Study of the solid state of carbamazepine after processing with gas anti-solvent technique

The purpose of this study was to investigate the influence of supercritical CO2 processing on the physico-chemical properties of carbamazepine, a poorly soluble drug. The gas anti-solvent (GAS) technique was used to precipitate the drug from three different solvents (acetone, ethylacetate and dichloromethane) to study how they would affect the final product. The samples were analysed before and after treatment by scanning electron microscopy analysis and laser granulometry for possible changes in the habitus of the crystals. In addition, the solid state of the samples was studied by means of X-ray powder diffraction, differential scanning calorimetry, diffuse reflectance Fourier-transform infrared spectroscopy and hot stage microscopy. Finally, the in vitro dissolution tests were carried out. The solid state analysis of both samples untreated and treated with CO2, showed that the applied method caused a transition from the starting form III to the form I as well as determined a dramatic change of crystal morphology, resulting in needle-shaped crystals, regardless of the chosen solvent. In order to identify which process was responsible for the above results, carbamazepine was further precipitated from the same three solvents by traditional evaporation method (RV-samples). On the basis of this cross-testing, the solvents were found to be responsible for the reorganisation into a different polymorphic form, and the potential of the GAS process to produce micronic needle shaped particles, with an enhanced dissolution rate compared to the RV-carbamazepine, was ascertained.

Physiological condition and water source use of Sonoran Desert riparian trees at the Bill Williams River, Arizona, USA

We investigated the environmental water sources used in mid-summer by three Sonoran Desert phreatophytic riparian tree species, Salix gooddingii, Populus fremontii, and the exotic Tamarix spp., at sites that differed in water table depth. Salix gooddingii was most sensitive to water table decline, as evidenced by lower predawn water potentials. Although P. fremontii was less sensitive to water table decline than S. gooddingii, its leaf gas exchange was the most responsive to atmospheric water stress imposed by high leaf-to-air vapor pressure deficit. Tamarix spp. was least sensitive to water table decline and showed no reduction of predawn water potential over the measured range of depth to groundwater. Comparison between D/H of xylem and sampled environmental water sources suggest that S. gooddingii and P. fremontii used groundwater at most sites with no change in water source as depth to groundwater varied. In contrast, xylem D/H of Tamarix spp. was depleted in deuterium compared to groundwater at most sites, suggesting use of water from an unsampled source, or discrimination against deuterium during water uptake. This study highlights the difficulty in sampling all water sources in large-scale studies of riparian ecosystems with complex subsurface hydrogeology.

Physiologically-based kinetic modeling of vapours toxic to the respiratory tract

The respiratory tract is frequently identified as a site of toxicity for inhaled xenobiotic chemicals. Usually, these observations come from controlled animal studies. For these studies to be of quantitative value to human health risk assessment, species-specific factors governing dosimetry of inhaled substances must be taken into account. Toxicokinetics of vapours in the respiratory tract are defined by absorption, distribution, metabolism, and excretion, as they are in other tissues; however, these concepts take on new dimensions when considering respiratory tract toxicants, especially those that elicit portal of entry effects by directly interacting with the tissue lining the respiratory tract. Species-specific factors related to anatomy, physiology and biochemistry govern inter-species extrapolation of toxicokinetics. This article discusses critical factors of respiratory tract kinetics that should be considered when developing physiological-based toxicokinetic (PBTK) models for inhaled vapours. Important considerations such as impact of regional airflow-delivery, water solubility, reactivity, and rates of local biotransformation on respiratory tract tissue dosimetry are highlighted. These factors can be accounted for only to a limited extent when using default approaches to extrapolate dosimetry of inhaled substances across species. On the other hand, PBTK modeling has the flexibility to accommodate many of the critical determinants of respiratory tract toxicity. PBTK models can also help identify the most critical toxicokinetic data necessary to replace defaults. PBTK approaches have led to more informed estimates of human target tissue dose, and therefore human health risk, especially where these risk assessments have been based on extrapolation of animal dosimetry studies. Experience derived from the development of more intensive case studies have, in turn, enabled simplified approaches to the use of PBTK modeling for respiratory tract toxicants. Whether simplified or highly complex, PBTK modeling approaches are proven to be of great utility to risk assesors interested in applying quantitative information to informed risk assessment evaluations.

