Physiological and histological alterations in the bronchial mucociliary clearance system of rabbits following intermittent oral or nasal inhalation of sulfuric acid mist

Challenges in inhaled product development and opportunities for open innovation

Dosimetry, safety and the efficacy of drugs in the lungs are critical factors in the development of inhaled medicines. This article considers the challenges in each of these areas with reference to current industry practices for developing inhaled products, and suggests collaborative scientific approaches to address these challenges. The portfolio of molecules requiring delivery by inhalation has expanded rapidly to include novel drugs for lung disease, combination therapies, biopharmaceuticals and candidates for systemic delivery via the lung. For these drugs to be developed as inhaled medicines, a better understanding of their fate in the lungs and how this might be modified is required. Harmonized approaches based on 'best practice' are advocated for dosimetry and safety studies; this would provide coherent data to help product developers and regulatory agencies differentiate new inhaled drug products. To date, there are limited reports describing full temporal relationships between pharmacokinetic (PK) and pharmacodynamic (PD) measurements. A better understanding of pulmonary PK and PK/PD relationships would help mitigate the risk of not engaging successfully or persistently with the drug target as well as identifying the potential for drug accumulation in the lung or excessive systemic exposure. Recommendations are made for (i) better industry-academia-regulatory co-operation, (ii) sharing of pre-competitive data, and (iii) open innovation through collaborative research in key topics such as lung deposition, drug solubility and dissolution in lung fluid, adaptive responses in safety studies, biomarker development and validation, the role of transporters in pulmonary drug disposition, target localisation within the lung and the determinants of local efficacy following inhaled drug administration.

Health effects of sulfur-related environmental air pollution. III. Nonspecific respiratory defense capacities

Recently concern has been raised about health effects related to environmental sulfur and/or acidic aerosols. To assess long-term effects on respiratory lung function, 8 beagle dogs were exposed over a period of 13 mo for 16.5 h/day to 1.0 microm neutral sulfite aerosol with a particle associated sulfur(IV) concentration of 0.32 mg m(-3) and for 6 h/day to 1.1 microm acidic sulfate aerosol providing an hydrogen ion concentration of 15.2 micromol m(-3) for inhalation. Prior to exposure the dogs were kept under clean air conditions for 16 mo to establish physiological baseline values for each dog. A second group of eight dogs (control) was kept for the entire study under clean air conditions. Nonspecific defense mechanisms in the airways and in the peripheral lung were studied during chronic exposure of the combination of neutral sulfur(IV) and acidic sulfur(VI) aerosols. No functional changes of tracheal mucus velocity were found, in agreement with unchanged morphometry of the airways. However, the exposure resulted in changes of several alveolar macrophage (AM) mediated particle clearance mechanisms: (1) Based on in vivo clearance analysis and cultured AM studies using moderately soluble cobalt oxide particles, intracellular particle dissolution was significantly reduced since phagolysosomal proton concentration was decreased. We deduce exposure-related malfunction of proton pumps bound to the phagolysosomal membrane as a result of an increase of cytosolic proton concentration. (2) Based on in vivo clearance analysis using insoluble polystyrene particles, AM-mediated particle transport from the lung periphery toward ciliated terminal bronchioli and further to the larynx was significantly reduced. Activation of epithelial type II cells at the entrance of alveoli was inferred from observed type II cell proliferation at those alveolar ridges and enhanced secretion of alkaline phosphatase in the fluid of bronchoalveolar lavages. As a result, hypersecretion of chemotactic mediators by activated type II cells at these loci led to the observed decrease of particle transport toward ciliated bronchioli. (3) Based on in vivo clearance analysis using insoluble polystyrene particles, particle transport from the alveolar epithelium into interstitial tissues was increased and (4) particle transport to the tracheobronchial lymph nodes was significantly enhanced. Particle transport into interstitial tissues is the most prominent clearance pathway from the canine alveolar epithelium. We conclude that the deteriorated particle transport toward ciliated terminal bronchioli resulted in an enhanced particle transport across the epithelial membrane into interstitial tissues and the lymphatic drainage. The observed alterations in alveolar macrophage-mediated clearance mechanisms during chronic exposure of these air pollutants indicate an increased risk of health.

