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Published ahead of print on October 11, 2007, doi:10.1164/rccm.200602-301OC
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American Journal of Respiratory and Critical Care Medicine Vol 177. pp. 426-432, (2008)
© 2008 American Thoracic Society
doi: 10.1164/rccm.200602-301OC

Original Article

Deposition, Retention, and Translocation of Ultrafine Particles from the Central Airways and Lung Periphery

Winfried Möller1, Kathrin Felten1, Knut Sommerer2, Gerhard Scheuch2, Gabriele Meyer3, Peter Meyer1, Karl Häussinger3 and Wolfgang G. Kreyling1

1 Inflammatory Lung Diseases Clinical Cooperation Group, Institute for Inhalation Biology, and Aerosols and Health Focus Network, GSF-National Research Center for Environment and Health, Gauting and Neuherberg, Germany; 2 Inamed Research GmbH, Gauting, Germany; and 3 Asklepios Hospital for Respiratory Diseases, Gauting, Germany

Correspondence and requests for reprints should be addressed to Dr. Winfried Möller, Ph.D., GSF–National Research Center for Environment and Health Institute for Inhalation Biology, Robert Koch Allee 29, D-82131 Gauting, Germany. E-mail: moeller{at}gsf.de


   ABSTRACT
TOP
 ABSTRACT
AT A GLANCE COMMENTARY
 METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Rationale: Little is known about clearance of ultrafine carbon particles from the different regions of the human lung. These particles may accumulate and present a health hazard because of their high surface area.

Objectives: Technetium Tc 99m (99mTc)–radiolabeled 100-nm-diameter carbon particles were inhaled by healthy nonsmokers, asymptomatic smokers, and by patients with chronic obstructive pulmonary disease (COPD).

Methods: Using a bolus inhalation technique, particle deposition was targeted either to the airways or to the lung periphery, and retention, clearance, and translocation were measured using retained radiotracer imaging.

Measurements and Main Results: In vitro studies revealed that mean leaching of soluble 99mTc-radiotracer from the carbon particles was 4.1 (2.6 [SD]) % after 24 hours. Cumulative 99mTc activity in urine at 24 hours was 1.1 (1.3) % of activity deposited in the lungs. In the lung periphery, particle retention was not affected by smoking or pulmonary disease; retention was 96 (3) % after 24 hours. The small amount of clearance could be attributed to leaching of the 99mTc label, suggesting negligible particle clearance. In healthy nonsmokers, retention of particles targeted to the airways was 89 (6) and 75 (10) % after 1.5 and 24 hours, respectively. Radiolabel activity did not accumulate in the liver.

Conclusions: Within the limits of detection of our experimental system, most inhaled ultrafine carbon particles are retained in the lung periphery and in the conducting airways without substantial systemic translocation or accumulation in the liver at 48 hours. Repeated exposure may result in significant pulmonary accumulation of ultrafine particles.

Key Words: air pollution • clearance • translocation • ultrafine particles



   AT A GLANCE COMMENTARY
TOP
 ABSTRACT
 AT A GLANCE COMMENTARY
METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Scientific Knowledge on the Subject
It has been suggested that ultrafine particles may play a key role in the morbidity and mortality of inhaled air pollution. The fate of inhaled ultrafine particles in the different regions of the human lung is unclear and systemic translocation to other organs, such as the liver or the heart, is under debate.

What This Study Adds to the Field
Inhaled ultrafine carbon particles are retained long term in the human lung and may accumulate. Mucociliary clearance does not remove all particles deposited in the airways and is impaired in smokers and in patients with chronic obstructive pulmonary disease. Our study does not show significant systemic translocation.

 
A growing body of epidemiological studies has shown consistent associations between exposure to particulate urban air pollution and acute increases in morbidity and mortality rates, especially for persons with obstructive pulmonary and cardiovascular diseases (1–3). In almost all these studies, the sources of air pollution were widely spread over the urban area: carbonaceous aerosols from incomplete combustion processes, including exhaust emissions from motor vehicles and fossil fuel domestic heating.

