Concentration-Time Product (CT) as an Expression of Dose in Sublethal Exposures to Phosgene

Clinical course of acute chemical lung injury caused by 3-chloropentafluoropene

Perfluoroallyl chloride (PFAC), a fluorine-containing compound, has very severe toxicity, but this toxicity is not well characterised. We report a fatal case of acute chemical lung injury caused by the inhalation of PFAC. A 39-year-old man, working at a chemical factory, inhaled PFAC gas and died 16 days later of acute lung injury with severe pneumothorax. We present his clinical course together with thoracic CT findings, autopsy and analysis of PFAC in blood and urine samples with gas chromatograph-mass spectrometry. Previously, a fatal case of PFAC was reported in 1981 but PFAC was not identified in any of the patient's samples. In our patient, we identified PFAC in both blood and urine samples. Our toxicological analysis may be used as a reference to detect PFAC toxicity in the future. Our study should be helpful for diagnosing lung injury induced by a highly toxic gas, such as PFAC.

Toxic effects following phosgene exposure of human epithelial lung cells in vitro using a CULTEX® system

The aim of the present study was to investigate toxic effects following phosgene exposure of human epithelial lung cells (A549) in vitro using a CULTEX® system. In particular, toxic effects regarding early biomarkers emerging during the latency period following exposure might be of great value for medical treatment. Cells cultured on semi-permeable membranes were directly exposed at the liquid-air interface to different concentrations of phosgene, or dry medical air. Cell membrane integrity (leakage of LDH), metabolic activity (reduction of Alamar Blue), oxidative damage (GSH, and HO-1, in cell lysates), and release of IL-8, were studied. For most of the above-mentioned biological end-point markers, significant changes could be assessed following a 20 min exposure to 1.0 ppm and 2.0 ppm phosgene. Moreover, except for IL-8, all biological marker profiles showed to be in line with results obtained by others in animal studies. The C×t value of 40 ppm min appeared to be constant. The overall results suggest that at 4 h post-exposure a maximal level of toxicity was achieved. Our results demonstrate the suitability of a CULTEX® system to detect toxic effects induced by phosgene on human epithelial lung cells, which may contribute to the discovery of early biomarkers for new medical countermeasures.

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.

Overview of the Standing Operating Procedure (SOP) for the development of Provisional Advisory Levels (PALs)

Provisional Advisory Levels (PALs) are concentrations in air and drinking water for priority toxic chemicals. This article summarizes the Standing Operating Procedure (SOP) currently in place for the data-driven development of chemical-specific PALs. To provide consistency and transparency, and to avoid faults of arbitrariness, the SOP was developed for guidance in deriving PAL values. Three levels (PAL 1, PAL 2, and PAL 3), distinguished by severity of toxic effects, are developed for 24-hour, 30-day, 90-day, and 2-year durations of potential drinking water and inhalation exposures for the general public. The SOP for PAL development focuses on (1) data acquisition and analysis, (2) identification of a chemical-specific critical effect, (3) selection of a quantitative point-of-departure (POD), (4) uncertainty analysis and adjustments, (5) exposure duration adjustment and extrapolation, (6) identification of special concerns and issues, and (7) verification, documentation, and dissemination of PALs. To avoid uncompromising rigidity in deriving PAL values and to allow for incorporation of new or refined methodologies, the overall procedure is fluid and subject to modification. The purpose of this publication is to provide a summary of this SOP.