Health effects of sulfur-related environmental air pollution. V. Lung structure

The lungs of 8 male beagle dogs were examined morphologically and morphometrically after exposure for 13 mo to a respirable sulfur(IV) aerosol at a mass concentration of 1.53 mg m(-3) (16.5 h/day), and to an acidic sulfate aerosol carrying 15.2 micromol m(-3) hydrogen ions into the lungs (6 h/day). An additional eight dogs served as unexposed controls. Standard morphometric analyses of both the surface epithelia of the conducting airways and the alveolar region were performed. These analyses showed no difference between the exposure group and control group. However, there was a tendency to an increase in the volume density of bronchial glands in the exposure group. Five of eight exposed animals showed thickened ridges (knob-like structures) at the entrance to alveoli in the alveolar duct and alveolar sac. Transmission electron microscopy revealed that the thickening was mainly due to type II cell proliferation. As the previous experiment using sulfite aerosol only showed no alterations in the proximal alveolar regions, the changes observed may be considered as effects of acidic sulfate aerosol alone or in combination with sulfite. These findings suggest that sulfur aerosols have the potential to induce epithelial alterations in the proximal alveolar region, which is a primary target for air pollutants.

Chronic exposure of rats to ozone and sulfuric acid aerosol: biochemical and structural responses

Groups of rats were exposed to either 0.12 or 0.20 ppm of ozone, 20, 100, or 150 ppm of sulfuric acid aerosol (0.4-0.8 microm diameter), or their mixtures in whole body exposure chambers for up to 90 days. Matched control animals were exposed to filtered air in comparable chambers. The rats were examined biochemically and morphometrically for centriacinar fibrosis or other indicators of pollutant-induced changes in the terminal bronchiole-alveolar duct junction region of the lung at the end of the exposures. By evaluating different markers of lung injury, we had previously demonstrated a synergistic interaction between ozone and sulfuric acid aerosol after acute exposures to these same concentrations of the pollutants. The present experiments were designed to answer the question of whether there was any interaction between ozone and respirable sized aerosols of sulfuric acid, synergistic or antagonistic, after chronic exposures. Exposure of rats to 0.12 or 0.20 ppm of ozone elicited tissue and cellular changes at the bronchiole-alveolar duct junction. Concurrent exposure to sulfuric acid aerosol did not affect the extent or magnitude of these changes. Intermittent exposure (12 h per day) to ozone, with or without the acid aerosol, elicited a greater response than did continuous exposure (24 h per day). No consistent effects of exposure to sulfuric acid aerosol alone were observed, either morphometrically or biochemically. The biochemical data were consistent with the morphometric analyses, showing trends towards or significantly increased lung 4-hydroxyproline content in the rats exposed to ozone, with or without sulfuric acid aerosol, in the intermittent exposure experiment, but not after continuous exposure. No interactive effects between ozone and sulfuric acid aerosol were observed with any of the biochemical parameters examined. We conclude that ozone and sulfuric acid aerosols do not exhibit synergistic interactions after chronic exposures (90 days) of rats to the concentrations tested in this study, which correspond to concentrations showing synergistic interactions in previously performed acute studies. We also observed that exposure of rats to ozone for 12 h per day elicited greater lung changes, which we interpret to indicate a mild fibrotic response, than did exposure of rats for 24 h per day, whether or not there was accompanying exposure to the acid aerosol.

Effects of exposure to sulfuric acid-aerosol on airway responsiveness in guinea pigs: concentration and time dependency

We investigated the concentration and time dependency of the effects of exposure to sulfuric acid (H2SO4) aerosol on airway responsiveness. Two hundred and sixteen male Hartley guinea pigs were used. The animals were divided into 3 groups (n = 72/group), with 1 group being exposed to filtered air and the other 2 to 1.0 mg/m3 or 3.2 mg/m3 H2SO4 aerosol. In each group, the animals were divided into 4 subgroups (n = 18/subgroup), with exposure terms of 3 d (24 h/d), 7 d, 14 d, and 30 d. Specific airway resistance (SRaw) under room air (SRaw0) and airway responsiveness were determined 1 wk before the beginning of exposure and on the day of termination of the exposure. Specific airway resistance values under room air (SRaw0) prior to and after exposure were compared. There was no significant change in SRaw0 after the exposure to filtered air, 1.0 mg/m3, or 3.2 mg/m3 H2SO4 aerosol. Our results also showed that exposure to filtered air or 1 mg/m3 H2SO4 aerosol did not cause any significant change in airway responsiveness to inhaled histamine aerosol, expressed as the effective concentration of histamine (EC200His) that produced a doubling of SRawNaCl (SRaw after exposure to aerosol of 0.9% NaCl saline). On the contrary, exposure to 3.2 mg/m3 H2SO4 aerosol induced transient airway hyporesponsiveness after a 3-d exposure [EC200His prior to and after exposure: 1.35 +/- 0.28 and 2.23 +/- 0.22 mM (p < .01), respectively] and then transient hyperresponsiveness after a 14-d exposure [EC200His prior to and after exposure: 1.65 +/- 0.21 and 0.95 +/- 0.23 mM (p < .01), respectively]. Overall, the present results revealed that (1) 1.0 mg/m3 or 3.2 mg/m3 H2SO4 aerosol had no significant effect on SRaw0 during a 30-d exposure period, (2) a high concentration (3.2 mg/m3) of H2SO4 aerosol affected airway responsiveness during the 30-d exposure, while a low concentration (1.0 mg/m3) of H2SO4 aerosol did not, and (3) the effect of exposure to 3.2 mg/m3 H2SO4 aerosol on airway responsiveness was transient and stimulatory or inhibitory, depending on the duration of exposure.

