An 'injury-time integral' model for extrapolating from acute to chronic effects of phosgene

Pathogenetic role of alveolar surfactant depleted by phosgene: Biophysical mechanisms and peak inhalation exposure metrics

In contrast to water-soluble respiratory tract irritants in their gas phase, the physicochemical properties of 'hydrophilicity' vs. 'lipophilicity' are the preponderant factors that dictate the site of major retention of the gas at the portal of entry. The lipophilic physical properties of phosgene gas facilitate retention in the alveolar region lined with amphipathic pulmonary surfactant (PS). The relationship between exposure and adverse health outcomes is complex, may vary over time, and is dependent on the biokinetics, biophysics, and pool size of PS relative to the inhaled dose of phosgene. Kinetic PS depletion is hypothesized to occur as inhalation followed by inhaled dose-dependent PS depletion. A kinetic model was developed to better understand the variables characterizing the inhaled dose rates of phosgene vs. PS pool size reconstitution. Modeling and empirical data from published evidence revealed that phosgene gas unequivocally follows a concentration x exposure (C × t) metric, independent of the frequency of exposure. The modeled and empirical data support the hypothesis that the exposure standards of phosgene are described best by a C × t time-averaged metric. Modeled data favorably duplicate expert panel-derived standards. Peak exposures within a reasonable range are of no concern.

Phosgene inhalation toxicity: Update on mechanisms and mechanism-based treatment strategies

Phosgene (carbonyl dichloride) gas is an indispensable high-production-volume chemical intermediate used worldwide in numerous industrial processes. Published evidence of human exposures due to accidents and warfare (World War I) has been reported; however, these reports often lack specificity because of the uncharacterized exposure intensities of phosgene and/or related irritants. These may include liquid or solid congeners of phosgene, including di- and triphosgene and/or the respiratory tract irritant chlorine which are often collectively reported under the umbrella of phosgene exposure without any appreciation of their differences in causing acute lung injury (ALI). Among these irritants, phosgene gas is somewhat unique because of its poor water solubility. This prevents any appreciable retention of the gas in the upper airways and related trigeminal sensations of irritation. By contrast, in the pulmonary compartment, amphiphilic surfactant might scavenge this lipophilic gas. The interaction of phosgene and the surfactant may affect basic physiological functions controlled by Starling's and Laplace's laws, which can be followed by cardiogenic pulmonary edema. The phenotypic manifestations are dependent on the concentration × exposure duration (C × t); the higher the C × t is, the less time that is required for edema to appear. It is hypothesized that this type of edema is caused by cardiovascular and colloid osmotic imbalances to initial neurogenic events but not because of the injury itself. Thus, hemodynamic etiologies appear to cause imbalances in extravasated fluids and solute accumulation in the pulmonary interstitium, which is not drained away by the lymphatic channels of the lung. The most salient associated findings are hemoconcentration and hypoproteinemia. The involved intertwined pathophysiological processes coordinating pulmonary ventilation and cardiopulmonary perfusion under such conditions are complex. Pulmonary arterial catheter measurements on phosgene-exposed dogs provided evidence of 'cor pulmonale', a form of acute right heart failure produced by a sudden increase in resistance to blood flow in the pulmonary circulation about 20 h postexposure. The objective of this review is to critically analyze evidence from experimental inhalation studies in rats and dogs, and evidence from accidental human exposures to better understand the primary and secondary events causing cardiopulmonary dysfunction and an ensuing life-threatening lung edema. Mechanism-based diagnostic and therapeutic approaches are also considered for this form of cardiogenic edema.

Phosgene-induced acute lung injury (ALI): differences from chlorine-induced ALI and attempts to translate toxicology to clinical medicine

BACKGROUND

Phosgene (carbonyl dichloride) gas is an indispensable chemical inter-mediate used in numerous industrial processes. There is no clear consensus as to its time- and inhaled-dose-dependent etiopathologies and associated preventive or therapeutic treatment strategies.

METHODS

Cardiopulmonary function was examined in rats exposed by inhalation to the alveolar irritant phosgene or to the airway irritant chlorine during and following exposure. Terminal measurements focused on hematology, protein extravasation in bronchoalveolar lavage (BAL), and increased lung weight. Noninvasive diagnostic and prognostic endpoints in exhaled breath (carbon dioxide and nitric oxide) were used to detect the clinically occult stage of pulmonary edema.