How exposure to inhaled pollutants might cause adverse cardiovascular effects is still under debate, but ultrafine particles (UPs; diameter <= 100 nm) may play a causal role. Although the UP fraction contributes little to the total particle mass concentration, it contains a very large number of particles (2). Studies have shown that high doses of UPs can trigger toxicological responses, such as oxidative stress formation and the induction of inflammation, increased blood coagulation, impaired cellular defense, and modulation of the immune system (reviewed in References 4 and 5).

Estimation of the doses of inhaled fine particles and UPs, which may have toxic properties, requires knowledge of several mechanisms: regional deposition, retention and clearance, toxicology, solubility, systemic translocation, metabolism and accumulation in organs, and the excretion pathways via urine and feces (4). After inhalation, UPs may follow different pathways from those of fine particles, including translocation to the blood circulation and hence other organs (6, 7). Human studies give conflicting results: whereas Nemmar and colleagues (7) reported rapid uptake of radiolabeled carbon UPs into the bloodstream and deposition in the liver, Brown and colleagues (8) and Mills and colleagues (9) found no UPs outside the lungs detectable by gamma camera (<1% of inhaled activity).

Here, we report on lung deposition, retention and clearance, and translocation of 100-nm radiolabeled carbon particles in three study groups: healthy volunteers, cigarette smokers, and patients with chronic obstructive pulmonary disease (COPD). Cigarette smokers and patients with COPD are believed to be especially susceptible to inhaled particles. Because the conducting airways and the alveoli are anatomically different structures with different clearance mechanisms, we targeted UPs to these regions separately using the aerosol bolus technique. Because interpretation of the studies cited above is confounded by the high rates of leaching of the radiolabel from the particles, we improved UP production to keep leaching rates at the 1–2% level.

Some of the results of this study have been reported previously in the form of an abstract (10).


   METHODS
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 ABSTRACT
 AT A GLANCE COMMENTARY
 METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Subjects and Study Protocol
A total of 9 healthy never-smokers, 10 asymptomatic smokers, and 7 patients with COPD participated in the study. Respiratory symptoms were identified using a questionnaire (11), and pulmonary function parameters were measured by spirometry and body plethysmography (Jäger Masterlab, Würzburg, Germany) (12). Physiological dead space was measured using a fast mass spectrometer (13, 14) combined with the inhalation of a tracer gas mixture (see online supplement). The protocol was approved by the ethics committee of the Medical School of the Ludwig Maximilian University (Munich, Germany), and informed consent was obtained from each subject.

Figure 1 shows the protocol for aerosol inhalation and retention measurements. Each subject inhaled 100-cm3 aerosol boluses of technetium Tc 99m (99mTc)–radiolabeled carbon UP ({approx}2 MBq) twice, once as shallow boluses to target the conducting airways (AW), and the second time as deep boluses to target the alveoli (AL). There was a period of at least 2 weeks between the two aerosol inhalations. Using the 99mTc activity, UP retention was measured in a gamma-spectroscopy lung counter (LuCo; GSF-National Research Center for Environment and Health, Neuherberg, Germany) at 10 minutes, 1.5, 5.5, 24, and 48 hours after inhalation. Posterior gamma camera (DIACAM; Siemens, Erlangen, Germany) images were obtained after 45 minutes and 5 hours. Blood samples (8 ml) were taken from the arm vein 1 and 5 hours after UP inhalation. The 99mTc activity concentration in the blood samples was measured in a shielded well-type gamma counter. This concentration was multiplied by the total blood volume (assuming the total blood pool to be 7% of the body weight) to estimate the total 99mTc activity in the blood. Total urine was collected during the first 24 hours after aerosol inhalation and its activity was measured in the LuCo.