An internal dose model for interspecies extrapolation of immediate incapacitation risk from inhalation of fire gases

A quantitative mathematical model assesses incapacitation risk in humans from toxic gas inhalation. A body-mass-normalized internal dose for each gas is calculated from an inhalation equation in which ventilation is a function of species, activity, and the gases inhaled. Uptake in the dead space considers U.S. Environmental Protection Agency (EPA) gas categories. The probability of incapacitation is a function of normalized internal dose and follows a cumulative distribution curve whose parameters are found from small-animal incapacitation data. No internal interaction of gases is modeled, and probabilities are combined independently. The model compares favorably with combined gas and large-animal incapacitation data.

Sensing and transfer of respiratory gases at the fish gill

The gill is both a site of gas transfer and an important location of chemoreception or gas sensing in fish. While often considered separately, these two processes are clearly intricately related because the gases that are transferred between the ventilatory water and blood at the gill are simultaneously sensed by chemoreceptors on, and within, the gill. Modulation of chemoreceptor discharge in response to changes in O(2) and CO(2) levels, in turn, is believed to initiate a series of coordinated cardiorespiratory reflexes aimed at optimising branchial gas transfer. The past decade has yielded numerous advances in terms of our understanding of gas transfer and gas sensing at the fish gill, particularly concerning the transfer and sensing of carbon dioxide. In addition, recent research has moved from striving to construct a single model that covers all fish species, to recognition of the considerable inter-specific variation that exists with respect to the mechanics of gas transfer and the cardiorespiratory responses of fish to changes in O(2) and CO(2) levels. The following review attempts to integrate gas transfer and gas sensing at the fish gill by exploring recent advances in these areas.

[Sorption-frequency probes and biosensors for gas and liquid composition monitoring]

The review deals with the problems of development of analytical equipment based on piezoelectric crystals. Consideration is given to the philosophy of determining inorganic and organic compounds, biologically active compounds, viruses, and bacteria with piezoelectric resonators. Described are methods of immobilization of the biological component, immersion-and-drying analysis, and sensor-assisted detection in the soluble phase.

Mechanisms of intestinal gas retention in humans: impaired propulsion versus obstructed evacuation

To explore the clinical role of intestinal gas dynamics, we investigated two potential mechanisms of gas retention, defective propulsion and obstructed evacuation. In healthy subjects, a gas mixture was continuously infused into the jejunum (4 ml/min) 1) during a 2-h control period of spontaneous gas evacuation and 2) during a 2-h test period either with impaired gut propulsion caused by intravenous glucagon (n = 6) or with obstructed (self-restrained) anal evacuation (n = 10) while anal gas evacuation, symptom perception (0-6 scale), and abdominal girth were measured. Impaired gut propulsion and obstructed evacuation produced similar gas retention (558 +/- 68 ml and 407 +/- 85 ml, respectively, vs. 96 +/- 58 ml control; P < 0.05 for both) and abdominal distension (8 +/- 3 mm and 6 +/- 3 mm, respectively, vs. 1 +/- 1 mm control; P < 0.05 for both). However, obstructed evacuation increased symptom perception (2.3 +/- 0.6 score change; P < 0.05), whereas gas retention in the glucagon-induced hypotonic gut was virtually unperceived (-0.4 +/- 0.7 score change; not significant). In conclusion, the perception of intestinal gas accumulation depends on the mechanism of retention.

