Kinetics and removal formula of methyl mercaptan by ethanol absorption without neglecting solute accumulation

The wet scrubbing process is commonly adopted for organic odor treatment. In this study, methyl mercaptan (CH₃SH) was selected as a representative hydrophobic organic odorant which was treated using an ethanol solution in a scrubbing tower. Results showed that the ethanol solution can retain the ideal CH₃SH removal effect for 2.0 h. The following experimental conditions were set: intake load of 4,700 m³ m^-2 h^-1, spraying load of 5,100 L m^-2 h^-1, and volume ratio of ethanol/water at 1:5. The solute accumulation of CH₃SH in the scrubbing liquid exceeded 3.01 × 10^-4 kmol CH₃SH/kmol ethanol when the scrubbing tower operated for more than 2.0 h. The mathematical formula which neglected solute accumulation in the ethanol solution exhibited poor adaptability to the removal effect of CH₃SH by ethanol absorption. The CH₃SH removal effect of solute accumulation in the ethanol solution was explored in long-term operation. Meanwhile, the CH₃SH removal rate formula which considered solute accumulation in the ethanol solution could be calculated as η = a'-b'X₂/Y₁. The kinetic parameters of the formula fitting results were phase equilibrium constant m 0.0076, and overall mass transfer coefficient K_Y 4.98 kmol m^-2 h^-1 in the scrubbing tower. These findings can serve as a reference for engineering design and operation for the removal of CH₃SH by ethanol absorption.

Airlift bioreactor system for simultaneous removal of hydrogen sulfide and ammonia from synthetic and actual waste gases

The effectiveness of an airlift reactor system in simultaneously removing hydrogen sulfide (H₂S) and ammonia (NH₃) from synthetic and actual waste gases was investigated. The effects of various parameters, including the ratio of inoculum dilution, the gas concentration, the gas retention time, catalyst addition, the bubble size, and light intensity, on H₂S and NH₃ removal were investigated. The results revealed that optimal gas removal could be achieved by employing an activated inoculum, using a small bubble stone, applying reinforced fluorescent light, adding Fe₂O₃ catalysts, and applying a gas retention time of 20 s. The shock loading did not substantially affect the removal efficiency of the airlift bioreactor. Moreover, more than 98.5% of H₂S and 99.6% of NH₃ were removed in treating actual waste gases. Fifteen bands or species were observed in a profile from denaturing gradient gel electrophoresis during waste gas treatment. Phylogenetic analysis revealed the phylum Proteobacteria to be predominant. Six bacterial strains were consistently present during the entire operating period; however, only Rhodobacter capsulatus, Rhodopseudomonas palustris, and Arthrobacter oxydans were relatively abundant in the system. The photosynthetic bacteria R. capsulatus and R. palustris were responsible for H₂S oxidation, especially when the reinforced fluorescent light was used. The heterotrophic nitrifier A. oxydans was responsible for NH₃ oxidation. To our knowledge, this is the first report on simultaneous H₂S and NH₃ removal using an airlift bioreactor system. It clearly demonstrates the effectiveness of the system in treating actual waste gases containing H₂S and NH₃.

A novel enhanced diffusion sampler for collecting gaseous pollutants without air agitation

A novel enhanced diffusion sampler for collecting gaseous phase polycyclic aromatic hydrocarbons (PAHs) without air agitation is proposed. The diffusion of target compounds into a sampling chamber is facilitated by continuously purging through a closed-loop flow to create a large concentration difference between the ambient air and the air in the sampling chamber. A glass-fiber filter-based prototype was developed. It was demonstrated that the device could collect gaseous PAHs at a much higher rate (1.6 ± 1.4 L/min) than regular passive samplers, while the ambient air is not agitated. The prototype was also tested in both the laboratory and field for characterizing the concentration gradients over a short distance from the soil surface. The sampler has potential to be applied in other similar situations to characterize the concentration profiles of other chemicals.