Assessment of toxicologic interactions resulting from acute inhalation exposure to sulfuric acid and ozone mixtures

Studies examining effects of air pollutants often use single compounds, while "real world" exposures are to more than one chemical. Thus, it is necessary to assess responses following inhalation of chemical mixtures. Rabbits were exposed for 3 hr to sulfuric acid aerosol at 0, 50, 75, or 125 micrograms/m3 in conjunction with ozone at 0, 0.1, 0.3, or 0.6 ppm, following which broncho-pulmonary lavage was performed. Various pulmonary response endpoints related to general cytotoxicity and macrophage function were examined. In addition, a goal of the study was to define an improved approach to the analysis of data sets involving binary pollutant mixtures. Results were evaluated using analysis of variance with multiple linear contrasts to determine the significance of any effect in the pollutant-exposed groups compared to sham control animals and to assess the type, and extent, of any toxicological interaction between acid and ozone. Interaction was considered to occur when the effects of combined exposure were either significantly greater or less than additive. Pollutant exposures had no effect on lavage fluid levels of lactate dehydrogenase, prostaglandins E2 and F2 alpha, nor on the numbers, viability, or types of immune cells recovered by lavage. Phagocytic activity of macrophages was depressed at the two highest acid levels and at all levels of ozone. Exposure to all mixtures showed significant antagonism. Superoxide production by stimulated macrophages was depressed by acid exposure at the two highest concentrations, while ozone alone had no effect. Significant antagonistic interaction was observed following exposure to mixtures of 75 or 125 micrograms/m3 acid with 0.1 or 0.3 ppm ozone. The activity of tumor necrosis factor elicited from stimulated macrophages was depressed by acid at 75 and 125 micrograms/m3 while ozone had no effect. Exposure to mixtures of 125 micrograms/m3 acid with 0.3 or 0.6 ppm ozone resulted in synergistic interaction. This study provided additional evidence for antagonism between two common air pollutants and demonstrated that the type of interaction between sulfuric acid and ozone depended upon the endpoint but that the magnitude of any interaction was not always related to the exposure concentrations of the constituent pollutants.

Long-term intermittent exposure to sulfuric acid aerosol, ozone, and their combination: alterations in tracheobronchial mucociliary clearance and epithelial secretory cells

Understanding the effects from long-term exposure to individual ambient air pollutants and mixtures of pollutants is necessary for adequate assessment of health risk. This study examined quantitative and temporal alterations in tracheobronchial mucociliary clearance function and bronchial epithelial secretory cells in rabbits exposed to sulfuric acid (125 micrograms/m3), ozone (0.1 ppm), and their combination for 2 h/d, 5 d/wk for up to 1 yr; some animals were allowed a 6-month post-exposure period. Clearance times were altered during exposure to sulfuric acid or to the mixture, and became progressively slower following the end of exposures to each of the pollutant atmospheres. There was no indication of any interaction in terms of clearance response between the acid and ozone in the group exposed to the mixture. Histological examination of intrapulmonary conducting airways was performed after 4, 8, or 12 months of exposure, and after the post-exposure period. Sulfuric acid resulted in an increase in the number of secretory cells in small airways by 12 months of exposure. Ozone and the mixture resulted in an increase in secretory cell number by 4 months, but the response became attenuated with continued exposure. There was evidence for synergistic interaction between ozone and acid at 4 months, and antagonistic interaction at subsequent times. No inflammation or other biologically significant histological effects were found in any of the animals.