Ventilation-perfusion inhomogeneity increases gas uptake in anesthesia: computer modeling of gas exchange

Ventilation-perfusion (VA/Q) inhomogeneity was modeled to measure its effect on overall gas exchange during maintenance-phase N(2)O anesthesia with an inspired O(2) concentration of 30%. A multialveolar compartment computer model was used based on physiological log normal distributions of VA/Q inhomogeneity. Increasing the log standard deviation of the distribution of perfusion from 0 to 1.75 paradoxically increased O(2) uptake (VO(2)) where a low mixed venous partial pressure of N(2)O [high N(2)O uptake (VN(2)O)] was specified. With rising mixed venous partial pressure of N(2)O, a threshold was observed where VO(2) began to fall, whereas VN(2)O began to rise with increasing VA/Q inhomogeneity. This phenomenon is a magnification of the concentrating effects that VO(2) and VN(2)O have on each other in low VA/Q compartments. During "steady-state" N(2)O anesthesia, VN(2)O is predicted to paradoxically increase in the presence of worsening VA/Q inhomogeneity.

Nasal tissue dosimetry-issues and approaches for "Category 1" gases: a report on a meeting held in Research Triangle Park, NC, February 11-12, 1998

Three organizations, the Basic Acrylic Monomer Manufacturers (BAMM), Methacrylate Producers Association (MPA), and Vinyl Acetate Toxicology Group (VATG), have sponsored development of physiologically based pharmacokinetic (PBPK) models for nasal tissue dosimetry with, respectively, acrylic acid (AA), methyl methacrylate (MMA), and vinyl acetate (VA). These compounds cause lesions in nasal epithelial tissues and are classified as "Category 1" gases within the U.S. EPA (1994) classification scheme. The National Center for Environmental Assessment in the U.S. EPA Office of Research and Development also has continuing interests in refining its methods for dosimetry adjustments when data on mode of action are available for Category 1 gases. A round-table discussion was held in Research Triangle Park, NC, on 11-12 February 1998, to develop a broader appreciation of the key processes and parameters required in developing nasal tissue dosimetry models. The discussions at the round table drew on these three case studies and several background presentations to assess the manner in which chemical-specific and mode-of-action data can be incorporated into nasal dosimetry models. The round table had representation from the U.S. EPA, academia, and industry. This article outlines the presentations and topical areas discussed at the round table and notes recommendations made by participants to extend models for nasal dosimetry and to develop improved data for modeling. The contributions of several disciplines-toxicology, engineering, and physiologically based pharmacokinetic (PBPK) modeling-were evident in the discussions. The integration of these disciplines in creating opportunities for dosimetry model applications in risk assessments has several advantages in the breadth of skills upon which to draw in model development. A disadvantage is in the need to provide venues and develop cross-discipline dialogue necessary to ensure the understanding of cultural attitudes, terminology, and methods. The round-table discussions were fruitful in achieving such enhanced understanding and communication. Subsequent elaboration of these models will benefit from the interactions of these groups at the round table. The round-table discussions have already led to model improvements-as noted in several recently published articles. Participants emphasized several generic data needs in relation to nasal vapor uptake studies in human subjects, to broader discussion of tissue diffusion models, and to extensions to other classes of gases. The round-table articles that are published separately in this issue and the discussions, captured in this overview, provide a glimpse of the state of the science in nasal dosimetry modeling and a clear indication of the growth of and continuing opportunities in this important research area.

A hybrid computational fluid dynamics and physiologically based pharmacokinetic model for comparison of predicted tissue concentrations of acrylic acid and other vapors in the rat and human nasal cavities following inhalation exposure