Non-isothermal pyrolysis of de-oiled microalgal biomass: Kinetics and evolved gas analysis

Non-isothermal (β=5, 10, 20, 35°C/min) pyrolysis of de-oiled microalgal biomass (DMB) of Chlorella variabilis was investigated by TGA-MS (30-900°C, Argon atmosphere) to understand thermal decomposition and evolved gas analysis (EGA). The results showed that three-stage thermal decomposition and three volatilization zone (100-400°C, 400-550°C and 600-750°C) of organic matters during pyrolysis. The highest rate of weight-loss is 8.91%/min at 302°C for 35°C/min heating-rate. Kinetics of pyrolysis were investigated by iso-conversional (KAS, FWO) and model-fitting (Coats-Redfern) method. For Zone-1and3, similar activation energy (E_a) is found in between KAS (α=0.4), FWO (α=0.4) and Avrami-Erofe'ev (n=4) model. Using the best-fitted kinetic model Avrami-Erofe'ev (n=4), E_a values (R²=>0.96) are 171.12 (Zone-1), 404.65 (Zone-2) and 691.42kJ/mol (Zone-3). EGA indicate the abundance of most gases observed consequently between 200-300°C and 400-500°C. The pyrolysis of DMB involved multi-step reaction mechanisms for solid-state reactions having different E_a values.

Physical model for absorption of intraocular gas

PURPOSE

To present a physical model for intraocular absorption of an inert gas used as a tamponade.

METHODS

The absorption kinetics of gas in contact with the retinal surface is examined, including the changing geometry of the shrinking gas bubble inside the eye.

RESULTS

An analytic solution is derived that predicts how the bubble dimensions change with time, yielding a formula for the lifetime of the gas bubble. Comparison to an experimental measurement shows that the analytic solution accurately replicates the time evolution of the bubble geometry. The result is also compared to an alternative exponential model, which does not predict a finite bubble lifetime.

CONCLUSIONS

Further experiments are needed to discriminate between the surface absorption and exponential models.

Application of Glycyrrhiza glabra root as a novel adsorbent in the removal of toluene vapors: equilibrium, kinetic, and thermodynamic study

The aim of this paper is to investigate the removal of toluene from gaseous solution through Glycyrrhiza glabra root (GGR) as a waste material. The batch adsorption experiments were conducted at various conditions including contact time, adsorbate concentration, humidity, and temperature. The adsorption capacity was increased by raising the sorbent humidity up to 50 percent. The adsorption of toluene was also increased over contact time by 12 h when the sorbent was saturated. The pseudo-second-order kinetic model and Freundlich model fitted the adsorption data better than other kinetic and isotherm models, respectively. The Dubinin-Radushkevich (D-R) isotherm also showed that the sorption by GGR was physical in nature. The results of the thermodynamic analysis illustrated that the adsorption process is exothermic. GGR as a novel adsorbent has not previously been used for the adsorption of pollutants.

Composite films of arabinoxylan and fibrous sepiolite: morphological, mechanical, and barrier properties

Hemicelluloses represent a largely unutilized resource for future bioderived films in packaging and other applications. However, improvement of film properties is needed in order to transfer this potential into reality. In this context, sepiolite, a fibrous clay, was investigated as an additive to enhance the properties of rye flour arabinoxylan. Composite films cast from arabinoxylan solutions and sepiolite suspensions in water were transparent or semitransparent at additive loadings in the 2.5-10 wt % range. Scanning electron microscopy showed that the sepiolite was well dispersed in the arabinoxylan films and sepiolite fiber aggregation was not found. FT-IR spectroscopy provided some evidence for hydrogen bonding between sepiolite and arabinoxylan. Consistent with these findings, mechanical testing showed increases in film stiffness and strength with sepiolite addition and the effect of poly(ethylene glycol) methyl ether (mPEG) plasticizer addition. Incorporation of sepiolite did not significantly influence the thermal degradation or the gas barrier properties of arabinoxylan films, which is likely a consequence of sepiolite fiber morphology. In summary, sepiolite was shown to have potential as an additive to obtain stronger hemicellulose films although other approaches, possibly in combination with the use of sepiolite, would be needed if enhanced film barrier properties are required for specific applications.