Therapeutic Treatments of Phosgene-Induced Lung Injury

A series of studies was performed to address treatment against the former chemical warfare edemagenic gas phosgene. Both in situ and in vivo models were used to assess the efficacy of postexposure treatment of phosgene-induced lung injury using clinically existing drugs. The degree of efficacy was judged by examining treatment effects on pulmonary edema formation (PEF) as measured by wet/dry weight (WW/DW) ratios, real-time (in situ) lung weight gain (LWG), survival rates (SR), odds ratios, and glutathione (GSH) redox states. Drugs included N-acetylcysteine (NAC), ibuprofen (IBU), aminophylline (AMIN), and isoproterenol (ISO). Using the in situ isolated perfused rabbit lung model (IPRLM), intratracheal (IT) NAC (40 mg/kg bolus) delivered 45-60 min after phosgene exposure (650 mg/m(3)) for10 min lowered pulmonary artery pressure, LWG, leukotrienes (LT) C(4)/D(4)/E(4), lipid peroxidation, and oxidized GSH. We concluded that NAC protected against phosgene-induced lung injury by acting as an antioxidant by maintaining protective levels of GSH, reducing both lipid peroxidation and production of arachidonic acid metabolites. Also in IPRLM, administration of AMIN (30 mg/kg) 80-90 min after phosgene exposure significantly reduced lipid peroxidation and perfusate LTC(4)/D(4)/E(4), reduced LWG, and prevented phosgene-induced decreases in lung tissue cAMP. These data suggest that protective mechanisms observed with AMIN involve decreased LTC(4)/D(4)/E(4) mediated pulmonary capillary permeability and attenuated lipid peroxidation. Direct antipermeability effects of AMIN-induced upregulation of cAMP on cellular contraction may also be important in protection against phosgene-induced lung injury. Posttreatment with ISO in the IPRLM by either combined intravascular (iv; infused into pulmonary artery at 24 microg/min infused) + IT (24 microg bolus) or IT route alone 50-60 min after phosgene exposure significantly lowered pulmonary artery pressure, tracheal pressure, and LWG. ISO treatment significantly enhanced GSH products or maintained protective levels when compared with results from phosgene-exposed only rabbits. These data suggest that protective mechanisms for ISO involve reduction in vascular pressure, decreased LTC(4)/D(4)/E(4)-mediated pulmonary capillary permeability, and favorably maintained lung tissue GSH redox states. For in vivo male mouse (CD-1, 25-30 g) studies IBU was administered ip within 20 min after a lethal dose of phosgene (32 mg/m(3) for 20 min) at 0 (saline), 3, 9, or 15 mg/mouse. Five hours later, a second IBU injection was given but at half the original doses (0, 1.5, 4.5, and 7.5 mg/mouse); therefore, these treatment groups are now referred to as the 0/0, 3/1.5, 9/4.5, and 15/7.5 mg IBU/mouse groups. SRs and odds ratios were calculated for each dose at 12 and 24 h. The 12-h survival was 63% for 9/4.5 mg IBU and 82% for the 15/7.5 mg IBU groups, compared with 25% for saline-treated phosgene-exposed mice. At 24 h, those survival rates were reduced to 19%, 19%, and 6%, respectively. In the 15/7.5 mg IBU group, lung WW/DW ratios were significantly lower than in saline-treated mice at 12 h. Lipid peroxidation was lower only for the 9/4.5 mg IBU dose; however, nonprotein sulfhydryls (a measure of GSH) were greater across all IBU doses. The odds ratio was 5 for the 9/4.5 IBU group at 12 h and 13 for the 15/7.5 mg IBU group, compared with 3.5 for both groups at 24 h. IBU posttreatment increased the survival of mice at 12 h by reducing PEF, lipid peroxidation, and GSH depletion. In conclusion, effective treatment of phosgene-induced lung injury involves early postexposure intervention that could reduce free radical species responsible for lipid peroxidation, correct the imbalance in the GSH redox state, and prevent the release of biological mediators such as leukotrienes, which are accountable for increased permeability.

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.

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.

Evaluation of acute inhalation toxicity for chemicals with limited toxicity information

A large reference database consisting of acute inhalation no-observed-adverse-effect levels (NOAELs) and acute lethality data for 97 chemicals was compiled to investigate two methods to derive health-protective concentrations for chemicals with limited toxicity data for the evaluation of one-hour intermittent inhalation exposure. One method is to determine threshold of concern (TOC) concentrations for acute toxicity potency categories and the other is to determine NOAEL-to-LC(50) ratios. In the TOC approach, 97 chemicals were classified based on the Globally Harmonized System of Classification and Labeling of Chemicals proposed by the United Nations into different acute toxicity categories (from most toxic to least toxic): Category 1, Category 2, Category 3, Category 4, and Category 5. The tenth percentile of the cumulative percentage distribution of NOAELs in each category was determined and divided by an uncertainty factor of 100 to derive the following health-protective TOC concentrations: 4microg/m(3) for chemicals classified in Category 1; 20microg/m(3) for Category 2; 125microg/m(3) for both Categories 3 and 4; and 1000microg/m(3) for Category 5. For the NOAEL-to-LC(50) ratio approach, 55 chemicals with NOAEL exposure durations < or = 24 hour were used to calculate NOAEL-to-LC(50) ratios. The tenth percentile of the cumulative percentage distribution of the ratios was calculated and divided by an uncertainty factor of 100 to produce a composite factor equal to 8.3x10(-5). For a chemical with limited toxicity information, this composite factor is multiplied by a 4-hour LC(50) value or other appropriate acute lethality data. Both approaches can be used to produce an estimate of a conservative threshold air concentration below which no appreciable risk to the general population would be expected to occur after a one-hour intermittent exposure.