RESULTS

The first event observed in rats following high but sublethal acute exposure to phosgene was the stimulation of alveolar nociceptive vagal receptors. This afferent stimulation resulted in dramatic changes in cardiopulmonary functions, ventilation: perfusion imbalances, and progressive pulmonary edema and phospholipoproteinosis. Hematology revealed hemoconcentration to be an early marker of pulmonary edema and fibrin as a discriminating endpoint that was positive for the airway irritant chlorine and negative for the alveolar irritant phosgene.

CONCLUSIONS

The application of each gas produced typical ALI/ARDS (acute lung injury/acute respiratory distress syndrome) characteristics. Phosgene-induced ALI showed evidence of persistent apnea periods, bradycardia, and shifts of vascular fluid from the peripheral to the pulmonary circulation. Carbon dioxide in expired gas was suggestive of increased ventilation dead space and appeared to be a harbinger of progressively developing lung edema. Treatment with the iNOS inhibitor aminoguanidine aerosol by inhalation reduced the severity of phosgene-induced ALI when applied at low dose-rates. Symptomatic treatment regimens were considered inferior to causal modes of treatment.

Pathology, toxicology, and latency of irritant gases known to cause bronchiolitis obliterans disease: Does diacetyl fit the pattern?

Bronchiolitis obliterans (BO) is a rare disease involving concentric bronchiolar fibrosis that develops rapidly following inhalation of certain irritant gases at sufficiently high acute doses. While there are many potential causes of bronchiolar lesions involved in a variety of chronic lung diseases, failure to clearly define the clinical features and pathological characteristics can lead to ambiguous diagnoses. Irritant gases known to cause BO follow a similar pathologic process and time course of disease onset in humans. Studies of inhaled irritant gases known to cause BO (e.g., chlorine, hydrochloric acid, ammonia, nitrogen oxides, sulfur oxides, sulfur or nitrogen mustards, and phosgene) indicate that the time course between causal chemical exposures and development of clinically significant BO disease is typically limited to a few months. The mechanism of toxic action exerted by these irritant gases generally involves widespread and severe injury of the epithelial lining of the bronchioles that leads to acute respiratory symptoms which can include lung edema within days. Repeated exposures to inhaled irritant gases at concentrations insufficient to cause marked respiratory distress or edema may lead to adaptive responses that can reduce or prevent severe bronchiolar fibrotic changes. Risk of BO from irritant gases is driven substantially by toxicokinetics affecting concentrations occurring at the bronchiolar epithelium. Highly soluble irritant gases that cause BO like ammonia generally follow a threshold-dependent cytotoxic mechanism of action that at sufficiently high doses results in severe inflammation of the upper respiratory tract and the bronchiolar epithelium concurrently. This is followed by acute respiratory distress, pulmonary edema, and post inflammatory concentric fibrosis that become clinically obvious within a few months. In contrast, irritant gases with lower solubility like phosgene also follow a threshold-dependent mechanism of cytotoxicity action but can exhibit more insidious and isolated bronchiolar tissue damage with a similar latency to fibrosis. To date, animal and human studies on the highly soluble gas, diacetyl, have not identified a coherent pattern of pathology and latency that would be expected based on studies of other known causes of bronchiolitis obliterans disease.

A pragmatic approach to assess the exposure of the honey bee (Apis mellifera) when subjected to pesticide spray

Plant protection spray treatments may expose non-target organisms to pesticides. In the pesticide registration procedure, the honey bee represents one of the non-target model species for which the risk posed by pesticides must be assessed on the basis of the hazard quotient (HQ). The HQ is defined as the ratio between environmental exposure and toxicity. For the honey bee, the HQ calculation is not consistent because it corresponds to the ratio between the pesticide field rate (in mass of pesticide/ha) and LD50 (in mass of pesticide/bee). Thus, in contrast to all other species, the HQ can only be interpreted empirically because it corresponds to a number of bees/ha. This type of HQ calculation is due to the difficulty in transforming pesticide field rates into doses to which bees are exposed. In this study, we used a pragmatic approach to determine the apparent exposure surface area of honey bees submitted to pesticide treatments by spraying with a Potter-type tower. The doses received by the bees were quantified by very efficient chemical analyses, which enabled us to determine an apparent surface area of 1.05 cm(2)/bee. The apparent surface area was used to calculate the exposure levels of bees submitted to pesticide sprays and then to revisit the HQ ratios with a calculation mode similar to that used for all other living species. X-tomography was used to assess the physical surface area of a bee, which was 3.27 cm(2)/bee, and showed that the apparent exposure surface was not overestimated. The control experiments showed that the toxicity induced by doses calculated with the exposure surface area was similar to that induced by treatments according to the European testing procedure. This new approach to measure risk is more accurate and could become a tool to aid the decision-making process in the risk assessment of pesticides.

