Normal Breathing Conditions Research Articles

Fluid flow and particle deposition analysis in a realistic extrathoracic airway model using unstructured grids

An efficient Lagrangian particle tracking module is developed for unstructured grids to analyze the deposition patterns in a CT based realistic extrathoracic airway model. Inhalation flow rates of 15, 30 and 60 l/min are considered with particle diameters ranging between 2 and 20 μ m . The complex flow patterns with skewed velocity profiles and flow separations are discussed. While most idealized mouth–throat models predict no transition for a flow rate of 15 l/min, we observe increase in kinetic energy levels soon after glottis, indicating the sensitivity to geometrical complexity at low flow rates. Mouth cavity acts as an effective filter and is responsible for a major percentage of total deposition for all three flow rates. Combining our model depositions with central bronchial tree depositions indicates that for normal breathing conditions, inspiratory deposition for the particle size 5 μ m , which is usually referred to as the upper limit of the respirable range for inhalation drugs, does not exceed 12%.

A cyclic model for particle motion in the pulmonary acinus

A simplified model for the pulmonary alveolus that imitates the rhythmical expansion of the alveolus and the periodic shear flow in the adjacent airway is explored. The model consists of two eccentric cylinders and incompressible fluid that occupies the gap between them. The two cylinders undergo a simultaneous rhythmical expansion and contraction (mimicking the alveolus expansion) while the inner cylinder performs a periodic rotation about its axis (inducing shear flow mimicking airway ductal flow). An analytical solution is obtained for the creeping flow induced by the simultaneously expanding cylinders. It is shown that above a certain critical value of rotation to expansion velocity ratio, the flow exhibits characteristic features such as a saddle point and closed streamlines about a centre, similar to those existing inside a single alveolus during inhalation and exhalation. Poincare maps of the trajectories of fluid particles demonstrate that, under various flow conditions, chaotic trajectories may exist, provided that expansion and rotation are slightly out of phase. This is similar to normal breathing conditions where the periodic expansion of the alveolus and the tidal flow (i.e. shear flow above the mouth of the alveolus) may be slightly out of phase. A novel definition of overall convective mixing efficiency is also suggested that inherently discounts reversible processes that do not contribute to mixing. It is demonstrated that two different convective mechanisms, related to the irreversibility of exhalation and inhalation and the onset of chaos, govern mixing efficiency in lung alveoli.

Comparison of Diaphragmatic EMG and Oesophageal Pressure in Obstructed and Unobstructed Breathing during Sleep. Korrelation von diaphragmalem EMG und Osophagusdruckmessung bei obstruierter und unobstruierter Atmung im Schlaf

The importance of quantitative changes in respiratory effort during routine polysomnographic investigation has been recognized since the description of the upper airway resistance syndrome. In clinical settings, oesophageal manometry is rerely conducted on a routine basis. We therefore developed a new technique for semiquantitative assessment of respiratory effort using diaphragmatic EMG (EMGdi) monitoring and tested its efficacy. Five patients with obstructive sleep apnoea syndrome and five subjects without sleep-disordered breathing were investigated in a private sleep disorders laboratory. EMGdi was sampled at 0.25 kHz and processed to a moving average. For each inspiratory effort during apnoea/hypopnoea and unobstructed breathing, we calculated maximum EMGdi and Pes. Data were normalized calculating the percentage difference between the first obstructed and each subsequent inspiratory effort during respiratory events and control periods. A total of 583 periods with unobstructed breathing and 477 apnoeas and hypopnoeas were scored to compare the evolution between Pes and EMGdi. There was a significant difference in the evolution of both respiratory parameters in unobstructed breathing versus apnoeas and hypopnoeas. EMGdi and Pes evolution corresponded in each condition investigated. In unobstructed periods, the increase of Pes averaged +2.6±24.1% versus −1.1±37.4% for EMGdi (P=0.616). In apnoeas and hypopnoeas, the respective values were +90.2±38.0% for Pes and +82.4±31.6% for EMGdi (P=0.7330). EMGdi can be used in lieu of oesophageal manometry to describe relative changes in respiratory effort under conditions of normal and obstructed breathing during sleep.

