Pulmonary structural and extracellular matrix alterations in Fischer 344 rats following subchronic phosgene exposure

Phosgene-Induced acute lung injury: Approaches for mechanism-based treatment strategies

Phosgene (COCl2) gas is a chemical intermediate of high-volume production with numerous industrial applications worldwide. Due to its high toxicity, accidental exposure to phosgene leads to various chemical injuries, primarily resulting in chemical-induced lung injury due to inhalation. Initially, the illness is mild and presents as coughing, chest tightness, and wheezing; however, within a few hours, symptoms progress to chronic respiratory depression, refractory pulmonary edema, dyspnea, and hypoxemia, which may contribute to acute respiratory distress syndrome or even death in severe cases. Despite rapid advances in medicine, effective treatments for phosgene-inhaled poisoning are lacking. Elucidating the pathophysiology and pathogenesis of acute inhalation toxicity caused by phosgene is necessary for the development of appropriate therapeutics. In this review, we discuss extant literature on relevant mechanisms and therapeutic strategies to highlight novel ideas for the treatment of phosgene-induced acute lung injury.

Derivation of thresholds for inhaled chemically reactive irritants: Searching for substance-specific common denominators for read-across prediction

Emergency response planning guideline values are used to protect the public when there has been a short-term chemical release. These values serve the purpose of identifying areas where a hazard exists if the concentration of hazardous chemicals is exceeded for the specified exposure duration. This paper focuses on carbonyl chlorides, a class of highly irritant/corrosive chemical intermediates characterized by the reactive moiety R-COCl. Despite their unifying property of reacting with nucleophilic biopolymers/peptides lining the airways of the respiratory tract, their adverse outcome pathway (AOP), in addition to surface area dose, appears to be dominated by their site(s) of major deposition (liquid) or retention (gas) within the respiratory tract. Thus, the physicochemical properties "phase" and "lipophilicity" become more decisive for the AOP than the chemical structure. This complicates the grouping of portal-of-entry irritant chemicals for the read-across prediction of chemicals, especially those with semivolatile properties. Phosgene (COCl₂) served as a template to predict emergency response planning levels 2 (non-incapacitating, reversible injury) and 3 (nonlethal) for related chemicals such as SOCl₂, formates, and acid chlorides. A rationale and guide to the systematic characterization of uncertainties associated with the lung region, water solubility of the vapor phase, and chemical specificity is given. The approach described in this paper highlights the regional differences and outcomes that are phenotypically described as irritation of the respiratory tract. Especially for such a data-lean group of chemicals, reliable read-across predictions could reduce the uncertainty associated with the derivation of values used for emergency-related risk assessment and management. Likewise, the approach suggested could improve the grouping and categorization of such chemicals, providing a means to reduce animal testing with potentially corrosive chemicals. Overall, the course taken for read-across predictions provided valid estimates as long as emphasis was directed to the physicochemical properties determining the most critical regional injury within the respiratory tract.

Human risk assessment of inhaled irritants: Role of sensory stimulations from spatially separated nociceptors

Contemporary approaches to human health risk assessment for respiratory tract irritants are variable and controversial. This manuscript provides an in-depth analysis and assessment of the applicability of the classical respiratory depression 50 % (RD₅₀) assay with focus on the Log-linear extrapolation of the non-sensory irritant threshold (RD₀ or RD₁₀) relative to the contemporary Point of Departure (POD) U.S.-EPA benchmark approach. Three prototypic volatile chemically reactive irritants are used to exemplify the pros and cons of this alternative approach. These irritants differ in physicochemical properties affecting water-solubility and lipophilicity. Depending on these variables, a vapor may preferentially be retained in the extrathoracic region (ET), the tracheobronchial region (TB), and the pulmonary region (PU); although a smooth transition between these regions occurs at increasingly high concentrations. Each region has its specific nociceptors sensing irritants and regional-specific response to injury. The alternative approach using rats identified the chemical-specific critical region of respiratory tract injury. Statistically derived PODs on ET-TB related sensory irritation provide important information for ET-TB irritants but not for PU irritants. The POD of ET-TB irritants from acute and repeated studies decreased substantially. In summary, statistically derived PODs improve the risk assessment of respiratory tract irritants; however, those from repeated exposures should be given preference to those from acute exposures.

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.