Functional genomic assessment of phosgene-induced acute lung injury in mice

In this study, a genetically diverse panel of 43 mouse strains was exposed to phosgene and genome-wide association mapping performed using a high-density single nucleotide polymorphism (SNP) assembly. Transcriptomic analysis was also used to improve the genetic resolution in the identification of genetic determinants of phosgene-induced acute lung injury (ALI). We prioritized the identified genes based on whether the encoded protein was previously associated with lung injury or contained a nonsynonymous SNP within a functional domain. Candidates were selected that contained a promoter SNP that could alter a putative transcription factor binding site and had variable expression by transcriptomic analyses. The latter two criteria also required that ≥10% of mice carried the minor allele and that this allele could account for ≥10% of the phenotypic difference noted between the strains at the phenotypic extremes. This integrative, functional approach revealed 14 candidate genes that included Atp1a1, Alox5, Galnt11, Hrh1, Mbd4, Phactr2, Plxnd1, Ptprt, Reln, and Zfand4, which had significant SNP associations, and Itga9, Man1a2, Mapk14, and Vwf, which had suggestive SNP associations. Of the genes with significant SNP associations, Atp1a1, Alox5, Plxnd1, Ptprt, and Zfand4 could be associated with ALI in several ways. Using a competitive electrophoretic mobility shift analysis, Atp1a1 promoter (rs215053185) oligonucleotide containing the minor G allele formed a major distinct faster-migrating complex. In addition, a gene with a suggestive SNP association, Itga9, is linked to transforming growth factor β1 signaling, which previously has been associated with the susceptibility to ALI in mice.

Interrelating the acute and chronic mode of action of inhaled methylenediphenyl diisocyanate (MDI) in rats assisted by computational toxicology

Polymeric methylenediphenyl diisocyanate (MDI) is a high production volume chemical intermediate consisting of monomeric 4,4'-MDI, its 2,2'- and 2,4'-isomers, and higher oligomeric homologues. The toxicity of pMDI has systematically been investigated in previous regulatory and mechanistic studies. One cornerstone of toxicological risk assessment is to understand the critical Mode of Action (MoA) of inhaled MDI aerosol. This paper compares the no-observed-adverse effect levels (NOAELs) in rats from two published whole-body exposure chronic inhalation bioassays with the lung irritation-based point of departures (PODs) from acute and subacute nose-only inhalation studies. Acute irritation was related to elevated concentrations of protein in bronchoalveolar lavage fluid (short-term studies), whilst the chronic events were characterized by histopathology. In the chronic bioassay the exposure duration was either 6 or 18h/day while in all other studies a 6h/day regimens were applied. The major objective of this paper is to analyze the interrelationship of acute pulmonary irritation and the acute-on-chronic manifestations of pulmonary disease following recurrent chronic inhalation exposure. This included considerations on the most critical metrics of exposure with regard to the acute concentration×exposure duration per day (C×T(day)) and the chronic cumulative dose metrics. In summary, this analysis supports the conclusion that the C×T(day) relative to the acute pulmonary irritation threshold is more decisive for the chronic outcome than the concentration per se or the time-adjusted cumulative dose. For MDI aerosols, the acute threshold C×T(day) was remarkably close to the NOAELs of the chronic inhalation studies, independent on their differing exposure mode and regimens. This evidence is supportive of a simple, direct MoA at the site of initial deposition of aerosol. Accordingly, for chemicals reactive to the endogenous nucleophilic agents contained in the lining fluid of the lung, one unifying essential prerequisite for pulmonary injury appears to be a C×T(day) that exhausts the homeostatic pool of MDI-scavenging agents. In the case that threshold is exceeded, the secondary compensatory chronic response may then cause additional superimposed types of chronic pathologies.