To assist in interspecies dosimetry comparisons for risk assessment of the nasal effects of organic acids, a hybrid computational fluid dynamics (CFD) and physiologically based pharmacokinetic (PBPK) dosimetry model was constructed to estimate the regional tissue dose of inhaled vapors in the rat and human nasal cavity. Application to a specific vapor would involve the incorporation of the chemical-specific reactivity, metabolism, partition coefficients, and diffusivity (in both air and tissue phases) of the vapor. This report describes the structure of the CFD-PBPK model and its application to a representative acidic vapor, acrylic acid, for interspecies tissue concentration comparisons to assist in risk assessment. By using the results from a series of short-term in vivo studies combined with computer modeling, regional nasal tissue dose estimates were developed and comparisons of tissue doses between species were conducted. To make these comparisons, the assumption was made that the susceptibilities of human and rat olfactory epithelium to the cytotoxic effects of organic acids were similar, based on similar histological structure and common mode of action considerations. Interspecies differences in response were therefore assumed to be driven primarily by differences in nasal tissue concentrations that result from regional differences in nasal air flow patterns relative to the species-specific distribution of olfactory epithelium in the nasal cavity. The results of simulations with the seven-compartment CFD-PBPK model suggested that the olfactory epithelium of the human nasal cavity would be exposed to tissue concentrations of acrylic acid similar to that of the rat nasal cavity when the exposure conditions are the same. Similar analysis of CFD data and CFD-PBPK model simulations with a simpler one-compartment model of the whole nasal cavities of rats and humans provides comparable results to averaging over the compartments of the seven-compartment model. These results indicate that the general structure of the hybrid CFD-PBPK model applied in this assessment would be useful for target tissue dosimetry and interspecies dose comparisons for a wide variety of vapors. Because of its flexibility, this CFD-PBPK model is envisioned to be a platform for the construction of case-specific inhalation dosimetry models to simulate in vivo exposures that do not involve significant histopathological damage to the nasal cavity.

Mass transport analysis: inhalation rfc methods framework for interspecies dosimetric adjustment

In 1994, the U.S. Environmental Protection Agency introduced dosimetry modeling into the methods used to derive an inhalation reference concentration (RfC). The type of dosimetric adjustment factor (DAF) applied had to span the range of physicochemical characteristics of the gases listed on the Clean Air Act Amendments in 1991 as hazardous air pollutants (HAPs) and accommodate differences in available data with respect to their toxicokinetic properties. A framework was proposed that allowed for a hierarchy of dosimetry model structures, from optimal to rudimentary, and a category scheme that provided for limiting model structures based on physicochemical and toxicokinetic properties. These limiting cases were developed from restricting consideration to specific properties relying on an understanding of the generalized system based on mass transport theory. Physiochemical characteristics included the solubility and reactivity (e.g., propensity to dissociate, oxidize, or serve as a metabolic substrate) of the gas and were used as major determinants of absorption. Dosimetric adjustments were developed to evaluate portal of entry (POE) effects as well as remote (systemic) effects relevant to the toxicokinetic properties of the gas of interest. The gas categorization scheme consisted of defining three gas categories: (1) gases that are highly soluble and/or reactive, absorbing primarily in the extrathoracic airways; (2) gases that are moderately soluble and/or reactive, absorbing throughout the airways, as well as accumulating in the bloodstream; and (3) gases that have a low water solubility and are lipid soluble such that they are primarily absorbed in the pulmonary region and likely to act systemically. This article presents the framework and the mass transport theory behind the RfC method. Comparison to compartmental approaches and considerations for future development are also discussed.

Sites and mechanisms for uptake of gases and vapors in the respiratory tract

Inhalation is a common route by which individuals are exposed to toxicants. The air contains a multitude of gases and vapors that are brought into the respiratory tract with each breath. Depending upon the physical and chemical characteristics of the toxicant, the respiratory tract can be considered as a target organ in addition to a portal of entry. Sufficient information is not always available on the fate or effects of an inhaled gas or vapor. Two physiochemical principles, water solubility and reactivity, can be used to predict the site of uptake of gases and vapors in the respiratory tract and potential mechanisms for reaction with respiratory tract tissue and absorption into the blood. Four model compounds, formaldehyde, ozone, dibasic esters, and butadiene are discussed as examples of how knowledge of aqueous solubility and chemical reactivity can help toxicologists predict sites and mechanisms by which inhaled gases and vapors interact with respiratory tract tissues.