Figure 1
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  		Figure 1. Study protocol, including particle inhalation, lung counter (LuCo) and gamma camera (GC) measurements, and blood and urine sampling. UP = ultrafine particle.

 
Preparation of UP and Aerosol Bolus Inhalation
The experimental setup of the generation, monitoring, and inhalation system has been described in detail elsewhere (15). Radiolabeled carbon UPs were generated using a Technegas Generator (Vita Medical Ltd., Lucas Heights, Australia), to which modifications were made to obtain particles of defined size and low radiolabel leaching. Immediately after generation, the aerosol was diluted 1:10 into a flexible metal-coated, electrically conducting bag. Leaching of the radiolabeled from filter samples of the particles was measured in vitro using 0.9% saline.

Particles were administered with a respiratory aerosol probe (see online supplement). The respiratory aerosol probe (16) allows aerosol inhalation under controlled breathing conditions, and the application of an aerosol bolus to a predefined lung depth. For shallow aerosol bolus administration to the airways, the bolus penetration front depth (VF), was adjusted to the phase I dead space volume (VDP1), of each subject. For deep aerosol bolus administration to the alveoli, an VF of 800 ml was used. An 8-second breath hold was performed at the end of inhalation to increase particle deposition. The fraction deposited was assessed using filter samples of the inhaled and exhaled aerosol (see online supplement).

Lung Counter Imaging
The LuCo (see online supplement) consists of four 5 x 3–in collimated and shielded NaI crystals, two of which are located above and two below the body (17). The LuCo makes sensitive measurements by gamma spectroscopy of radioactivity in the head, lung, abdomen (gastrointestinal tract, including liver), and in the bladder separately, with little interference from activity elsewhere in the body.

Gamma Camera Imaging
At 45 minutes and at 5 hours after particle administration, the distribution of the inhaled particles in the lung was assessed by gamma camera imaging (see online supplement). Subjects sat on a chair upright with the back toward the gamma camera head. Gamma camera images were corrected for background activity and radiotracer decay.

The ventilated area of the lung in each subject was assessed by full-breath 81mKr gas inhalation. Regions of interest were generated individually at the 10–15% maximum intensity level of each lung lobe (see Figure 2). Central and peripheral regions of interest were generated, and the ratios of count rates between central and peripheral regions (C/P) were calculated for the Kr gas ventilation image and for the shallow and deep aerosol bolus administrations (see online supplement). Normalized C/P ratios were calculated by dividing each aerosol C/P ratio by the Kr gas ventilation C/P ratio (18, 19).


Figure 2
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  		Figure 2. Gamma camera images (posterior) of full-breath 81mKr gas inhalation and of 99mTc-labeled deep and shallow aerosol bolus inhalation of a healthy nonsmoker (NS) and a patient with chronic obstructive pulmonary disease (COPD). Yellow boundaries indicate whole lung regions of interest (ROIs), whereas green boundaries indicate central ROIs, which cover an area of 30.9% of the total lung ROIs.

 
Estimation of Particle Accumulation in the Liver
To estimate 99mTc activity in the liver, the horizontal activity profile across the abdomen (covering liver and stomach) was analyzed and compared with the background activity profile (see online supplement). To obtain lower attenuation than that caused by the abdominal part of the skeleton, an additional anterior gamma camera scan was performed 2 hours after particle inhalation.

Statistical Analysis
All data are expressed here as mean (±SD). Group differences were calculated by a two-sided t test (WinSTAT add-in package for Microsoft Excel, version 2005.1; R. Fitch Software, Bad Krozingen, Germany), using a significance level of P less than 0.05. Pearson correlation analysis was performed to analyze correlations between parameters.


