 19.87 and ␤ ⫽ – 0.7466 with r 2 ⫽ 0.46. This
equation has the right behavior by approaching 1 for small
particles and 0 for large particles. While some studies in
humans suggested that inhalability reached a plateau for sufficiently large particles (Aitken et al., 1999; Chung and DunnRankin, 1992; Erdal and Esmen, 1995), this equation showed
that inhalability became negligible around 50 ␮m.
The inhalability fraction computed by Equation 3 was compared with the results of Menache et al. (1995a) in Figure 7 for
various particle sizes. Equation 3 was plotted for slow, normal,
(2)
where DF and ⌬ are the measured and theoretical (100%
inhalable) lung regional or total deposition fractions (see Appendix for derivation). Values for ⌬ can be found from studies
where 100% of particles are inhalable or, alternatively, from
experimentally validated mathematical models.
Anjilvel and Asgharian (1995) introduced a mathematical
FIG. 7. Comparison of the inhalability fraction prediction of this study
with that of Menache et al. (1995). Minute ventilations of 3.33 cm 3/s, 7.14
cm 3/s, and 14 cm 3/s, corresponding to slow, normal, and fast breathing, were
used in Equation 3 to calculate particle inhalability.
110
ASGHARIAN, KELLY, AND TEWKSBURY
and fast breathing, corresponding to a minute ventilation of
3.33 cm 3/s, 7.14 cm 3/s, and 14 cm 3/s respectively. The inhalability prediction of Menache et al. (1995a) gave a higher
prediction of inhalability, because their expression was based
on the deposition data of Raabe et al. (1988), which overestimated deposition (Fig. 4). Raabe et al. (1988) also used a
different exposure system for which particle inhalability could
have been different, depending on the orientation of incoming
exposure air with respect to the nose of the animals.
The study presented here provides data on lobar and regional
deposition of particles in Long-Evans rats. The results can be
used to advance and validate lung deposition models and
correlate dose with biological effects in animals. The information on particle inhalability helps with data extrapolation to
humans, where it has an impact on multiple disciplines (e.g.,
the regulatory arena for desiring to minimize the deposition of
the inhaled pollutant and the pharmaceutical industry for maximizing deposition of a therapeutic drug by selecting certain
particle sizes).
Summary
FIG. A-1. Compartmental representation of the respiratory tract (see Appendix, Equation A-1). The respiratory tract is divided into head and lung
compartments with an additional compartment accounting for the inability of
the particles to enter the head due to limitations on inhalability. Airborne
particle concentration at the entrance and exit of each compartment is specified
in the figure.
IF ⫽
Ci
Ca
Particles enter the head airways at C i and exit at C 1, enter the lung and exit at
concentration C 2, and enter the head region again before exiting the respiratory
tract at a concentration C e. Since C a is larger than C i, measured deposition
fraction, DF, is smaller than theoretical deposition fraction ⌬ when a fraction
of the airborne particles enters the respiratory tract. Deposition fractions for the
entire respiratory tract are calculated as below:
Ce
Ci
(A2)
Ci ⫺ Ce
Ca
(A3)
⌬t ⫽ 1 ⫺
Deposition of radiolabeled monodisperse particles in the
lungs of Long-Evans rats was measured following a short
exposure to particles ranging in size from 0.9 to 4.2 ␮m in a
Cannon nose-only exposure system. The breathing rates for all
the animals were measured during the exposure and were used
to calculate the lobar and regional deposition fractions. Deposition patterns were found to be similar among the various
lobes, giving a maximum deposition for particles near 3.5 ␮m.
The caudal and accessory lobes gave the highest and lowest
deposition of particles in the right lung, while the medial and
cranial had similar deposition falling between the two limits.
Our deposition fractions agreed with the literature values for
particles smaller than 3 ␮m but fell short of those for larger
particles, because the breathing parameters used to calculate
deposition were low and resulted in overestimation of deposition values. The particle deposition fraction decreased with
particle size because of an inhalability limitation that increased
with particle size. A new expression based on the deposition
results was introduced for inhalability fraction prediction. This
expression predicted smaller inhalability of particles than the
previously used results of Menache et al. (1995a). The difference in inhalability results was attributed to the difference in
deposition datasets used to construct the inhalability curves.
(A1)
DF t ⫽
where subscript t denotes total deposition fraction. Taking the ratio of equations A2 and A3 and replacing C i/C a with IF from equation 1 gives
IF ⫽
DF t
⌬t
(A4)
The theoretical and measured deposition fractions in the lung compartment are
⌬L ⫽
C1 ⫺ C2
Ci
(A5)
DF L ⫽
C1 ⫺ C2
Ca
(A6)
Taking the ratio and inserting the definition of inhalability fraction yields
IF ⫽
DF L
⌬L
(A7)
Similarly, the inhalability fraction using the head compartment deposition
fractions can be shown to be
IF ⫽
DF h
⌬h
(A8)
APPENDIX
An increase in particle size makes it more difficult for a particle to enter the
respiratory tract on inhalation. Only a fraction of particles in the inhaled air
will be able to make it into the respiratory tract. The transport of the particles
in a compartmental representation of the respiratory tract is depicted in Figure
A-1. Due to partial inhalability, particle concentration initially decreases from
the concentration in the exposure air, C a, to that entering the body, C i. Particle
inhalability fraction is thus
The measurements for regional and total deposition fraction, along with the
predicted deposition fractions for 100% particle inhalability, were used in
equations A4, A7, and A8 to calculate inhalability fractions.
ACKNOWLEDGMENTS
This research has been supported in part by grant R827996-010 from the
Science to Achieve Results (STAR) program of the United States Environ-
DEPOSITION AND INHALABILITY IN RATS
mental Protection Agency and in part by core funding of the CIIT Centers for
Health Research. We are also grateful to Dr. Barbara Kuyper for her editorial
assistance in the preparation of this manuscript
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