A preoperative sleep study with nasal airway occlusion in pharyngeal flap surgery

Purpose Obstructive sleep apnea is a major complication of pharyngeal flap surgery. The purpose of the present study is to predict preoperatively the risk of upper airway obstruction after surgery. Material and methods We performed an overnight sleep study preoperatively and postoperatively in 16 pediatric patients considered for pharyngeal flap surgery. Preoperative sleep study was done for two nights, once in normal breathing condition and once with complete nasal occlusion by packing of nostril with tampon gauze. Results In preoperative sleep recordings in normal breathing condition, all subjects had a normal apnea hypopnea index (AHI) less than 5/h. In preoperative recording with complete nasal occlusion, five patients exceeded 5/h in AHI. In particular, for two patients who had AHI higher than 15/h, we gave up a surgery in one case and performed pharyngeal flap operation for the other following a tracheotomy for severe disturbance of oral breathing. The remaining 14 subjects underwent surgery without airway obstructive complications. There was strong correlation between preoperative AHI with nasal tampon gauze and AHI at two weeks postoperatively (r = 0.88 P < .0001). There was no significant correlation between preoperative AHI in normal breathing condition and postoperative AHI (P > .05). Conclusions These results exhibit preoperative sleep study with complete nasal airway occlusion represent postoperative breathing condition well during early postoperative period. Preoperative sleep study with complete nasal airway occlusion with nasal tampons could be useful for predicting the risk of upper airway obstruction secondary to pharyngeal flap surgery.

Exaggerated perception of normal physiological responses to stress and hypercapnia in young women with numerous functional somatic symptoms

Objective This study tested whether functional somatic symptoms are associated with exaggerated increases in self-reported anxiety and somatic complaints in response to stress and CO 2-enriched air breathing, and whether this association exists in parallel to or in the absence of exaggerated physiological responses. Methods Out of 499 young somatically healthy undergraduate women, 18 participants high in functional somatic symptoms (HSS group) and 18 participants low in symptoms (LSS) were selected. They were submitted to mental stress, mild physical exercise and relaxation during conditions of normal breathing, breathing compressed normal air, and breathing compressed 5% CO 2-enriched air. In all conditions, self-reported anxiety and somatic symptoms and respiratory and autonomic responses were assessed. Results HSS participants reported, as compared to LSS, more tenseness, anxiety, and somatic symptoms at baseline and increased responses to mental stress and during 5% CO 2 breathing, but not in response to exercise. However, no evidence was found for a corresponding exaggerated respiratory or autonomic response. Conclusion A young, female, and nonclinical population with numerous functional somatic symptoms and high levels of anxiety is characterized by an exaggerated perception of a normal physiological response.

Increased sensations of intensity of breathlessness impairs maintenance of intense intermittent exercise.

To identify the reserve of an individual's tolerance of the sensation of breathlessness and metabolic stress in maintaining intense intermittent exercise at exhaustion under conditions of normal breathing, the contribution of the effect of modest inspiratory load on these two responses to the change in the exercise sustainability (Ex(sus)) were examined. Seven men repeatedly performed 12 s exercise at 160% maximal aerobic power output followed by passive recovery for 18 s under normal and ventilatory muscle loaded (VML) breathing conditions until exhaustion. In the VML trial, ventilatory muscle work at exhaustion was double that of the normal control. The control Ex(sus) was reduced [mean (SEM)] [31.7 (6.6)%] while the slope of the time course for the rating of the perceived magnitude of breathing effort (RPMBE/Time), which reflected the intensity of breathlessness, was increased [164.8 (32.2)%] from control and the RPMBE at exhaustion was higher than corresponding control value [144.4 (21.8)%]. Moreover, increases in plasma ammonia and uric acid concentrations, which indicated metabolic stress, were increased [168.1 (28.0)% and 251.7 (57.4)%, respectively], with no change in total oxygen uptake from control when the control exercise was repeated with an identical duration of VML exercise. It was found that the reduction in Ex(sus) in the VML trial was correlated to the increase in their sensations of the intensity of breathlessness (RPMBE/Time: r=0.81; RPMBE at exhaustion: r=0.97, P<0.05). The reduction in Ex(sus), however, was not correlated to the increase in metabolite concentrations. These findings implied that there was no substantial reserve of tolerance of the sensation of breathlessness relative to that of metabolic stress in subjects maintaining intense intermittent exercise at exhaustion under normal conditions of breathing.

Numerical simulation of respiratory flow patterns within human lung.