   RESULTS
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 ABSTRACT
 AT A GLANCE COMMENTARY
 METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Lung Function and Dead Space Data
Table 1 summarizes the lung function, dead space, and aerosol inhalation data. All nonsmokers and smokers had FEV1 greater than 80% predicted and FEV1/FVC greater than 70%; all patients with COPD had FEV1 less than 80% predicted and FEV1/FVC less than 70%. Mean dead space volumes showed no differences between the three study groups. The VDP1s were about 60% of the Fowler dead spaces (VDFs). In all subjects, VF for the shallow bolus was within VDP1.


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  		TABLE 1. ANTHROPOMETRIC DATA OF STUDY GROUPS AND DATA OF SHALLOW AND DEEP AEROSOL BOLUS APPLICATION

 
Properties of Technegas Particles
The mean mobility count median diameter of the aerosol in the three study groups was in the approximate range of 90–110 nm (Table 1). The aerosol has a geometric standard distribution, {sigma}g, of 1.6 and the mean number concentration in the bag was below 106 cm–3. There was no statistically significant difference in count median diameter between the three study groups, or between shallow and deep bolus administration.

Leaching
Leaching of the radiolabel from the carbon matrix was assessed by three methods: leaching was measured (1) in vitro using a filter sandwich method, (2) in vivo by the amounts of activity in the blood (which may contain a fraction of translocated particles), and (3) in vivo by the amounts excreted in urine. All three parameters showed no correlation with study group or bolus depth (AW or AL). The leaching data were therefore pooled from all studies (Figure 3). Mean in vitro leaching after 1 and 24 hours in saline was 2.2 (1.8 [SD]) % and 4.1 (2.6) %, respectively. Activity in the blood after 1 and after 5 hours was 0.2 (0.2) % and 0.2 (0.2) % of the deposited activity, respectively. Within 24 hours after particle inhalation, 1.1 (1.3) % of the deposited activity was excreted in urine.


Figure 3
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  		Figure 3. Leaching of the radiotracer from the carbon matrix, assessed by three different methods: fraction of deposited activity in the blood pool 1 and 5 hours after particle inhalation; fraction of deposited activity excreted via urine during the first 24 hours after particle inhalation; and in vitro leaching after 1 and 24 hours incubation of a filter sandwich in 0.9% saline. Error bars denote SD.

 

Figure 4
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  		Figure 4. Clearance of carbon particles after shallow bolus inhalation in healthy nonsmokers, asymptomatic smokers, and in patients with chronic obstructive pulmonary disease (COPD) during a 48-hour postinhalation period. Error bars denote SD.

 
Total Particle Deposition
In all study groups, mean particle deposition was lower (37–43%) after shallow than after deep (50–64%) bolus inhalation (Table 1); there was no difference between study groups after shallow bolus inhalation. After deep bolus inhalation, mean particle deposition was significantly higher in smokers and in patients with COPD compared with healthy nonsmokers.

Aerosol Distribution in the Lung
The gamma camera images in Figure 2 show the distribution of aerosol deposition in the lung after deep and shallow aerosol bolus inhalation compared with Kr gas ventilation in a nonsmoker and in a patient with COPD. The distributions as measured by the normalized C:P ratio at 1 hour after aerosol inhalation are shown in Table 1. C:P ratios after 4.5 hours were not significantly different from those after 1 hour in any study group. In healthy nonsmokers, C:P was 0.83 (0.09) and 2.01 (0.52) at 1 hour after deep and shallow bolus inhalation, respectively. After deep bolus administration C:P showed no differences between study groups. After shallow bolus inhalation C:P was lower in smokers and in patients with COPD than in healthy nonsmokers.

Retention and Clearance in the Airways
Airway retention after shallow bolus inhalation shows two phases in all three study groups (Figure 4 and Table 1). Fast mucociliary clearance removes particles within the first 24 hours. There was no further significant clearance between 24 and 48 hours. Retention in smokers was significantly higher than in healthy nonsmokers (P < 0.01) after 1.5 hours, but not at later time points. Retention in patients with COPD was significantly higher than in healthy nonsmokers (P < 0.01) at all four measurement times, and 24-hour retention (Ret24) was significantly higher than in smokers (P < 0.05). Airway retention did not show a correlation with in vitro leaching, blood activity, or urine excretion. C:P ratios show a significant negative correlation with Ret24 (Pearson correlation coefficient, R = –0.65; P < 0.01; see online supplement).