The ability of apoplastic ascorbate to protect poplar leaves against ambient ozone concentrations: a quantitative approach

Shoots of a sensitive (Populus nigra 'Brandaris') and a more tolerant (Populus euramericana 'Robusta') poplar clones were exposed for 30 days to Filtered Air or ambient O3-concentrations in fumigation cabinets. At regular intervals were determined: gas exchange of the leaves, the internal air space (Vair) and apoplastic water volume (Vapo) and the reduced (ASA) and oxidized (DHA) ascorbate concentration in the apoplast and in the mesophyll cells. The apoplastic ASA-concentration was 0.2 mM at the start of the experiment for both cultivars, while the effective cell wall thickness, estimated from Vapo, varied from 0.3 to 0.6 micron. Model calculations revealed that only 30% of the O3 molecules entering the apoplast was intercepted at these values. The O3-treatment induced a decline in stomatal conductance, an increase in Vapo and in the apoplastic ASA-concentration. As a result the estimated O3-flux to the cell membrane strongly declined. However, these responses occurred after the O3-induced reduction in photosynthesis. Moreover, they did not prevent early senescence of the leaves at a prolonged exposure. Therefore, it is concluded that the increase in apoplastic ASA-concentration was rather a general stress reaction of the affected poplar leaf than a (specific) defence reaction induced by O3. Our results suggest that other factors than the scavenging efficiency of apoplastic ASA were responsible for the difference in O3 sensitivity between both poplar cultivars.

Simple box model for dense-gas dispersion in a straight sloping channel

A box model for instantaneous release and subsequent one-dimensional spreading of isothermal dense gases on sloping surfaces is presented. A numerical solution and an approximate analytical solution of the model equations are compared to the experimental data obtained in a sloping heavy-gas channel of the Institute of Fluid Dynamics at ETH-Zürich. The influence of the rear wall of the containment from where the cloud is released is analysed. Different entrainment assumptions, in particular the scaling of the entrainment parameters, are discussed. The numerical values of the entrainment parameters are tuned by computer optimization in order to obtain best agreement of the theoretical results with experimental data.

Histopathological changes in gills of the estuarine crab Chasmagnathus granulata (Crustacea-Decapoda) following acute exposure to ammonia

Histopathological effects of ammonia on the gills of the estuarine crab Chasmagnathus granulata (Dana, 1851) were evaluated after acute exposure to ammonia concentrations around LC(50) value (17.85 Mm). Disruption of pilaster cells and a subsequent collapse of gill lamellae were the main effects observed. Epithelial necrosis and hyperplasia were also detected. Significant (P<0.05) increases in pCO(2) and lactate, and significant decreases of pO(2) were detected in the haemolymph of ammonia-exposed crabs. These changes suggest that the observed histopathological damage affected gas exchange, possibly leading to death.

Consequence analysis in LPG installation using an integrated computer package

This paper presents the prototype of the computer code, Atlantide, developed to assess the consequences associated with accidental events that can occur in a LPG storage plant. The characteristic of Atlantide is to be simple enough but at the same time adequate to cope with consequence analysis as required by Italian legislation in fulfilling the Seveso Directive. The application of Atlantide is appropriate for LPG storage/transferring installations. The models and correlations implemented in the code are relevant to flashing liquid releases, heavy gas dispersion and other typical phenomena such as BLEVE/Fireball. The computer code allows, on the basis of the operating/design characteristics, the study of the relevant accidental events from the evaluation of the release rate (liquid, gaseous and two-phase) in the unit involved, to the analysis of the subsequent evaporation and dispersion, up to the assessment of the final phenomena of fire and explosion. This is done taking as reference simplified Event Trees which describe the evolution of accidental scenarios, taking into account the most likely meteorological conditions, the different release situations and other features typical of a LPG installation. The limited input data required and the automatic linking between the single models, that are activated in a defined sequence, depending on the accidental event selected, minimize both the time required for the risk analysis and the possibility of errors. Models and equations implemented in Atlantide have been selected from public literature or in-house developed software and tailored with the aim to be easy to use and fast to run but, nevertheless, able to provide realistic simulation of the accidental event as well as reliable results, in terms of physical effects and hazardous areas. The results have been compared with those of other internationally recognized codes and with the criteria adopted by Italian authorities to verify the Safety Reports for LPG installations. A brief of the theoretical basis of each model implemented in Atlantide and an example of application are included in the paper.