Attempts to counteract phosgene-induced acute lung injury by instant high-dose aerosol exposure to hexamethylenetetramine, cysteine or glutathione

Phosgene is an important high-production-volume intermediate with widespread industrial use. Consistent with other lung irritants causing ALI (acute lung injury), mode-of-action-based countermeasures remain rudimentary. This study was conducted to analyze whether extremely short high-level exposure to phosgene gas could be mitigated using three different inhaled nucleophiles administered by inhalation instantly after exposure to phosgene. Groups of young adult male Wistar rats were acutely exposed to carbonyl chloride (phosgene) using a directed-flow nose-only mode of exposure of 600 mg/m³ for 1.5 min (225 ppm × min). Immediately after exposure to phosgene gas the rats were similarly exposed to three strong nucleophiles with and without antioxidant properties for 5 or 15 min. The following nucleophiles were used: hexamethylenetetramine (HMT), l-cysteine (Cys), and l-glutathione (GSH). The concentration of the aerosol (mass median aerodynamic diameter 1.7-2 µm) was targeted to be in the range of 1 mg/L. Cys and GSH have antioxidant properties in addition. The calculated alveolar molar dosage of phosgene was 9 µmol/kg. At 15-min exposure duration, the respective inhaled dose of HMT, Csy, and GSH were 111, 103, and 46 µmol/kg, respectively. The alveolar dose of drugs was ~10-times lower. The efficacy of treatment was judged by protein concentrations in bronchoalveolar lavage fluid (BALF) collected 1 day post-exposure. In spite of using optimized aerosolization techniques, none of the nucleophiles chosen had any mitigating effect on BALF-protein extravasation. This finding appear to suggest that inhaled phosgene gas acylates instantly nucleophilic moieties at the site of initial deposition and that the resultant reaction products can not be reactivated even following instant inhalation treatment with competing nucleophilic agents. In spite of using maximal technically attainable concentrations, it appears to be experimentally challenging to deliver such nucleophiles to the lower respiratory tract at high dosages.

Air pollution toxicology--a brief review of the role of the science in shaping the current understanding of air pollution health risks

Human and animal toxicology has had a profound impact on our historical and current understanding of air pollution health effects. Early animal toxicological studies of air pollution had distinctively military or workplace themes. With the discovery that ambient air pollution episodes led to excess illness and death, there became an emergence of toxicological studies that focused on industrial air pollution encountered by the general public. Not only did the pollutants investigated evolve from ambient mixtures to individual pollutants but also the endpoints and outcomes evaluated became more sophisticated, resulting in our present state of the science. Currently, a large toxicological database exists for the effects of particulate matter and ozone, and we provide a focused review of some of the major contributions to the biological understanding for these two "criteria" air pollutants. A limited discussion of the toxicological advancements in the scientific knowledge of two hazardous air pollutants, formaldehyde and phosgene, is also included. Moving forward, the future challenge of air pollution toxicology lies in the health assessment of complex mixtures and their interactions, given the projected impacts of climate change and altered emissions on ambient conditions. In the coming years, the toxicologist will need to be flexible and forward thinking in order to dissect the complexity of the biological system itself, as well as that of air pollution in all its varied forms.

Acute Nose-Only Exposure of Rats to Phosgene. Part I: Concentration × Time Dependence of LC50s, Nonlethal-Threshold Concentrations, and Analysis of Breathing Patterns

Groups of young adult Wistar rats were acutely exposed to phosgene gas using a directed-flow nose-only mode of exposure. The exposure durations used were 10, 30, 60, and 240 min and the corresponding C x t products bracketed a range from 1538 to 2854 mg/m3 x min. The postexposure period was 2 wk. Subgroups of rats were subjected to respiratory function measurements. With few exceptions, mortality occurred within 24 h after exposure. The median lethal concentration (LC50) and the estimated nonlethal threshold concentrations (LC01) for 10, 30, 60, and 240 min were 253.3 (105.3), 54.5 (29.2), 31.3 (21.1), and 8.6 (5.3) mg/m3, respectively. With regard to the fixed outcome Cn x t product, the exponent n was found to be approximately 0.9 for both the LC50 and the LC01. Due to an apparent rodent-specific transient depression in ventilation, results from 10-min exposures were excluded for the calculation of average C x t products. The average LCt50 (and confidence interval 95%) and LCt01 were 1741 (1547-1929) mg/m3 x min and 1075 mg/m3 x min, respectively, with a LCt50/LCt01 ratio of 1.6. Respiratory function measurements revealed an increased apnea time (AT), which is typical for lower respiratory tract irritants. This response was associated with transiently decreased respiratory minute volumes. Borderline, although distinct, changes in AT occurred at 1.2 x 30 mg/m3 x min and above, which did not show evidence of recovery during a 30-min postexposure period at 47.6 x 30 mg/m3 x min and above. In an ancillary study, one group of rats was exposed to 1008 mg/m3 x min (at 4.2 mg/m3 for 240 min; postexposure period 4 wk). Emphasis was on the time course of nonlethal endpoints (bronchoalveolar lavage, BAL) and histopathology of the lungs of rats sacrificed at the end of the 4-wk postexposure period. The climax of BAL protein was on the first postexposure day and exceeded approximately 70 times the control without causing mortality. The changes in BAL protein resolved within 2 wk. Histopathology did not show evidence of lung remodeling or progressive, potentially irreversible changes 4 wk postexposure. In summary, the analysis of the C x t dependent mortality revealed a steep C x t mortality relationship. The C x t product in the range of the nonlethal threshold concentration (1008 mg/m3 x min) caused pulmonary injury as indicated by markedly increased protein in BAL. Changes resolved almost entirely within the 4-wk postexposure period.