Retention and Clearance in the Lung Periphery
There was no significant difference between the three study groups in alveolar retention of the UPs after deep bolus inhalation (Table 1). Among all healthy subjects, but not among patients with COPD, 24- and 48-hour retention showed significant negative correlations with in vitro leaching after 1 hour (R = –0.49; P = 0.025) and after 24 hours (R = –0.53; P = 0.016), with blood activity after 1 hour (R = –0.45; P = 0.039) and after 5 hours (R = –0.46; P = 0.035), and with cumulative urine excretion after 24 hours (R = –0.45; P = 0.041). There was no significant correlation between deep aerosol bolus retention (Ret24) and aerosol distribution in the lung (C/P).

Particle Translocation to the Liver
The analysis of the activity profiles of the gamma camera images of the abdomen after peripheral bolus administration (covering the liver and stomach) were not significantly different from the background profile (see online supplement). From analysis of the lower detection limit of the gamma camera and attenuation by the liver and overlaying tissue, the system should be able to detect an activity accumulation of about 10 kBq in the liver (see online supplement). This corresponds to about 0.5% of the deposited activity, which is therefore an upper limit on the translocation fraction to the liver.


   DISCUSSION
TOP
 ABSTRACT
 AT A GLANCE COMMENTARY
 METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Total and Regional Particle Deposition
Using inhalation of shallow (bolus front depth VF ~ 150 ml) and deep (VF ~ 800 ml) aerosol boluses, particles were targeted either to the AW or to the AL. A possible penetration and deposition of particles in the AL after shallow bolus inhalation was discussed recently (20, 21), and is considered negligible here because, in all subjects, VF was within the VDP1. Deposition of UPs is mainly by diffusion, and hence proportional to the square root of the time available. Therefore, deposition in the airways during deep bolus inhalation is also considered negligible because of the large airway calibers and the short transit times.

Studies using fine particles (in the micron size range) showed increased particle deposition in smokers and in patients with COPD, which was attributed to increased impaction at bifurcations caused by airway obstructions, (22, 23). Increased UP deposition in patients with COPD and with asthma was also found in previous studies (8, 24, 25). However, in those studies, continuous breathing of full tidal volume breaths of aerosol was used, and therefore they cannot discriminate deposition between airways and lung periphery.

After shallow bolus inhalation in our study, deposition was not significantly different between study groups, even though the lower C:P in smokers and patients with COPD than in healthy nonsmokers suggested deeper penetration of the bolus. Presumably, this is because diffusion is not an effective deposition mechanism in larger airways, and is not sensitive to differences in airway caliber between study groups. A deeper penetration of the shallow aerosol bolus, as suggested by the lower C/P in smokers and in patients with COPD, should result in increased particle deposition, but could not be confirmed in our study.

After deep bolus inhalation, deposition was significantly greater in smokers and in patients with COPD than in healthy nonsmokers. Because of the smaller dimensions in the lung periphery, deposition by diffusion is more effective than it is in the airways. Therefore, even minor narrowing of small airways may increase UP deposition. It was similarly suggested that the high fine particle deposition after deep bolus inhalation observed in women with asymptomatic, nonspecific airway hyperresponsiveness (26) resulted from early small airway inflammations. In addition, increased aerosol bolus dispersion, as demonstrated in smokers and in patients with COPD (27), can cause deeper aerosol penetration to smaller calibers, resulting in an increase of particle deposition. Therefore, the higher UP deposition rate, as demonstrated in our study in smokers and in patients with COPD, may result from obstructions in small airways.