Stretching the stress boundary: Linking air pollution health effects to a neurohormonal stress response

Inhaled pollutants produce effects in virtually all organ systems in our body and have been linked to chronic diseases including hypertension, atherosclerosis, Alzheimer's and diabetes. A neurohormonal stress response (referred to here as a systemic response produced by activation of the sympathetic nervous system and hypothalamus-pituitary-adrenal (HPA)-axis) has been implicated in a variety of psychological and physical stresses, which involves immune and metabolic homeostatic mechanisms affecting all organs in the body. In this review, we provide new evidence for the involvement of this well-characterized neurohormonal stress response in mediating systemic and pulmonary effects of a prototypic air pollutant - ozone. A plethora of systemic metabolic and immune effects are induced in animals exposed to inhaled pollutants, which could result from increased circulating stress hormones. The release of adrenal-derived stress hormones in response to ozone exposure not only mediates systemic immune and metabolic responses, but by doing so, also modulates pulmonary injury and inflammation. With recurring pollutant exposures, these effects can contribute to multi-organ chronic conditions associated with air pollution. This review will cover, 1) the potential mechanisms by which air pollutants can initiate the relay of signals from respiratory tract to brain through trigeminal and vagus nerves, and activate stress responsive regions including hypothalamus; and 2) the contribution of sympathetic and HPA-axis activation in mediating systemic homeostatic metabolic and immune effects of ozone in various organs. The potential contribution of chronic environmental stress in cardiovascular, neurological, reproductive and metabolic diseases, and the knowledge gaps are also discussed. This article is part of a Special Issue entitled Air Pollution, edited by Wenjun Ding, Andrew J. Ghio and Weidong Wu.

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.

Evaluation of effects of long term exposure on lethal toxicity with mammals

The relationship between exposure time (LT50) and lethal exposure concentration (LC50) has been evaluated over relatively long exposure times using a novel parameter, Normal Life Expectancy (NLT), as a long term toxicity point. The model equation, ln(LT50) = aLC50(ν) + b, where a, b and ν are constants, was evaluated by plotting lnLT50 against LC50 using available toxicity data based on inhalation exposure from 7 species of mammals. With each specific toxicant a single consistent relationship was observed for all mammals with ν always <1. Use of NLT as a long term toxicity point provided a valuable limiting point for long exposure times. With organic compounds, the Kow can be used to calculate the model constants a and v where these are unknown. The model can be used to characterise toxicity to specific mammals and then be extended to estimate toxicity at any exposure time with other mammals.

Acute nose-only inhalation exposure of rats to di- and triphosgene relative to phosgene

Groups of young adult Wistar rats were acutely exposed to trichloromethyl chloroformate (diphosgene) and bis(trichloromethyl) carbonate (triphosgene) vapor atmospheres using a directed-flow nose-only mode of exposure. The exposure duration used was 240 min. The median lethal concentration (LC50) of diphosgene and triphosgene was 13.9 and 41.5 mg/m3, respectively. Based on the molar exposure concentrations, the LC50s of phosgene (previously published), diphosgene, and triphosgene were 0.07, 0.07, and 0.14 mmol/m3, respectively. Although the principal toxic mode of action of the volatile diphosgene was similar to phosgene gas, the vapor phase of triphosgene appeared to be different to that of phosgene and diphosgene based on a more persistent occurrence of signs of respiratory distress and a biphasic onset of mortality. While all substances caused mortality within 1 day postexposure, triphosgene induced a second phase of mortality 11?14 days postexposure. The vapor saturation concentration of triphosgene at ambient temperature is ?100 times its LC50. In summary, triphosgene-induced lung injury patterns are different from that of phosgene and diphosgene. More research is needed to close the substantial data gaps of triphosgene.

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.

Developing Health-Based Pre-Planning Clearance Goals for Airport Remediation Following a Chemical Terrorist Attack: Decision Criteria for Multipathway Exposure Routes

In the event of a chemical terrorist attack on a transportation hub, post-event remediation and restoration activities necessary to attain unrestricted facility re-use and re-entry could require hours to multiple days. While timeframes are dependent on numerous variables, a primary controlling factor is the level of pre-planning and decision-making completed prior to chemical release. What follows is the second of a two-part analysis identifying key considerations, critical information and decision criteria to facilitate post-attack and post-decontamination consequence management activities. Decision criteria analysis presented here provides first-time, open-literature documentation of multi-pathway, health-based remediation exposure guidelines for selected toxic industrial compounds, chemical warfare agents, and agent degradation products for pre-planning application in anticipation of a chemical terrorist attack. Guideline values are provided for inhalation and direct ocular vapor exposure routes as well as percutaneous vapor, surface contact, and ingestion. Target populations include various employees as well as transit passengers. This work has been performed as a national case study conducted in partnership with the Los Angeles International Airport and The Bradley International Terminal. All recommended guidelines have been selected for consistency with airport scenario release parameters of a one-time, short-duration, finite airborne release from a single source followed by compound-specific decontamination.