Does Haber's law apply to human sensory irritation?

Irritation of the eyes, nose, and throat by airborne chemicals--also referred to as "sensory irritation"--is an important endpoint in both occupational and environmental toxicology. Modeling of human sensory irritation relies on knowledge of the physical chemistry of the compound(s) involved, as well as the exposure parameters (concentration and duration). A reciprocal relationship between these two exposure variables is postulated under Haber's law, implying that protracted, low-level exposures may be toxicologically equivalent to brief, high-level exposures. Although time is recognized as having an influence on sensory irritation, the quantitative predictions of Haber's Law have been addressed for only a handful of compounds in human experimental studies. We have conducted a systematic literature review that includes a semiquantitative comparison of psychophysical data extracted from controlled human exposure studies versus. the predictions of Haber's law. Studies containing relevant data involved exposures to ammonia (2), chlorine (2), formaldehyde (1), inorganic dusts such as calcium oxide (1), and the volatile organic compound 1-octene (1). With the exception of dust exposure, varying exposure concentration has a proportionally greater effect on sensory irritation than does changing exposure duration. For selected time windows, a more generalized power law model (c(n) x t = k) rather than Haber's law per se (c x t = k) yields reasonably robust predictions. Complicating this picture, however, is the frequent observation of intensity-time "plateauing," with time effects disappearing, or even reversing, after a relatively short period, depending on the test compound. The implications of these complex temporal dynamics for risk assessment and standard setting have been incompletely explored to date.

Acute inhalative exposure assessment: Derivation of guideline levels with special regard to sensitive subpopulations and time scaling

Risk assessment for acute airborne exposure to volatile organic compounds (VOCs), including exposure to chemical warfare agents, requires consideration of local and systemic effects at high concentrations. The operating procedure developed by the US Acute Exposure Guideline Level (AEGL) committee has gained special attention, in part because of the international collaboration in the project. The procedure defines three levels (AEGL-1: discomfort; AEGL-2: irreversible or other serious, long-lasting adverse effects; AEGL-3: life-threatening effects or death) for different exposure times (10 and 30 min, and 1, 4 and 8 h). In this article, the methodology for deriving AEGL values is reported. Extending the areas covered by the existing AEGL methodology, sensitive subpopulations are dealt with in more detail. Sensitive persons are expected to suffer from stronger effects when exposed to a given external concentration. Using a kinetic model with the sample substance dichloromethane (DCM), the higher internal exposure of children is quantified and compared to a healthy, young adult. The difference is shown to depend on age, on dose, and on duration of exposure. Furthermore, several ways are presented to derive AEGL values for exposure times which differ from the exposure duration in animal studies ('time scaling'). In comparison to the conventional procedure, the alternative approaches are based on mechanistic models of the toxicodynamic effect. Use of these models results in AEGL values which are biologically justified.

The development of acute exposure guideline levels for hazardous substances

The National Advisory Committee for Acute Exposure Guideline Levels for Hazardous Substances (NAC/AEGL) was created to develop guideline levels for short-term exposures to airborne concentrations for approximately 400-500 high priority, acutely hazardous substances. The program should be completed within the next 10 years. These Acute Exposure Guideline Levels (AEGLs) are being applied to a wide range of planning, response, and prevention applications both within the United States and abroad. The NAC/AEGL Committee seeks to develop the most scientifically credible, acute (short-term) exposure guideline levels possible within the constraints of data availability, resources and time. The program begins with comprehensive data gathering, data evaluation and data summarization. The resulting Technical Support Documents (TSD) are first reviewed by a small review committee; (chemical manager, two chemical reviewers and the author), then by the full AEGL committee. After that review, a summary is published in the Federal Register for Public comment. When these comments have been addressed, the TSDs are sent to the National Research Council's (NRC) Subcommittee on AEGLs for a peer review. Following acceptance by the NRC, they are published by the Academy. The NAC/AEGL Committee currently comprises representatives of federal, state, and local agencies and representatives from France, Germany, and the Netherlands, private industry, medicine, academia and other organizations in the private sector that will derive programmatic or operational benefits from the existence of the AEGL values. AEGL values are determined for three different health effect end-points. These values are intended for the general public where they are applicable to emergency (accidental) situations. Threshold exposure values are developed for five exposure periods (10 and 30 min, 1 h, 4 h, 8 h). Each threshold value is distinguished by varying degrees of severity of toxic effects, as initially conceived by the American Industrial Hygiene Association's Emergency Response Planning Committee, subsequently defined in the NAS' National Research Council publication of the Guideline for Developing Community Emergency Exposure Levels for Hazardous Substances and further categorized in the Standing Operating Procedures of the NAC/AEGL Committee. To date, the committee has reviewed almost 100 chemicals.