Aerosol Distribution in the Lung
Shallow aerosol bolus inhalation.
C/P ratios larger than unity, as seen in all study groups, indicate a central airway aerosol deposition. Previously, C/P ratios larger than unity were mainly reported after rapid inhalation of large particles (>4 µm in diameter) (19, 28), which causes high particle deposition at bifurcations due to impaction. They were also seen after shallow bolus inhalation of 3.5-µm diameter particles (29). C/P was significantly smaller in smokers and in patients with COPD than in healthy nonsmokers, indicating a wider spreading of the shallow aerosol bolus, as can be seen in the gamma camera image in Figure 2. The Kr ventilation scan indicates that this was caused partly by ventilation inhomogeneities.

Deep aerosol bolus inhalation.
In all three study groups C/P was smaller than unity after the deep bolus administration. This indicates a less homogeneous aerosol distribution than obtained during Kr gas ventilation, with preferential particle deposition in peripheral lung regions. This was confirmed by an analysis of the horizontal activity distributions over the chest (see online supplement). C/P ratios smaller than unity have not been reported before, and may demonstrate regional targeting using the aerosol bolus technique.

Retention and Clearance in the Airways
After shallow aerosol bolus inhalation, our data show two clearance phases within the two-day measurement period: a fast phase during the first 5–10 hours, and a subsequent slow phase. In healthy nonsmokers, about 25% of the deposited particles cleared within 1 day by mucociliary action, and 75% of the particles were persistent in the airways.

The fraction cleared rapidly was significantly smaller in patients with COPD, and somewhat smaller in asymptomatic smokers, than in healthy nonsmokers. This may partly result from deeper penetration of part of the bolus in patients with COPD, as indicated by a lower C/P ratio. It also reflects impairment of mucociliary function due to smoking and chronic inflammation of the airways. COPD is composed of multiple components, including pulmonary inflammation, airway remodeling, and mucociliary dysfunction resulting from mucus hypersecretion coupled with a decrease in mucus transport. Impairment of UP clearance in patients with COPD was also shown in a previous study within the first few hours after deposition (8), but after 24 hours, there was no difference in particle retention. In that study, particles were inhaled during full tidal breathing, depositing particles in both the airways and the alveoli. The particles also showed high 99mTc leaching rates (about 20%), giving rise to uncertainty on the retained particle fraction.

Mucociliary clearance was once thought to clear all particles deposited in the airways within 24 hours. However, many studies using shallow bolus administration of larger particles, targeting particle deposition primarily to the airways, have shown that a large fraction of deposited particles cleared slowly. Prolonged UP retention in the airways after shallow bolus inhalation in two healthy subjects was reported previously (30). Our study using 100-nm carbon particles supports that study and provides data on a larger population of healthy subjects and patients with COPD.

Several mechanisms have been proposed, which can lead to incomplete mucociliary clearance and prolonged particle retention in the airways. They include surface forces caused by the surfactant on top of the mucus layer (31) displacing particles in the mucus and periciliary fluid toward the epithelium, and discontinuities of the mucus layer (32, 33) allowing particle deposition directly onto the periciliary fluid and penetration between cilia. All these mechanisms allow particle uptake by macrophages, dendritic cells, or the airway epithelium (34, 35). A recent study indicated that the kinetics of micron-sized, long-term retained iron oxide particles in the airways are similar to the kinetics of alveolar clearance (21). This hypothesis is supported by histological studies on hamsters, which showed that most particles retained after 24 hours in the airways were found in airway macrophages (36). Although macrophage uptake of UPs seems to be less effective compared with micron-sized particles (37), it may still play an important role in retention. In addition, surfactant, including suspended proteins, may play a role in particle uptake by epithelial and by phagocytic cells in wetting and coating the particles (opsonization) (38, 39).