Acute Nose-Only Exposure of Rats to Phosgene. Part II. Concentration × Time Dependence of Changes in Bronchoalveolar Lavage During a Follow-Up Period of 3 Months

Groups of young adult male Wistar rats were acutely exposed to phosgene gas for either 30 or 240 min using a directed-flow nose-only mode of exposure. In 30-min exposed rats the concentrations were 0.94, 2.02, 3.89, 7.35, and 15.36 mg/m3, which relate to C x t products of 28.2, 60.6, 116.7, 220.5, and 460.8 mg/m3 x min. In 240-min exposed rats the concentrations were 0.96, 0.387, 0.786, 1.567, and 4.2 mg/m3, which relate C x t products of 47.0, 92.9, 188.6, 376, and 1008 mg/m3 x min. Six rats/group were sacrificed on postexposure days 1, 3, 7, 14, and 84, while the rats of the 1008 mg/m3 x min group where sacrificed on postexposure days 1, 7, 14, and 28. The focus of measurements was directed toward indicators of inflammatory response and increased transmucosal permeability in bronchoalveolar lavage (BAL), including lung weights. Lungs from rats sacrificed at the end of the postexposure period were additionally examined by histopathology. Mortality did not occur at any C x t product. The most pronounced changes were related to C x t-dependent increases in the following markers in BAL: protein, soluble collagen, polymorphonuclear leukocytes (PMN) counts, and alveolar macrophages with foamy appearance. These indicators were maximal on the first postexposure day, while total cell counts and alveolar macrophages containing increased phospholipids reached their climax around post-exposure day 3. At 1008 mg/m3 x min the most sensitive indicators in BAL, that is, protein, PMN, and collagen, resolved within 2 wk, whereas at lower C x t products they reached the level of the control by postexposure day 7. At 1008 mg/m3 x min (day 28), histopathology revealed a minimal to slight hypercellularity in terminal bronchioles with focal peribronchiolar inflammatory infiltrates and focal septal thickening. At lower C x t products (day 84) the rats from all groups were indistinguishable and Sirius red-stained lungs did not provide evidence of late-onset sequelae, such as fibrotic changes or collagen deposition. At similar C x t products the changes in BAL were slightly less pronounced using 30-min exposure periods when compared to 240-min exposure periods. In summary, the phosgene-induced transmucosal permeability caused a C x t-dependent increase of several BAL indicators, of which those of protein, PMN, and soluble collagen were most pronounced. Exposure intensities up to 116.7 mg/m3 x min did not cause changes different from those observed in controls, while at 188.6 mg/m3 x min distinct differences to the control existed. Despite the extensively increased airway permeability, histopathology did not provide evidence of lung tissue remodeling or irreversible sequelae.

Acute Head-Only Exposure of Dogs to Phosgene. Part III. Comparison of Indicators of Lung Injury in Dogs and Rats