A computational fluid dynamics (CFD) modelling approach is used to study the unsteady respiratory airflow dynamics within a human lung. The three-dimensional asymmetric bifurcation model of the central airway based on the morphological data given by Horsfield et al. (J. Appl. Physiol. 67 (1971) 207) was used in the present study to simulate the oscillatory respiratory. The single bifurcation was found to be sufficient to give a number of results which both qualitatively and quantitatively agreed well with other published experimental and CFD results. Numerical simulation were made for two breathing conditions: (a) resting or normal breathing condition and (b) maximal exercise condition. The respiratory flow results for the both conditions are found strongly dependent on the convective effect and the viscous effect with some contribution of the unsteadiness effect. The secondary motions were stronger for the normal breathing condition as compared with the maximal exercise condition. The difference between the two cases is the flow separation regions found close to the carinal ridge for maximal exercise condition. For normal breathing condition no separation regions was observed in this region.

Transient airflow structures and particle transport in a sequentially branching lung airway model

Considering oscillatory laminar incompressible three-dimensional flow in triple planar and nonplanar bifurcations representing generations three to six of the human respiratory system, air flow fields and micron-particle transport have been simulated under normal breathing and high-frequency ventilation (HFV) conditions. A finite-volume code (CFX4.3 from AEA Technology, Pittsburgh, PA) and its user-enhanced FORTRAN programs were validated with experimental velocity data points for a single bifurcation. The airflow structures and micron-particle motion in the triple bifurcations were analyzed for a representative normal breathing cycle as well as HFV condition. While both the peak inspiratory and expiratory velocity profiles for the low Womersley case (α=0.93) agree well with those of instantaneously equivalent steady-state cases, some differences can be observed between flow acceleration and deceleration at off-peak periods or near flow reversal, especially during inspiratory flow. Similarly, the basic features of instantaneous particle motion closely resemble the steady-state case at equivalent inlet Reynolds numbers. The preferential concentration of particles caused by the coherent vortical structures was found in both inhalation and exhalation; however, it is more complicated during expiration. The effects of Womersley number and non-planar geometries as well as the variations in secondary flow intensity plus pressure drops across various bifurcations under normal breathing and HFV conditions were analyzed as well. This work may elucidate basic physical insight of aerosol transport relevant in dosimetry-and-health-effect studies as well as for drug aerosol delivery analyses.

Prostate immobilization using a rectal balloon.

We use a rectal balloon for prostate immobilization during intensity modulated radiotherapy (IMRT) prostate treatment. To improve the accuracy of our prostate planning target volume, we have measured prostate displacements using computed tomography (CT)‐CT fusion on patients that previously received gold seed implants. The study consists of ten patients that were scanned twice per week during the course of IMRT treatment. In addition to biweekly scans, breathing studies were performed on each patient to estimate organ motion during treatment. The prostate displacement in the anterior‐posterior and the lateral direction is minimal, on the order of measurement uncertainty (~1mm). The standard deviation of the superior‐inferior (SI) displacements is 1.78 mm. The breathing studies show that no organ displacement was detected during normal breathing conditions with a rectal balloon.PACS number(s): 87.53.–j, 87.90.+y

Prostate immobilization using a rectal balloon

We use a rectal balloon for prostate immobilization during intensity modulated radiotherapy (IMRT) prostate treatment. To improve the accuracy of our prostate planning target volume, we have measured prostate displacements using computed tomography (CT)-CT fusion on patients that previously received gold seed implants. The study consists of ten patients that were scanned twice per week during the course of IMRT treatment. In addition to biweekly scans, breathing studies were performed on each patient to estimate organ motion during treatment. The prostate displacement in the anterior-posterior and the lateral direction is minimal, on the order of measurement uncertainty . The standard deviation of the superior-inferior (SI) displacements is 1.78 mm. The breathing studies show that no organ displacement was detected during normal breathing conditions with a rectal balloon. PACS number(s): 87.53.–j, 87.90.+y

High-speed magnetic resonance imaging for four-dimensional treatment planning of conformal radiotherapy of moving body tumors

Purpose: High-speed magnetic resonance imaging (MRI) was applied to the determination of the planning target volume (PTV) of moving hepatobiliary tumors. Methods and Materials: Three moving tumors, including two metastatic hepatic tumors and one bile duct tumor, were examined using high-speed MRI and reference fiducial markers before external radiotherapy. Patients were examined for 30 seconds under conditions of normal breathing during the examination. The coordinates of the center of the tumor contours were shown on sagittal and coronal images displayed on the monitor. Results: The maximum length of movement was 10.6 ± 7.0 mm in a craniocaudal direction; 5.2 ± 1.8 mm in a lateral direction; and 4.6 ± 1.6 mm in a ventrodorsal direction. When the PTV was determined using MRI at exhalation phase with a 10-mm safety margin, clinical target volume (CTV) was not covered in 19% of all images in the 3 patients. With MRI at inhalation phase with a 10-mm safety margin, CTV was not covered in 36% of all images. Conclusion: Four-dimensional treatment planning using high speed MRI, and integrating time and spatial information, has the potential to determine the planning target volume of moving body tumors more precisely than does conventional CT planning.