Retention and Clearance in the Lung Periphery
Clearance in the lung periphery is much slower than from the airways, and results from completely different mechanisms, the most important being the actions of alveolar macrophages. After phagocytosis and uptake into phagolysosomes, bacteria or viruses are inactivated in the acidic environment and by radicals and chelating agents, and particles are dissolved. However, relatively insoluble substances can remain for years (40, 41). Some of the particle-laden macrophages reach the bronchial system, where they are cleared by mucociliary transport. Interspecies studies have shown that this process is much slower in humans than in rodents (20, 42).

Because of the stability of the carbon particles, long-term retention in the lung periphery is expected (43). Our data show about 3% clearance within 24 hours, with no effect of smoking or COPD. The retention values at 24 and 48 hours show negative correlations with all particle leaching parameters, suggesting that the small clearance observed here primarily reflects leaching of the radiolabel with little transport of particles from the lungs. The data are in agreement with results obtained in humans with micron-sized Teflon or iron oxide particles (40, 41).

Particle Translocation
The minimal clearance of the particles from the lung periphery within 48 hours suggests that there can be little particle translocation to the blood and other organs. This is supported by the low activities in the blood and urine. Because of the low blood activity (<1 Bq/ml), chromatography analysis, which might have provided information directly on the fractions of soluble and particle-bound 99mTc, could not be performed. Inhaled free pertechnetate (TcO4–) is excreted slowly (44, 45): based on these data, the total blood activity of about 0.23% after 5 hours detected here corresponds to about 2.5% of soluble pertechnetate, and the total fraction of activity excreted via urine of about 1.1% corresponds to about 3.5% of soluble pertechnetate. This is similar to the in vitro leaching results observed here; hence, we conclude that most of the activity found in blood and urine results from leaching.

In addition, significant fractions of particle-bound activity in the blood should be deposited in the liver (46, 47). However, the absence of increased liver activity in gamma camera images gave an upper limit on translocation of 0.5% of activity deposited in the lung. This is in agreement with other studies using gamma camera imaging (8, 9). Nemmar and coworkers (7), using Technegas 99mTc-carbon particles, found large amounts of activity in blood and in the liver as early as 10 minutes after particle inhalation. Using paper chromatography, they suggested that a fraction of the activity in the blood might be particle bound, but this was not verified by Mills (9). Because the Technegas aerosol used in their study was generated under the standard protocol, a much higher fraction of the radiolabel leaching from the particles and circulating in the body as pertechnetate is the most likely reason for the findings of Nemmar and colleagues, as indicated by our own studies (15).

Studies on UP translocation from rat lungs rats show conflicting results: Oberdörster reported high liver accumulation of carbon particles in rats (6), but Kreyling and colleagues (6) and Takenaka and colleagues (48) reported low translocation of iridium or gold UPs, respectively, to other organs in rats. These findings were in agreement with our own.

Conclusions
Our data show that most inhaled ultrafine carbon particles deposited in the lung periphery were retained at 48 hours. Furthermore, only 25% of inhaled UPs deposited in the airways were eliminated by mucociliary clearance in this period. Within the limits of detection of our experimental system, our findings did not support significant systemic particle translocation or accumulation of particles in the liver. Repeated inhalation of environmental UPs from combustion processes or cigarette smoke may result in accumulation within the lung, and could conceivably contribute to acute and chronic pulmonary diseases.


   Acknowledgments
 
The authors thank Dr. Mike Bailey (Health Protection Agency, UK) for helpful discussions and editorial remarks.


   FOOTNOTES
 
Supported in part by CEC grant FIGD-CT-2000-00053.

This article has an online supplement, which is accessible from this issue's table of contents at www.atsjournals.org

Originally Published in Press as DOI: 10.1164/rccm.200602-301OC on October 11, 2007

Conflict of Interest Statement: None of the authors has a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

Received in original form February 28, 2006; accepted in final form October 11, 2007


   REFERENCES
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 ABSTRACT
 AT A GLANCE COMMENTARY
 METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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