Resistance of hamster bronchiolar epithelium to neutrophil elastase: investigation by cell surface lectin cytochemistry

An intratracheal instillation of human neutrophil elastase (HNE) causes accumulation of an excess number of secretory granules in the epithelial secretory cells lining the hamster bronchus. This chronic lesion, which we refer to as secretory cell metaplasia (SCM), is not seen in the trachea or bronchioles. Because luminal cell surface lectin binding is much higher in the trachea than in the bronchus, we concluded that tracheal resistance may be due to a protective glycoconjugate coat. In the present ultrastructural study, we analyzed the lectin-binding capability of bronchiolar epithelial cells to determine whether their luminal cell surface glycoconjugate layer is similar to tracheal epithelial cells. None of the six ferritin-conjugated lectins showed higher binding in bronchioles compared to the bronchus, suggesting that a high level of surface oligosaccharides is not necessary for resistance to the metaplastic effects of HNE. HNE caused a significant reduction in bronchiolar surface binding of the gold-labeled, secretory cell-specific lectin, Helix pomatia agglutinin. The principal granulated secretory cell type in bronchioles was ultrastructurally similar to a form of bronchial Clara cell that converts to a mucous cell phenotype in response to HNE. The results suggest that absence of bronchiolar SCM is not attributable to a protective layer of cell surface oligosaccharides, a lack of cellular contact by HNE, or the presence of a morphologically distinct population of epithelial cells in bronchioles.

Pulmonary arachidonic acid metabolism following acute exposures to ozone and nitrogen dioxide

Ozone (O3) and nitrogen dioxide (NO2) are common air pollutants, and exposure to these gases has been shown to affect pulmonary physiology, biochemistry, and structure. This study examined their ability to modulate arachidonic acid metabolites (eicosanoids) in the lungs. Rabbits were exposed for 2 h to O3 at 0.1, 0.3, or 1 ppm; NO2 at 1, 3, or 10 ppm; or to a mixture of 0.3 ppm O3 and 3 ppm NO2. Groups of animals sacrificed either immediately or 24 h after each exposure underwent broncho-pulmonary lavage. Selected eicosanoids were assessed in lavage fluid by radioimmunoassay. Increases in prostaglandins E2 (PGE2) and F2 alpha (PGF2 alpha) were found immediately after exposure to 1 ppm O3. Exposure to 10 ppm NO2 resulted in a depression of 6-keto-PGF1 alpha, while thromboxane B2 (TxB2) was elevated after exposure to 1 ppm NO2 and depressed following 3 and 10 ppm. The O3/NO2 mixture resulted in synergistic increases in PGE2 and PGF2 alpha, with the response appearing to be driven by O3. This study has demonstrated that acute exposure to either O3 or NO2 can alter pulmonary arachidonic acid metabolism and that the responses to these oxidants differ, both quantitatively and qualitatively.

Sulfuric acid-induced changes in the physiology and structure of the tracheobronchial airways

Sulfuric acid aerosols occur in the ambient particulate mode due to atmospheric conversion from sulfur dioxide (SO2). This paper describes the response of the rabbit tracheobronchial tree to daily exposures to sulfuric acid (H2SO4) aerosol, relating physiological and morphological parameters. Rabbits were exposed to filtered air (sham control) or to submicrometer-sized H2SO4 at 250 micrograms/m3 H2SO4, for 1 hr/day, 5 days/week, with sacrifices after 4, 8, and 12 months of acid (or sham) exposure; some rabbits were allowed a 3-month recovery after all exposures ended. H2SO4 produced a slowing of tracheobronchial mucociliary clearance during the first weeks of exposure; this change became significantly greater with continued exposures and did not improve after exposures ended. Airway hyperresponsiveness was evident by 4 months of acid exposure; the condition worsened by 8 months of exposure and appeared to stabilize after this time. Standard pulmonary mechanics parameters showed no significant trends with repeated acid exposure, except for a decline in dynamic lung compliance in animals exposed to acid for 12 months. Lung tissue samples obtained from exposed animals showed a shift toward a greater frequency of smaller airways compared to control, an increase in epithelial secretory cell density in smaller airways, and a shift from neutral to acidic glycoproteins in the secretory cells. The effect on airway diameter resolved after the exposures ceased, but the secretory cell response did not return to normal within the recovery period. No evidence of inflammatory cell infiltration was found due to H2SO4 exposure. Thus, significant alterations in the physiology of the tracheobronchial tree have been demonstrated due to repeated 1-hr exposures to a concentration of H2SO4 that is one-fourth the current 8-hr threshold limit value for exposure in the work environment. The cumulative dose inhaled by the rabbits is similar to current peak daily doses from ambient exposure in North America. The results obtained in the rabbit model provide insight into early changes in the tracheobronchial tree due to repeated irritant exposure and may be involved in the pathogenesis of chronic airway disease.