Dorzolamide Hydrochloride and Visual Function in Normal Eyes

To determine by a pilot study whether standard treatment with the topical carbonic anhydrase inhibitor dorzolamide hydrochloride influences visual function under normal breathing conditions, during carbon dioxide inhalation, or during hyperventilation, and to establish criteria for future larger-scale studies. We enrolled 12 normal subjects into this randomized double-masked placebo-controlled crossover study. Each subject was treated with either dorzolamide 2% or placebo, three times daily, for 4 days. After a 2-week washout period, the alternative topical agent was used under identical testing conditions. On day 2 of each treatment phase, contrast sensitivities to sinusoidal gratings of 1 and 4 cycles per degree (cpd) were assessed. On day 4, mean deviation values from full-threshold 10-2 visual fields were obtained. Three sets of each visual function test were obtained before each treatment phase, and in sequence on each testing day, during normal breathing (baseline), inhalation of carbon dioxide-enriched air, and hyperventilation while intraocular pressure was monitored. Contrast sensitivity at 4 cpd decreased significantly (P < .01) during carbon dioxide supplementation with placebo but showed no significant change with dorzolamide. The decrease in contrast sensitivity accompanying hyperventilation was attenuated (by nearly 50% at 1 cpd) during dorzolamide treatment. Dorzolamide treatment was associated with higher perimetry mean deviation values under each treatment condition and was statistically significant (P < .05) at baseline. Dorzolamide appears to enhance contrast sensitivity in normal subjects during physiologic hypercapnia and hypocapnia at 4 and 1 cpd, respectively. Also, under normal breathing conditions, dorzolamide therapy increases perimetric light sensitivity.

Cardiorespiratory interactions during fixed-pace resistive breathing

We tested the hypothesis that the arterial baroreflex was important in the origin of respiratory sinus arrhythmia (RSA) under conditions of normal and resistive breathing. That is, mechanical effects of breathing [indicated by instantaneous lung volume (ILV)] would directly influence left ventricular stroke volume (LVSV), which in turn would influence systolic arterial blood pressure (SABP), causing variation in R-R interval through the baroreflex. Eight healthy young subjects (four men and four women) were monitored in the supine position while breathing with a fixed frequency (0.2 Hz) and tidal volume for 15 min through each of three resistances (R0, R1, and R2) producing inspiratory (-) and expiratory (+) pressures of +/- 1.6, +/- 5.4, and +/- 16.6 cmH2O, respectively. LVSV was estimated by stroke distance [(SDist); by Doppler ultrasound]. There were no differences across R0, R1, and R2 for the mean values of R-R interval, SDist, or SABP. Cross-spectral analysis showed that, at R0, each value of R-R interval, SDist, and SABP lagged ILV by approximately 80 degrees. At R1 and R2, phase was reduced from ILV to SDist and R-R interval, and the transfer magnitude for SDist (R2 only), SABP, and R-R interval increased. The transfer magnitude from SDist to SABP significantly increased as a function of resistance breathing, whereas that from SABP to R-R interval significantly decreased. There were no changes in phase relationships from SDist to SABP to R-R interval. Thus the magnitude of RSA (ILV to R-R interval) was increased, but the transfer through the arterial baroreflex (SABP to R-R interval) was reduced. Although factors other than the arterial baroreflex are probably involved in the genesis of RSA, the constant phase relationship across the levels of breathing resistance among SDist, SABP, and R-R interval suggests an important functional link caused by mechanical effects of breathing.