To better understand the relevance of phosgene-induced changes in bronchoalveolar lavage (BAL) fluid protein observed in acutely exposed rats, groups of beagle dogs were similarly exposed for 30 min to phosgene using a head-only mode of exposure. The actual exposure concentrations were 9, 16.5, and 35 mg/m3, with resultant C x t products of 270, 495, and 1050 mg/m3 x min. In rats, a C x t product of 270 mg/m3 x min caused a significant elevation of protein in the bronchoalveolar lavage (BAL) fluid, while the nonlethal threshold concentration (LCt01) was estimated to be 1075 mg/m3 x min. The endpoints examined in dogs focused on changes in BAL, lung weights, arterial blood gases, and lung histopathology approximately 24 h postexposure. Mortality did not occur at any C x t product. Increased lung weights and elevations in protein, soluble collagen, and polymorphonuclear leukocyte (PMN) counts in BAL were observed at 1050 mg/m3 x min with borderline changes at 495 mg/m3 x min. Following exposure to 1050 mg/m3 x min, the analysis of arterial blood gases provided evidence of a significantly decreased arterial pO2. Histopathology revealed a mild, although distinctive, inflammatory response at the bronchoalveolar level at 495 mg/m3 x min, whereas serofibrinous exudates and edema were observed at 1050 mg/m3 x min. The magnitude of effects correlated with the individual dogs' respiratory minute volume and breathing patterns (panting). Collectively, phosgene-induced indicators of acute lung injury appeared to be characterized best by protein in BAL fluid. With regard to both the inhaled dose and the associated increase of protein in BAL, the responses obtained in dogs appear to be more similar to humans. In contrast, elevations in BAL protein occurred in rats at three-fold lower concentrations when compared to dogs. The results of this study demonstrate that the magnitude of elevations of plasma exudate in BAL fluid following acute exposure to the pulmonary irritant phosgene is markedly more pronounced in rats when compared to the dog which is considered more human-like than rats. This is believed to be associated with the higher ventilation of small rodents and with rodent-specific sensory bronchopulmonary defense reflexes.

Workshop Summary: Phosgene-Induced Pulmonary Toxicity Revisited: Appraisal of Early and Late Markers of Pulmonary Injury From Animal Models With Emphasis on Human Significance

A workshop was held February 14, 2007, in Arlington, VA, under the auspices of the Phosgene Panel of the American Chemistry Council. The objective of this workshop was to convene inhalation toxicologists and medical experts from academia, industry and regulatory authorities to critically discuss past and recent inhalation studies of phosgene in controlled animal models. This included presentations addressing the benefits and limitations of rodent (mice, rats) and nonrodent (dogs) species to study concentration x time (C x t) relationships of acute and chronic types of pulmonary changes. Toxicological endpoints focused on the primary pulmonary effects associated with the acute inhalation exposure to phosgene gas and responses secondary to injury. A consensus was reached that the phosgene-induced increased pulmonary extravasation of fluid and protein can suitably be probed by bronchoalveolar lavage (BAL) techniques. BAL fluid analyses rank among the most sensitive methods to detect phosgene-induced noncardiogenic, pulmonary high-permeability edema following acute inhalation exposure. Maximum protein concentrations in BAL fluid occurred within 1 day after exposure, typically followed by a latency period up to about 15 h, which is reciprocal to the C x t exposure relationship. The C x t relationship was constant over a wide range of concentrations and single exposure durations. Following intermittent, repeated exposures of fixed duration, increased tolerance to recurrent exposures occurred. For such exposure regimens, chronic effects appear to be clearly dependent on the concentration rather than the cumulative concentration x time relationship. The threshold C x t product based on an increased BAL fluid protein following single exposure was essentially identical to the respective C x t product following subchronic exposure of rats based on increased pulmonary collagen and influx of inflammatory cells. Thus, the chronic outcome appears to be contingent upon the acute pulmonary threshold dose. Exposure concentrations high enough to elicit an increased acute extravasation of plasma constituents into the alveolus may also be associated with surfactant dysfunction, intra-alveolar accumulation of fibrin and collagen, and increased recruitment and activation of inflammatory cells. Although the exact mechanisms of toxicity have not yet been completely elucidated, consensus was reached that the acute pulmonary toxicity of phosgene gas is consistent with a simple, irritant mode of action at the site of its initial deposition/retention. The acute concentration x time mortality relationship of phosgene gas in rats is extremely steep, which is typical for a local, directly acting pulmonary irritant gas. Due to the high lipophilicity of phosgene gas, it efficiently penetrates the lower respiratory tract. Indeed, more recent published evidence from animals or humans has not revealed appreciable irritant responses in central and upper airways, unless exposure was to almost lethal concentrations. The comparison of acute inhalation studies in rats and dogs with focus on changes in BAL fluid constituents demonstrates that dogs are approximately three to four times less susceptible to phosgene than rats under methodologically similar conditions. There are data to suggest that the dog may be useful particularly for the study of mechanisms associated with the acute extravasation of plasma constituents because of its size and general morphology and physiology of the lung as well as its oronasal breathing patterns. However, the study of the long-term sequelae of acute effects is experimentally markedly more demanding in dogs as compared to rats, precluding the dog model to be applied on a routine base. The striking similarity of threshold concentrations from single exposure (increased protein in BAL fluid) and repeated-exposure 3-mo inhalation studies (increased pulmonary collagen deposition) in rats supports the notion that chronic changes depend on acute threshold mechanisms.