Mitigation of dense gas releases in buildings: use of simple models

When an accidental release of a hazardous material is considered within a safety case or risk assessment, its off-site effects are generally assessed by calculating the dispersion of vapour from the site. Although most installations handling flammables will be in the open air, many types of plant, particularly those handling toxics, are enclosed, partly to provide some form of containment and hence to mitigate the effects of any release. When such a release occurs within a building, the gas or vapour will undergo some mixing before emerging from any openings. The degree of mixing will depend upon the building geometry and the nature of the ventilation, which in turn may be modified by the leak. This situation is considered in this paper, with specific application to calculating the rate of release of a dense vapour from a building. All the calculations presented are based upon simple zone modelling, such that the region occupied by the vapour is assumed to be well mixed, and, in the isothermal case, either its concentration or its depth increases as it is fed by the gas leak. Transfer of air or gas/air mixture through the building openings is estimated by use of standard ventilation calculation methods. For the non-isothermal case, a preliminary model is presented in which it is assumed that there is complete mixing throughout the building and no wind-driven ventilation effects. A moderate release of chlorine is used as an example, and results are shown of the effects of various ventilation possibilities on the release rate to the atmosphere. In addition, comparisons are given between model results and experimental data, demonstrating the level of confidence which can be placed in the models, and also identifying areas where there is scope for further improvement.

Mitigation of dense gas releases within buildings: validation of CFD modelling

When an accidental release of a hazardous material is considered within a safety case or risk assessment, its off-site effects are generally assessed by calculating the dispersion of vapour from the site. Although most installations handling flammable materials will be in the open air, many types of plant, particularly those handling toxics, are enclosed, partly to provide some form of containment and hence, to mitigate the effects of any release. When such a release occurs within a building, the gas or vapour will undergo some mixing before emerging from any opening. The degree of mixing will depend upon the building geometry and the nature of the ventilation, which in turn may be modified by the leak. This situation is considered in this paper, with specific application to calculating the rate of release of a dense vapour from a building. The paper describes the application of computational fluid dynamics (CFD) techniques to modelling the release and mixing processes within buildings. Examples of validation calculations for simple geometric arrangements, as well as more complex geometries representative of an industrial site, are described. The results demonstrate the capabilities of CFD for this application but highlight the need for careful modelling of the near-wall flows and heat transfer, and need for an accurate fluid dynamics and thermodynamic representation of the release source.

Heat transfer in large-scale heavy-gas dispersion

Heavy-gas dispersion of practical interest is usually cold gas dispersion with the enthalpy deficit as the main cause of the density effect. New analysis of existing field experiment data suggests that heat transfer from the ground sometimes reduces this thermally induced density effect considerably. The limited heat capacity of the ground implies that heat transfer to a gas plume must disappear eventually, and our interpretation of Desert Tortoise measurements indicates that the surface heat flux decreased by 38% during a 3-min long release period.

TWODEE: the Health and Safety Laboratory's shallow layer model for heavy gas dispersion. Part 1. Mathematical basis and physical assumptions

The Major Hazard Assessment Unit of the Health and Safety Executive (HSE) provides advice to local planning authorities on land use planning in the vicinity of major hazard sites. For sites with the potential for large scale releases of toxic heavy gases such as chlorine this advice is based on risk levels and is informed by use of the computerised risk assessment tool RISKAT [C. Nussey, M. Pantony, R. Smallwood, HSE's risk assessment tool RISKAT, Major Hazards: Onshore and Offshore, October, 1992]. At present RISKAT uses consequence models for heavy gas dispersion that assume flat terrain. This paper is the first part of a three part paper. Part 1 describes the mathematical basis of TWODEE, the Health and Safety Laboratory's shallow layer model for heavy gas dispersion. The shallow layer approach used by TWODEE is a compromise between the complexity of CFD models and the simpler integral models. Motivated by the low aspect ratio of typical heavy gas clouds, shallow layer models use depth-averaged variables to describe the flow behaviour. This approach is particularly well suited to assess the effect of complex terrain because the downslope buoyancy force is easily included. Entrainment may be incorporated into a shallow layer model by the use of empirical formulae. Part 2 of this paper presents the numerical scheme used to solve the TWODEE mathematical model, and validated against theoretical results. Part 3 compares the results of the TWODEE model with the experimental results taken at Thorney Island [J. McQuaid, B. Roebuck, The dispersion of heavier-than-air gas from a fenced enclosure. Final report to the US Coast Guard on contract with the Health and Safety Executive, Technical Report RPG 1185, Safety Engineering Laboratory, Research and Laboratory Services Division, Broad Lane, Sheffield S3 7HQ, UK, 1985].