Mercury Release from Dental Amalgams: An in vitro Study Under Controlled Chewing and Brushing in an Artificial Mouth

The release of mercury vapor from class I amalgam restorations prepared in human molar teeth was studied during chewing simulations in an artificial mouth of a bi-axial servo-hydraulic mechanical test system. So that the total mercury released from the restoration over a fixed time could be determined, a closed chamber surrounded the envelope of chewing motion. In addition, the influence of sampling frequency on mercury release was corrected by the use of different sampling frequencies over a fixed time interval of mercury release measurement and extrapolation to zero sampling time. Thus, a combination of a closed environment and an extrapolation method to determine the mercury release under continuous sampling was used to determine the mercury released under normal breathing conditions. The measured mercury release rate data were used to calculate the potential daily mercury dose in a patient due to a single amalgam restoration, following the method previously outlined by Berglund. The mercury release from both a conventional and a high-copper amalgam was evaluated at different age intervals after the restoration was placed in the teeth. The results show that while the age of the amalgam and the amalgam type influence the extent of mercury release during the initial non-steady-state conditions, the steady-state value of mercury daily dose due to a single amalgam filling is 0.03 micrograms/day, which is well below the calculated threshold-limiting value (TLV) of 82.29 micrograms/day considered dangerous for occupational exposure in the United States.

Human lung impedance from spontaneous breathing frequencies to 32 Hz

Lung impedance (ZL) was measured from 0.1875 to 32 Hz in spontaneously breathing healthy subjects by spectral analysis of the pressure and flow signals generated simultaneously by the muscular generator of breathing and by a forced oscillation system. This method did not require cooperation from the subject to perform panting or special ventilatory maneuvers and therefore allowed us to analyze the frequency dependence of lung resistance, reactance, and elastance (-2 pi.frequency.reactance) at the physiological conditions of normal breathing. Resistance and elastance parameters were also computed by multiple linear regression of the time-domain pressure and flow data on a simple resistance-elastance model. Resistances and elastances computed at the breathing frequency by spectral analysis and by multiple linear regression were similar (nonsignificant differences < 4 and 10%, respectively). The results obtained when comparing ZL from the breathing component (0.1875-0.75 Hz) of the recorded signals and from the forced oscillation component (2-32 Hz) were fairly consistent. ZL (0.1875-10 Hz) was interpreted in terms of a model consisting of an airway compartment, including a resistance and an inertance, in series with a viscoelastic tissue compartment (J. Hildebrandt. J. Appl. Physiol. 28: 365-372, 1970) characterized by two parameters. The model analysis provided parameter values (resistance 2.49 +/- 0.58 hPa.l-1.s, inertance 1.70 +/- 0.29 Pa.l-1.s2, Hildebrandt parameters 4.87 +/- 2.28 and 0.73 +/- 0.99 hPa/l) consistent with the hypothesis that lung tissue in healthy humans during spontaneous breathing behaves as a viscoelastic structure with a hysteresivity of approximately 0.10.

Oxygenation of the Rat 9l Gliosarcoma and the Rat 13672 Mammary Carcinoma with Various Doses of a Hemoglobin Solution

Tumor oxygen tensions were measured using a computer controlled pO2 microelectrode in two preclinical solid tumor models, the rat 9L gliosarcoma and the rat 13672 mammary carcinoma. Tumor oxygenation profiles were determined under four conditions: 1) normal air breathing, 2) carbogen (95% O2/5% CO2) breathing, 3) after intravenous administration of a solution of ultrapurified polymerized bovine hemoglobin with normal air breathing and 4) after intravenous administration of a solution of ultrapurified polymerized bovine hemoglobin with carbogen breathing. Both tumors had severely hypoxic regions under normal air breathing conditions. Although carbogen breathing increased the oxygenation of the better oxygenated portions of the tumor, it did not impact on the severely hypoxic tumor regions. Administration of the hemoglobin solution was effective in increasing the oxygenation throughout both tumors under normal air breathing conditions. The addition of carbogen breathing to administration of the hemoglobin solution eliminated severe hypoxia in the 9L gliosarcoma and markedly reduced the severely hypoxic regions of the 13672 mammary carcinoma.