A tripartite synapse model in Drosophila

Tripartite (three-part) synapses are defined by physical and functional interactions of glia with pre- and post-synaptic elements. Although tripartite synapses are thought to be of widespread importance in neurological health and disease, we are only beginning to develop an understanding of glial contributions to synaptic function. In contrast to studies of neuronal mechanisms, a significant limitation has been the lack of an invertebrate genetic model system in which conserved mechanisms of tripartite synapse function may be examined through large-scale application of forward genetics and genome-wide genetic tools. Here we report a Drosophila tripartite synapse model which exhibits morphological and functional properties similar to those of mammalian synapses, including glial regulation of extracellular glutamate, synaptically-induced glial calcium transients and glial coupling of synapses with tracheal structures mediating gas exchange. In combination with classical and cell-type specific genetic approaches in Drosophila, this model is expected to provide new insights into the molecular and cellular mechanisms of tripartite synapse function.

Uptake, tissue distribution, and excretion of 14C-sulfur mustard vapor following inhalation in F344 rats and cutaneous exposure in hairless guinea pigs

Sulfur mustard (SM), a vessicating agent, has been used in chemical warfare since 1918. The purpose of this study was to quantitate SM vapor deposition, tissue distribution, and excretion following intratracheal inhalation in rats and cutaneous exposure in guinea pigs. 14C-SM vapors for inhalation studies were generated by metering liquid 14C-SM into a heated J tube. Vapors were transported via carrier air supplemented with oxygen and isoflurane to an exposure plenum. Anesthetized rats with transorally placed tracheal catheters were connected to the plenum port via the catheter hub for exposure (approximately 250 mg 14C-SM vapor/m(3); 10 min). For dermal exposure, 3 Teflon cups (6.6 cm(2) exposure area per cup) were applied to the backs of each animal and vapors (525 mg 14C-SM/m(3); 12 min) were generated by applying 6 μl 14C-SM to filter paper within each cup. Animals were euthanized at selected times up to 7 d postexposure. SM equivalents deposited in rats and guinea pigs were 18.1 ± 3 μg and 29.8 ± 5.31 μg, respectively. Inhaled SM equivalents rapidly distributed throughout the body within 2 h postexposure, with the majority (>70%) of material at that time located in carcass and pelt. In guinea pigs, >90% of deposited SM equivalents remained in skin, with minor distribution to blood and kidneys. Urine was the primary route of excretion for both species. Results indicate inhaled SM is rapidly absorbed from the lung and distributed throughout the body while there is limited systemic distribution following cutaneous exposure.

Gas Uptake in a Three-Generation Model Geometry During Steady Expiration: Comparison of Axisymmetric and Three-Dimensional Models

Mass transfer coefficients were predicted and compared for uptake of a formaldehyde-air gas system using an axisymmetric single path model (ASPM) and a three-dimensional computational fluid dynamics model (CFDM) in three-generation model geometry at steady expiratory flow. The flow and concentration fields in the ASPM were solved using Galerkin's finite-element method and in the CFDM using a commercial finite-element software, FIDAP. Numerical results were compared for two different inlet flow rates, wall mass transfer coefficients, and bifurcation angles. The mass transfer coefficients variation with bifurcation unit from the ASPM and CFDM compared qualitatively and quantitatively closely at all flows and lower wall mass transfer coefficients for both 40 degrees and 70 degrees bifurcation angles. However, at higher wall mass transfer coefficients, quantitatively they were within 40% for both the bifurcation angles. Also, at higher flow and wall mass transfer coefficients, they were off qualitatively for a 70 degrees bifurcation angle although the uptake compared qualitatively. This is due to the normalization of uptake within a bifurcation unit with the average of inlet and outlet average concentrations. Both CFDM and ASPM predict the same trends of increase in mass transfer coefficients with inlet flow and wall mass transfer coefficients. Also, the local values of the mass transfer coefficients compared closely at all conditions. These results validate the simplified ASPM and the complex CFDM. Mass transfer coefficients increase with bifurcation angles and with a flat inlet velocity profile compared to a parabolic velocity profile since the flow is non-fully developed and hence, the uptake increases.