TWODEE: the Health and Safety Laboratory's shallow layer model for heavy gas dispersion. Part 3: experimental validation (Thorney Island)

Part 1 of this three-part paper described the mathematical and physical basis of TWODEE, the Health and Safety Laboratory's shallow layer model for heavy gas dispersion. In part 2, the numerical solution method used to simulate the TWODEE mathematical model was developed; the flux correction scheme of Zalesak [S.T. Zalesak, Fully multidimensional flux-corrected transport algorithms for fluids, Journal of Computational Physics, 31 (1979) 335-362.] was used in TWODEE. This paper compares results of the TWODEE model to the experimental results taken at Thorney Island [J. McQuaid, B. Roebuck, The dispersion of heavier-than-air gas from a fenced enclosure. Final report to the U.S. Coast Guard on contract with the Health and Safety Executive. Technical Report RPG 1185, Safety Engineering Laboratory, Research and Laboratory Services Division, Broad Lane, Sheffield S3 7HQ, UK, 1985.]. There is no evidence to suggest that TWODEE predictions could be improved by changing any of the entrainment parameters from generally accepted values [R.K.S. Hankin, Heavy gas dispersion over complex terrain, PhD thesis, Cambridge University, 1997.]. The TWODEE model was broadly insensitive to the exact values of the entrainment parameters.

TWODEE: the Health and Safety Laboratory's shallow layer model for heavy gas dispersion. Part 2: outline and validation of the computational scheme

Part 1 of this three part paper described the mathematical and physical basis of TWODEE, the Health and Safety Laboratory's shallow layer model for heavy gas dispersion. In this part, the numerical solution method used to simulate the TWODEE mathematical model is developed. The boundary conditions for the leading edge, discussed in part 1, make demanding requirements on the computational scheme used. The flux correction scheme of Zalesak [S.T. Zalesak, Fully multidimensional flux-corrected transport algorithms for fluids, Journal of Computational Physics, 31 (1979) 335-362] is used in TWODEE as this has all the required properties. The TWODEE code is then tested against a number of theoretical and computational benchmark problems.

Analysis of vinyl acetate metabolism in rat and human nasal tissues by an in vitro gas uptake technique

Physiologically based pharmacokinetic (PBPK) models require estimates of catalytic rate constants controlling the metabolism of xenobiotics. Usually, these constants are derived from whole tissue homogenates wherein cellular architecture and enzyme compartmentation are destroyed. Since the nasal cavity epithelium is composed of a heterogeneous cell population measurement of xenobiotic metabolizing enzymes using homogenates could yield artifactual results. In this article a method for measuring rates of metabolism of vinyl acetate, a metabolism-dependent carcinogen, is presented that uses whole-tissue samples and PBPK modeling techniques to estimate metabolic kinetic parameters in tissue compartments. The kinetic parameter estimates were compared to those derived from homogenate experiments using two methods of tissue normalization. When the in vitro gas uptake constants were compared to homogenate-derived values, using a normalization procedure that does not account for tissue architecture, there was poor agreement. Homogenate-derived values from rat nasal tissue were 3- to 23-fold higher than those derived using the in vitro gas uptake method. When the normalization procedure for the rat homogenate-derived values took into account tissue architecture, a good agreement was observed. Carboxylesterase activity in homogenates of human nasal tissues was undetectable. Using the in vitro gas uptake technique, however, carboxylesterase activity was detected. Rat respiratory carboxylesterase and aldehyde dehydrogenase activities were about three and two times higher than those of humans, respectively. Activities of the rat olfactory enzymes were about equivalent to those of humans. K(m) values did not differ between species. The results suggest that the in vitro gas uptake technique is useful for deriving enzyme kinetic constants where effects of tissue architecture are preserved. Furthermore, the results suggest that caution should be exercised when scaling homogenate-derived values to whole-organ estimates, especially in organs of cellular heterogeneity.