Oxygenation of Human Tumor Xenografts in Nude Mice by a Perfluorochemical Emulsion and Carbogen Breathing

Human solid tumors (prostate carcinomas PC-3 and DU-145, breast carcinoma MX-1, cervical carcinoma ME-180, small cell lung carcinoma SW2, and glioblastoma T98G) were grown as xenografts in nude mice. Using the Eppendorf pO2 histograph microelectrode system, the oxygen profiles of the tumors were determined while the animals breathed air or carbogen (95% O2/5% CO2), and after administration of the perfluorochemical emulsion Oxygent-CA (8 ml/kg) under air breathing and carbogen breathing conditions. Under normal air breathing with or without Oxygent-CA administration the mean oxygen tensions were between 4.9 and 9.3 mmHg and each tumor had severely hypoxic regions where the pO2 was less than 5 mmHg. The severely hypoxic regions comprised 41-71% of the oxygen tension measurements under normal air breathing conditions. Carbogen breathing alone increased the mean oxygen tensions to 10.9-23.9 mmHg. Administration of Oxygent-CA and carbogen breathing increased the mean oxygen tensions over the levels of carbogen breathing alone to varying degrees. The highest mean oxygen tensions were 40.8 mmHg in the T98G glioblastoma and 24.5 mmHg in the ME-180 cervical carcinoma. Investigation of the use of Oxygent-CA/carbogen to increase the oxygenation of clinical tumors is warranted.

DECREASED TUMOR OXYGENATION AFTER CYCLOPHOSPHAMIDE, REOXYGENATION AND THERAPEUTIC ENHANCEMENT WITH A PERFLUBRON EMULSION CARBOGEN BREATHING

Oxygen profiles of the rat mammary 13672 carcinoma were determined using a pO2 histograph prior to treatment and 24 h and 48 h after i.p. administration of a single dose of cyclophosphamide (300 mg/kg). The tumors were severely hypoxic at 24 h post the administration of cyclophosphamide. There was little increase in oxygenation of the tumors at 48 h post therapy compared with 24 h post therapy indicating that reoxygenation after cyclophosphamide was occurring very slowly in this tumor. Carbogen breathing improved the oxygenation of the tumors under each of the conditions studied. Administration of the perflubron emulsion (8 ml/kg) produced little or no change in the oxygenation of the tumors under normal air breathing conditions. However, the addition of carbogen breathing to administration of the perflubron emulsion increased the oxygenation of the tumors to levels equal to or greater than carbogen breathing at the mean/median pO2's. Perhaps most significantly, administration of the perflubron emulsion with carbogen breathing increased the oxygenation of the most hypoxic regions of the tumors but carbogen breathing alone did not. The growth delay of the Lewis lung carcinoma increased with increasing dose.of the perflubron emulsion along with cyclophosphamide (3 x 150 mg/kg) and carbogen breathing (6 h). This combination treatment was most effective when the cyclophosphamide was prepared in the perflubron emulsion. The number of lung metastases decreased in a manner parallel with increased efficacy of the treatment toward the primary tumor.

The effect of acute hypoxia on the latency of the human auditory brainstem evoked response.

Recent studies have shown a decrease in the amplitude and an increase in the threshold of the cat's auditory brainstem evoked response (ABER) during severe hypoxia (PaO2 of 20 to 30 Torr). In this study we have examined the effects of euoxia (end tidal PO2 100 Torr) and mild hypoxia (end tidal PO2 of 45 to 50 Torr) on the latency of the ABER in 6 human subjects. Hypoxia resulted in a blood O2 saturation of between 75 to 85% and caused a significant prolongation of the latency of wave V of the ABER by 0.185 +/- 0.045 ms (Mean +/- S.D; p < 0.01). The prolongation of the ABER during severe hypoxia has previously been attributed to a change in peripheral sensitivity. Using the stimulus level/response latency relationship obtained for each subject under normal breathing conditions, the change in latency produced by mild hypoxia can be interpreted as a mean shift in auditory sensitivity of 5.1 +/- 3.4 dB. These results suggest that the auditory system is sensitive to much smaller changes in blood O2 saturation than previously thought.

Predictive Models for Deposition of Diesel Exhaust Particulates in Human and Rat Lungs

Diesel exhaust particulates are potentially hazardous when they are inhaled into the respiratory system. In this paper, theoretical models were developed to predict the deposition of such particles in humans and rats. It was shown that because of their submicron particle size, a significant fraction of diesel exhaust particulates can penetrate into the alveolar region of the lung and deposit there. Under normal breathing conditions, the alveolar deposition for humans was found to be between 7% and 13% depending upon particle size distribution. The alveolar deposition for rats was also investigated and it compared favorably with existing experimental data. Despite their considerable differences in airway size and respiratory condition, humans and rats exhibited similar deposition patterns during a breathing cycle; however, the average minute particle dose per unit pulmonary surface was found to be significantly higher for rats.