Incorporation of Acute Dynamic Ventilation Changes into a Standardized Physiologically Based Pharmacokinetic Model

A seven-compartment physiologically based pharmacokinetic (PBPK) model incorporating a dynamic ventilation response has been developed to predict normalized internal dose from inhalation exposure to a large range of volatile gases. The model uses a common set of physiologic parameters, including standardized ventilation rates and cardiac outputs for rat and human. This standardized model is validated against experimentally measured blood and tissue concentrations for 21 gases. For each of these gases, body-mass-normalized critical internal dose (blood concentration) is established, as calculated using exposure concentration and time duration specified by the lowest observed adverse effect level (LOAEL) or the acute exposure guideline level (AEGL). The dynamic ventilation changes are obtained by combining the standardized PBPK model with the Toxic Gas Assessment Software 2.0 (TGAS-2), a validated acute ventilation response model. The combined TGAS-2P model provides a coupled, transient ventilation and pharmacokinetic response that predicts body mass normalized internal dose that is correlated with deleterious outcomes. The importance of ventilation in pharmacokinetics is illustrated in a simulation of the introduction of Halon 1301 into an environment of fire gases.

Pitfalls and Related Improvements of In Vivo Gas Uptake Pharmacokinetic Experimental Systems

Gas uptake chamber studies have been widely used to study inhalation pharmacokinetics (PKs) in rodents, often for the ultimate purpose of developing physiologically-based pharmacokinetic (PBPK) models that can be used to describe human PKs and to support risk assessment for the chemical. In the course of our studies of gasoline PKs, we revisited several important issues heretofore not thoroughly addressed. Here, we report several refinements which will significantly improve future studies with this type of system, relating to the understanding of loss rates, the importance of carbon dioxide removal, and sampling of blood and chamber air at the same time. Losses of chemicals in gas uptake systems consist of leakage, adsorption to system components, and adsorption to the hair and skin (fur) of experimental animals. The loss rates were experimentally determined for a series of chemicals and mixtures including n-hexane, benzene, toluene, ethylbenzene, o-xylene, gasoline, and other gasoline components. The rate of loss to the animals' fur was similar to loss rates to system components and involved absorption to both hair and skin. Most of the absorption to fur was reversible when the chamber concentration was low enough. The amount of chemical that desorbed from the animal after an experiment was significant when compared to the amount of chemical in the chamber at the end of a gas uptake experiment, indicating that the rate of decline in concentrations can be influenced by a decrease in the fur absorption rate or desorption of chemicals. A modified gas uptake system design is described in which a steel ring improved the connections to an autosampler and allowed insertion of probes to monitor gases, such as carbon dioxide (CO2), in the chamber. When CO2 absorbent efficiency was inadequate, CO2 concentrations rose to levels that significantly affected the animals' ventilation rate. Using a real-time CO2 probe, an absorbent system was developed that adequately controlled CO2 levels in the chamber. Attention to details of absorptive loss and CO2 scrubbing can improve the reliability of kinetic constants inferred from closed chamber studies. We then describe a method for extending gas uptake experiments by simultaneously collecting blood to be analyzed for chemicals and/or metabolites.

Effect of a sustained inflation on regional distribution of gas and perfluorocarbon during partial liquid ventilation

OBJECTIVE

To study the effect of a sustained inflation (SI) maneuver on the regional distribution of gas and perfluorocarbon (PFC) during partial liquid ventilation (PLV) in normal pigs using computerized densitometry.

STUDY DESIGN

Observational study.

SETTING

Animal research laboratory.

PARTICIPANTS

Three healthy anaesthetized pigs.

INTERVENTIONS

Partial liquid ventilation, lung recruitment, CT densitometry.

METHODOLOGY

Lungs were filled with PFC to "liquid functional residual capacity (FRC)" (35-41 ml/kg) and CT images were recorded at a series of predetermined airway pressure levels (0, 20, 30, 40 cm H2O) both before and after SI to an airway pressure of 40 cm H2O for 30 sec. Anterior, middle, and posterior regions from upper (apical lung) to lower (basal lung) CT slices were analyzed at each pressure level for Hounsfield units to describe the relative distribution of gas and PFC before and after SI. Using an occlusion technique true gas volume above FRC was determined at each pressure level, before and after SI, and a pressure-volume (gas) envelope determined for each animal.

RESULTS

At low airway pressures (<20 cm H2O) gas was distributed predominantly to the anterior (non-dependent) part of the lung and PFC predominantly to the posterior (dependent) lung. Gas and liquid were more uniformly distributed throughout the lung at airway pressures >20 cm H2O. Generation of a pressure-volume (gas) envelope for each animal demonstrated an increase in total gas volume above FRC at each pressure level following recruitment of the lung with SI. However, marked regional differences were evident with the greatest effects of SI seen at higher airway pressures in posterior and basal regions.