N-acetylcysteine attenuates phosgene-induced acute lung injury via up-regulation of Nrf2 expression

Previous studies indicated that oxidative stress was involved in phosgene-induced acute lung injury (ALI) and many antioxidants had been used to prevent ALI. N-acetylcysteine (NAC) had been used to protect ALI induced by various types of oxidative stress. Considering the limited information of NAC on phosgene-induced ALI, the purpose of this study was to elucidate the molecular mechanisms of phosgene-induced ALI and the protective effects of NAC. This study discovered that intraperitoneal administration of NAC significantly alleviated phosgene-induced pulmonary edema, as confirmed by decreased lung wet to dry weight ratio and oxidative stress markers. The content of l-gamma-glutamyl-l-cysteinyl-glycine (glutathione; GSH) and the ratio of the reduced and disulfide forms (GSH/GSSG), significant indicators of the antioxidative ability, were apparently inhibited by phosgene exposure. However, both indicators could be reversed by NAC administration, indicating that dysregulation of redox status of glutathione might be the cause of phosgene-induced ALI. The nuclear factor (NF)-E2-related factor 2 (Nrf2), which has been proven to up-regulate the expression of glutathione reductase (GR), was obviously decreased by phosgene exposure. However, NAC administration elevated Nrf2 expression significantly. In conclusion, these data provided the first evidences showing that it was the transcriptional factor Nrf2 that connected phosgene-induced ALI with GSH metabolism. NAC protected against oxidative stress through acting on this newly disclosed Nrf2/GR/GSH pathway, by which NAC elevated the biosynthesis of protective GSH to repair and reconstitute the defense system destroyed by phosgene.

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.

Simple, rapid, and highly sensitive detection of diphosgene and triphosgene by spectrophotometric methods

Methods for the detection and estimation of diphosgene and triphosgene are described. These compounds are widely used phosgene precursors which produce an intensely colored purple pentamethine oxonol dye when reacted with 1,3-dimethylbarbituric acid (DBA) and pyridine (or a pyridine derivative). Two quantitative methods are described, based on either UV absorbance or fluorescence of the oxonol dye. Detection limits are approximately 4 micromol/L by UV and <0.4 micromol/L by fluorescence. The third method is a test strip for the simple and rapid detection and semi-quantitative estimation of diphosgene and triphosgene, using a filter paper embedded with dimethylbarbituric acid and poly(4-vinylpyridine). Addition of a test solution to the paper causes a color change from white to light blue at low concentrations and to pink at higher concentrations of triphosgene. The test strip is useful for quick on-site detection of triphosgene and diphosgene in reaction mixtures. The test is easy to perform and provides clear signal readouts indicative of the presence of phosgene precursors. The utility of this method was demonstrated by the qualitative determination of residual triphosgene during the production of poly(bisphenol-A carbonate).

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.

The application of non-default uncertainty factors in the U.S. EPA's Integrated Risk Information System (IRIS). Part I: UF(L), UF(S), and "other uncertainty factors"

The United States Environmental Protection Agency's Integrated Risk Information System (IRIS) includes hazard identification and dose-response assessment values developed by Agency scientists. Uncertainty factors (UFs) are used in the development of IRIS values to address the lack of information in five main areas. The standard UFs account for interspecies uncertainty (UF(A)) and intraspecies variability (UF(H)). The UF(A) addresses uncertainty related to the extrapolation of data from animals to humans, whereas the UF(H) addresses variability amongst individuals (i.e., intrahuman). Additional UFs have been employed to account for database incompleteness, extrapolations from a lowest-observed-adverse-effect level in the absence of a no-observed-adverse-effect level (UF(L)), and subchronic-to-chronic extrapolation (UF(S)). A sixth UF designated as "other uncertainty factors" (UF(O)) has also been applied in place of the UF(L) to account for uncertainty with the adversity of points of departure obtained using benchmark dose modeling. This review will discuss how UF(L), UF(S), and UF(O) have been applied in IRIS assessments, along with the rationale used to describe the choice of UF values that deviate from the standard default of 10.

Validation of Haber's Rule (dose×time=constant) in rats and mice for monochloroacetic acid and 2,3,7,8-tetrachlorodibenzo-p-dioxin under conditions of kinetic steady state

Haber's Rule and associated time to coma after monochloroacetic acid (MCA) exposure in male Sprague-Dawley (SD) rats and time to death after 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure in female Sprague-Dawley rats and male A/J mice were investigated at isoeffective or nearly isoeffective doses. Animals exposed to MCA received either single bolus intravenous (iv) doses or a loading dose rate via the iv route followed by a maintenance dose rate through subcutaneously implanted osmotic mini pumps. For TCDD, rats received a loading dose rate via bolus oral gavage followed by maintenance dose rates through iv injection every fourth day until death. Mice received both loading and maintenance (once a week) dose rates via oral gavage. Different dosing regimens were employed to demonstrate that the key to Haber's Rule lies not in the route of administration but in conducting experiments under conditions of kinetic steady state. Single doses of MCA produced inconsistent time responses but a reasonably constant c x t product (7657+/-391 mg/kg x min) which was not anticipated although it should have been expected because MCA's elimination half-life (2 h) is twice as long as its time to coma ( approximately 1h). Generation of kinetic steady state by infusion of MCA after iv injection of a loading dose rate resulted in a consistently decreasing time response with increasing dose which diminished the variability in the c x t (dose x time)=k relationship (8032+/-136 mg/kg x min). Both acute and chronic toxicity of TCDD under conditions of kinetic steady state yielded consistent time responses with inverse proportionality between dose and time leading to robust c x t=k products in both rats (1060+/-82 microg/kg x day) and mice (80+/-2 mg/kg x day).