Response of the tracheobronchial mucociliary clearance system to repeated irritant exposure: effect of sulfuric acid mist on function and structure

This study was designed to determine quantitative and temporal alterations in tracheobronchial mucociliary clearance function and structure due to repeated inhalation exposures to a common irritant, sulfuric acid mist. Rabbits were exposed to 250 micrograms/m3 sulfuric acid (0.3 micron) for 1 h/day, 5 days/week, for up to 1 year, with some animals allowed a 3-month recovery period following the end of the acid exposures. Control animals received temperature- and humidity-conditioned water vapor. At intervals of 2 to 4 weeks, animals inhaled a radioactively tagged tracer aerosol (ferric oxide microspheres, 4.5 micron), and its clearance via mucociliary transport from the thorax was monitored by external serial counting. Clearance became slower during the first month of acid exposure, and this slowing became progressive with time through the end of the 12-month exposure period. After cessation of acid exposure, clearance became extremely slow and did not return to normal by the end of the follow-up period. To assess specific histological changes in the tracheobronchial tree, groups of rabbits were killed after 4, 8, or 12 months of exposure and after the follow-up period. Tissue samples from each lung were embedded in plastic, sectioned at 3 micron, and stained with hematoxylin and eosin or alcian blue/periodic acid-Schiff (AB/PAS). Acid exposure changed the airway diameter distribution compared to the control; except for the follow-up group, all acid-exposed animals had a shift to smaller airways. Acid inhalation also caused an increase in epithelial secretory cell density and a shift from PAS to AB staining of glycoprotein within secretory cells, both of which were unresolved by 3 months after the exposures ceased. No evidence of inflammation was found in any of the animals. Thus, repeated exposures to H2SO4 resulted in a slowing of mucociliary clearance that was associated with alterations in airway morphometry and morphology. Such changes may be involved in the early pathogenesis of chronic airway disease.

Intermittent exposures to mixed atmospheres of nitrogen dioxide and sulfuric acid: effect on particle clearance from the respiratory region of rabbit lungs

Nitrogen dioxide (NO2) and sulfuric acid (H2SO4) are important constituents of the gas-particle complex in ambient air. The purpose of this study was to examine the effects of combined exposures upon the clearance of insoluble tracer particles from the respiratory region of the lungs. Rabbits were the animal model, and were exposed for 2 h/day for 14 days to either 0.3 ppm NO2, 1 ppm NO2, or 500 microgram/m3 H2SO4 alone, or to mixtures of the low and high NO2 concentrations with acid. Inhaled singly, both concentrations of NO2 accelerated clearance while H2SO4 retarded it, compared to control. Exposure to the combination of 0.3 ppm NO2 plus H2SO4 resulted in a response which was not different from that due to the acid alone. However, exposure to 1 ppm NO2 plus H2SO4 resulted in a clearance pattern which differed from that of both NO2 and H2SO4, but was more similar to that of the latter.

Early alveolar clearance in rabbits intermittently exposed to sulfuric acid mist

During the course of 1-h/d, 5-d/wk exposure to submicrometer sulfuric acid mist at 250 micrograms/m3, rabbits were exposed to a radioactively tagged polystyrene latex tracer aerosol to assess clearance from the alveolar region during the period 2-14 d after tracer exposure. The latex was administered on d 1, 57, and 240 following the start of the H2SO4 exposures. Early alveolar clearance was found to be accelerated during the first test, and this acceleration was maintained throughout the 8-mo monitoring period.