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.

Postexposure treatment with aminophylline protects against phosgene-induced acute lung injury

Pretreatment with aminophylline has been shown to protect against various types of acute lung injury. Mechanisms responsible for protection are multifactorial but are thought to involve upregulation of cAMP. While previous studies focused on pretreatment, the present investigation examined post-treatment in rabbits following exposure to a lethal dose of the oxidant gas phosgene. Rabbits, 2-3 kg, were exposed to a cumulative dose of phosgene to attain a c x t exposure effect of 1500 ppm.min. Lungs were isolated in situ and perfused for 90-100 min after exposure with Krebs-Henseleit buffer at 40 mL/min. Pulmonary artery pressure (Ppa), tracheal pressure (Pt), and lung weight gain (lwg) were measured continuously. Leukotrienes C4/D4/E4 were measured in the perfusate every 20 min during perfusion. At the immediate conclusion of the experiment, lung tissue was frozen in liquid N2 and analyzed for reduced GSH, GSSG, cAMP, and lipid peroxidation (TBARS). Post-treatment with aminophylline 80-90 min after exposure significantly lowered Ppa, Pt, and lwg. Aminophylline significantly reduced TBARS and perfusate LTC4/D4/E4, and prevented phosgene-induced decreases in lung tissue cAMP. These data suggest that protective mechanisms observed with aminophylline involve decreased LTC4/D4/E4-mediated pulmonary capillary permeability and attenuated lipid peroxidation. Direct antipermeability effects of cAMP on cellular contraction may also be important in protection against phosgene-induced lung injury.

Ibuprofen prevents oxidant lung injury and in vitro lipid peroxidation by chelating iron

Because ibuprofen protects from septic lung injury, we studied the effect of ibuprofen in oxidant lung injury from phosgene. Lungs from rabbits exposed to 2,000 ppm-min phosgene were perfused with Krebs-Henseleit buffer at 50 ml/min for 60 min. Phosgene caused no increase in lung generation of cyclooxygenase metabolites and no elevation in pulmonary arterial pressure, but markedly increased transvascular fluid flux (delta W = 31 +/- 5 phosgene vs. 8 +/- 1 g unexposed, P less than 0.001), permeability to albumin (125I-HSA) lung leak index 0.274 +/- 0.035 phosgene vs. 0.019 +/- 0.001 unexposed, P less than 0.01; 125I-HSA lavage leak index 0.352 +/- 0.073 phosgene vs. 0.008 +/- 0.001 unexposed, P less than 0.01), and lung malondialdehyde (50 +/- 7 phosgene vs. 24 +/- 0.7 mumol/g dry lung unexposed, P less than 0.01). Ibuprofen protected lungs from phosgene (delta W = 10 +/- 2 g; lung leak index 0.095 +/- 0.013; lavage leak index 0.052 +/- 0.013; and malondialdehyde 16 +/- 3 mumol/g dry lung, P less than 0.01). Because iron-treated ibuprofen failed to protect, we studied the effect of ibuprofen in several iron-mediated reactions in vitro. Ibuprofen attenuated generation of .OH by a Fenton reaction and peroxidation of arachidonic acid by FeCl3 and ascorbate. Ibuprofen also formed iron chelates that lack the free coordination site required for iron to be reactive. Thus, ibuprofen may prevent iron-mediated generation of oxidants or iron-mediated lipid peroxidation after phosgene exposure. This suggests a new mechanism for ibuprofen's action.

Pathogenesis of phosgene poisoning

Phosgene inhalation in concentrations greater than 1 ppm may produce a transient bioprotective vagus reflex with rapid shallow breathing in some individuals. Phosgene concentrations greater than 3 ppm are moderately irritating to eyes and upper airways. Toxic phosgene doses (greater than or equal to 30 ppm X min) inhaled into the terminal respiratory passages render the blood-air-barrier more permeable to blood plasma, which gradually collects in the lung. Some time passes, however, until the collection of fluid provokes signs and symptoms. This period in which the patient experiences relative well-being is known as the clinical latent phase. The clinical symptoms which follow and the pathological changes underlying them are discussed in detail; dose-effect relationships are demonstrated. The regression phase after poisoning has been overcome is briefly sketched.