Acute changes in lung histopathology and bronchoalveolar lavage parameters in mice exposed to the choking agent gas phosgene

Phosgene (CG) is a highly irritant gas widely used industrially as a chemical intermediate for the production of dyes, pesticides, and plastics, and can cause life-threatening pulmonary edema within 24 hours of exposure. This study was designed to investigate acute changes in lung tissue histopathology and selected bronchoalveolar lavage fluid (BALF) factors over time to determine early diagnostic indicators of exposure. Three groups of 40 male mice each were exposed to 32 mg/m3 (8 ppm) CG for 20 minutes, and 3 groups of 40 control male mice were exposed to filtered room air for 20 minutes, both exposures were followed by room air washout for 5 minutes. At 1, 4.8, 12, 24, 48, and 72 hours after exposure each group of mice was euthanized and processed for histopathology, bronchoalveolar lavage or gravimetric measurements, respectively. Over time, the histopathological lesions were characterized by acute changes consisting of alveolar and interstitial edema, fibrin and hemorrhage, followed by significant alveolar and interstitial flooding with inflammatory cell infiltrates and scattered bronchiolar and terminal airway epithelial degeneration and necrosis. From 48 to 72 hours, there was partial resolution of the edema and degenerative changes, followed by epithelial and fibroblastic regeneration centered on the terminal bronchiolar areas. Bronchoalveolar lavage was processed for cell differential counts, LDH, and protein determination. Comparative analysis revealed significant increases in both postexposure lung wet/dry weight ratios, and early elevations of BALF LDH and protein, and later elevations in leukocytes. This article describes the use of histopathology to chronicle the temporal pulmonary changes subsequent to whole body exposure to phosgene, and correlate these changes with BALF ingredients and postexposure lung wet weights in an effort to characterize toxic gas-induced acute lung injury and identify early markers of phosgene exposure.

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

The present study compares acute and subchronic episodic exposures to phosgene to test the applicability of the 'concentrationxtime' (CxT) product as a measure of exposure dose, and to relate acute toxicity and adaptive responses to chronic toxicity. Rats (male Fischer 344) were exposed (six hours/day) to air or 0.1, 0.2, 0.5 and 1.0 ppm of phosgene one time or on a repeated regimen for up to 12 weeks as follows: 0.1 ppm (five days/week), 0.2 ppm (five days/week), 0.5 ppm (two days/week), or 1.0 ppm (one day/week) (note that the CxT for the three highest exposures was the same). Animals were sacrificed at 4, 8, and 12 weeks during the exposure and after four weeks recovery. Bronchoalveolar lavage (BAL) was performed 18 hours after the last exposure for each time period and the BAL supernatant assayed for protein. Elevated BAL fluid protein was defined as 'acute injury', diminished response after repeated exposure was defined as 'adaptation', and increased lung hydroxyproline or trichrome staining for collagen was defined as 'chronic injury'. Results indicated that exposures that cause maximal chronic injury involve high exposure concentrations and longer times between exposures, not high CxT products. A conceptual model is presented that explains the lack of CxT correlation by the fact that adaptation reduces an 'injury-time integral' as phosgene exposure is lengthened from acute to subchronic. At high exposure concentrations, the adaptive response appears to be overwhelmed, causing a continued injury-time integral, which appears to be related to appearance of chronic injury. The adaptive response is predicted to disappear if the time between exposures is lengthened, leading to a continued high injury-time integral and chronic injury. It has generally been assumed that long, continuous exposures of rodents is a conservative approach for detecting possible chronic effects. The present study suggests that such an approach my not be conservative, but might actually mask effects that could occur under intermittent exposure conditions.

Phosgene exposure: mechanisms of injury and treatment strategies

Phosgene (carbonyl chloride, CAS 75-44-5) is a highly reactive gas of historical interest and current industrial importance. Phosgene has also proved to be a useful model for the study of those biochemical mechanisms that lead to permeability-type pulmonary edema (adult respiratory distress syndrome). In turn, the study of phosgene-induced adult respiratory distress syndrome has provided insights leading to revised treatment strategies for exposure victims. We summarized recent findings on the mechanisms of phosgene-induced pulmonary edema and their implications for victim management. In light of that research, we also provide a comprehensive approach to the management and treatment of phosgene exposure victims.