Airborne particles, their use in the respiratory system to measure air flow, function, and clearance

Nonhygroscopic monodisperse particles can be used to estimate airway dimensions within chosen regions of the respiratory tract. These dimensions correspond well with those measured in inflation-fixed lungs. The dispersion of a bolus of airborne particles on inhalation and exhalation is very sensitive to the dimensions of the airways through which the bolus passes, yielding indices of dispersion that provide sensitive indicators of changes in airway dimensions, eg, with smoking. The rates of clearance of particles from the lungs are determined using gamma-labelled particles whose lung retention is determined by external measurements. There are major differences between the deep lung and the ciliated airways. Changes in clearance rates are caused by disease or inhaled intoxicants such as cigarette smoke. At lower levels of irritant exposure, an acceleration of clearance rate is observed, while higher doses and longer periods of exposure produce decreased rates. It is suggested that aerosol techniques may have useful clinical applications; approaches are suggested for such applications.

Potential human health effects of acid rain: report of a workshop

This report summarizes the potential impact of the acid precipitation phenomenon on human health. There are two major components to this phenomenon: the predepositional phase, during which there is direct human exposure to acidic substances from ambient air, and the post-depositional phase, in which the deposition of acid materials on water and soil results in the mobilization, transport, and even chemical transformation of toxic metals. Acidification increases bioconversion of mercury to methylmercury, which accumulates in fish, increasing the risk to toxicity in people who eat fish. Increase in water and soil content of lead and cadmium increases human exposure to these metals which become additive to other sources presently under regulatory control. The potential adverse health effects of increased human exposure to aluminum is not known at the present time.

Comparative deposition of inhaled aerosols in experimental animals and humans: a review

The biological effects of inhaled aerosols are often related to their site(s) of deposition within the respiratory tract. However, deposition patterns may differ between humans and those experimental animals commonly used in inhalation toxicology studies, making cross-species risk extrapolations difficult. This paper reviews the factors that control deposition and synthesizes much of the available data on comparative regional deposition.

Interspecies comparisons of particle deposition and mucociliary clearance in tracheobronchial airways

Inhaled insoluble particles that deposit along normal healthy tracheobronchial airways of humans and other mammals are transported on the proximally moving mucous lining to the larynx, where they are swallowed. The transit time from the most distal ciliated airways varies from 0.1 to 1 d, with each individual having a relatively constant, characteristic time. The exact time course of clearance depends on the distributions of both particle deposition and mucus velocities along the airways. There are too few data on intrabronchial deposition and mucociliary transport rates for laboratory animals to permit a thorough intercomparison among species. However, enough is known about the relative lung sizes and anatomical differences among the various species to make some preliminary, but important, distinctions. As compared to commonly used experimental animals, humans have larger lungs and a more symmetric upper bronchial airway branching pattern. In addition, humans do considerable oral breathing, thus bypassing the effective air cleaning capability of the nasal airways. These differences contribute to a greater amount of upper bronchial airway particle deposition in humans, as well as to greater concentrations of deposition on localized surfaces near airway bifurcations. Airborne irritants that deposit in small ciliated airways may produce marked changes in mucociliary transport. Such materials include cigarette smoke, submicrometer-sized sulfuric acid mist, nitrogen dioxide, and ozone. For cigarette smoke and sulfuric acid, which have been studied for transient effects following single brief exposures in both humans and animals, the responses are similar. Upon repetitive exposures in animals, both of these irritants produce persistant alterations in clearance rates and airway morphometry. Studies of the effects of ozone on mucociliary clearance have, up to now, been limited to tests of the responses of rats to single exposures. The similarities between the known effects of various irritants suggests a nonspecific response.

Response of the bronchial mucociliary clearance system in rabbits to inhaled sulfite and sulfuric acid aerosols

A group of eight rabbits was exposed (orally) for 1 hr to a distilled water aerosol (sham control) or to submicrometer aerosols of either a transition metal sulfite complex (Fe(III)-S(IV], sodium sulfite (Na2SO3), or sulfuric acid (H2SO4). Mucociliary clearance was evaluated by external retention measurements of radioactively tagged tracer particles from the bronchial tree. Fe(III)-S(IV) in the range of 238 to 1227 micrograms/m3 (as SO2-3) produced no significant change from sham control in the mean residence time (MRT) of the tracer, indicating no effect upon mucociliary clearance rate. On the other hand, Na2SO3 at levels greater than or equal to 1200 micrograms/m3 (as SO2-3) resulted in clearance acceleration; the lack of effect of Fe(III)-S(IV) is possibly due to its stability. H2SO4 at 260 to 2155 micrograms/m3 produced a significant dose-related response, indicating clearance acceleration at low concentrations and a depression at higher levels.