Asphyxia due to oxygen deficiency by gaseous substances

The determination of the cause of death in asphyxiation gas cases is very difficult because of the variation in circumstances surrounding such deaths. To clarify the cause of death and to identify the factors involved in asphyxia, the symptoms during asphyxia, the concentration of gases at the respiratory arrest, the time to death and the concentration of the gaseous substances in the tissues were studied using rats and six gases. Three inhalations were used: (1) rapid asphyxia (2-3 min) in the exposure chamber in which the oxygen was depleted completely, (2) prolonged asphyxia (20-25 min) by gradually depleted oxygen, and (3) asphyxia by the inhalation of gases saturated with a critical gas concentration, maintaining the O2 at 20% (60 min). In the rapid asphyxia groups, respiratory arrest occurred within 30 to 40 s, followed by cardiac arrest 2 or 3 min thereafter. Severe convulsions were observed only with the use of nitrogen. In the prolonged asphyxia groups, respiratory arrest occurred at the concentration of 4-5% O2 with non-toxic gases (N2, CH4, N2O, and propane). The toxic gases CO2 and Freon-22 produced respiratory arrest at the concentration of 6.6-8.0% O2 (60-67% CO2) and 13-14% of O2 (30-35% Freon-22), respectively. Variations in the concentrations of the gases among the tissues was observed according to the type of asphyxia, type of gas and the duration of exposure. The concentration of the fat-soluble gases in the adipose tissue showed marked variation according to the duration of the exposure. The distribution pattern of methane was different from those of the other gases, in which the variation of concentrations among the tissues except lung were little in both rapid and prolonged asphyxia. These phenomena were considered to be attributable to the solubility of the gaseous substances in blood and tissues. Atrophy in the alveoli was observed after the rapid asphyxia with CO2 and N2O. Local hemorrhaging in the lungs was also observed, especially in CO2 asphyxia. The risks of oxygen-depletion asphyxia are the rapid reaction of loss of consciousness and respiratory and cardiac arrest. This paper presents valuable findings for the diagnosis of the cause of death and estimating the situation of the accident in cases of asphyxia.

Computer simulation of inspiratory nasal airflow and inhaled gas uptake in a rhesus monkey

There is increasing evidence that inspiratory airflow patterns play a major role in determining the location of nasal lesions induced in rats by reactive, water-soluble gases such as formaldehyde and chlorine. Characteristic lesion patterns have also been seen in inhalation toxicity studies conducted in rhesus monkeys, the nasal anatomy of which resembles that of humans. To examine the hypothesis that regions of high airflow-dependent uptake and lesions occur in similar nasal locations in the primate, airflow and gas uptake patterns were simulated in an anatomically accurate computer model of the right nasal airway of a rhesus monkey. The results of finite-element simulations of steady-state inspiratory nasal airflow for the full range of resting physiological flow rates are reported. Simulated airflow patterns agreed well with experimental observations, exhibiting secondary flows in the anterior nose and streamlined flow posteriorly. Simulated airflow results were used to predict gas transport to the nasal passage walls using formaldehyde as an example compound. Results from the uptake simulations were compared with published observations of formaldehyde-induced nasal lesions in rhesus monkeys and indicated a strong correspondence between airflow-dependent transport patterns and local lesion sites. This rhesus computer model will provide a means for confirming the extrapolation of toxicity data between species by extrapolating rat simulation results to monkeys and comparing these predictions with primate lesion data.