CONCLUSION

The healthy PFC filled lung demonstrates an increase in total gas volume following SI. CT densitometry suggests marked heterogeneity of gas/PFC distribution between different regions of lung and heterogeneity of response to SI.

A computer-controlled system for generation of chemical vapours in in vitro dermal uptake studies

BACKGROUND/AIMS

Recent work in our laboratory suggests that dermal absorption and desorption of volatile chemicals may be assessed in vitro by thermogravimetric analysis (TGA), i.e. by passing chemical vapour over a piece of skin while recording the weight increase at constant temperature and humidity. This paper describes a high-precision automated vapour-generating system for use with the TGA equipment.

METHODS AND RESULTS

The system consists of computer-controlled magnetic valves and mass flow meters that split and redirect a flow of pure, dry air through different stainless-steel gas wash bottles thermostated to 25.00+/-0.05 degrees C. Each wash bottle is filled with a neat volatile chemical and designed so that the air leaving reaches 100% saturation within seconds, as shown with cyclohexanone. The air leaving the wash bottles are combined and directed via stainless-steel liners to the skin piece in the TGA chamber. The liners are heated to 30 degrees C to prevent condensation of water or chemical. Special computer software was developed to allow automatic runs with different wash bottles (chemicals) and air flows over several days. A number of measurements were made to characterize the stability and reproducibility of the vapour-generating system.

CONCLUSIONS

We have developed a computer-controlled vapour-generating system for use in measurements of dermal absorption of chemicals by thermal gravimetry. The system has high stability and reproducibility and produces little noise.

Laminar airflow and nanoparticle or vapor deposition in a human nasal cavity model

The transport and deposition of nanoparticles, i.e., dp = 1-2 nm, or equivalent vapors, in the human nasal cavities is of interest to engineers, scientists, air-pollution regulators, and healthcare officials alike. Tiny ultrafine particles, i.e., dp < or = 5 nm, are of special interest because they are most rapidly absorbed and hence have an elevated toxic or therapeutic impact when compared to larger particles. Assuming transient laminar 3-D incompressible flow in a representative human nasal cavity, the cyclic airflow pattern as well as local and overall nanoparticle depositions were computationally simulated and analyzed. The focus was on transient effects during inhalation/exhalation as compared to the steady-state assumption typically invoked. Then, an equation for a matching steady-state inhalation flow rate was developed that generates the same deposition results as cyclic inhalation. Of special interest is the olfactory region where the narrow channel surfaces receive only about one-half of a percent of the inhaled nanoparticles because the airflow bypasses these recesses located in the superior-most portions in the geometrically complex nasal cavities.

Closed Cup Vapor Systems in Percutaneous Exposure Studies: What is the Dose?

Percutaneous vapor dosing studies have generally used saturated vapor concentration (SVC) measurements to estimate the exposure dose (Ct) of vapor produced from a volatile liquid within a closed system. The purpose of this study was to clarify whether the assumption was valid when translated to a biological system (pig skin) using sulfur mustard (SM) as a model skin penetrant. Three systems were evaluated, two containing skin and a control system (without skin). At set time points, samples from the headspace of each dosing system were extracted using a gas-tight syringe and analyzed by gas chromatography in conjunction with a flame-ionization detector. This demonstrated the rapid achievement of a constant vapor concentration within the biological and control systems and enabled a comparison with previously determined SVCs attained under ideal conditions. All three systems attained a constant vapor concentration within 2 min of exposure to SM. The control system reached an equilibrium vapor concentration of 1179 +/- 164 mg/m3, a value not significantly different from that derived from the SVC (1363 mg/m3). Because of absorption in the skin systems, SM vapor concentrations were significantly lower than that derived from the SVC and were dependent on the skin surface area within the dosing chamber (592 +/- 246 mg/m3 for a surface area of 10.15 cm2 and 740 +/- 224 mg/m3 for a surface area of 2.54 cm2). The assumption that SVC gives an acceptable measure of the Ct was shown to be valid by comparison with sulfur mustard recovered from the skin.

Characterization of 133Xe gas washout in pulmonary emphysema with dynamic 133Xe SPECT functional images

PURPOSE

To characterize regional ventilation impairment of pulmonary emphysema using dynamic 133Xe single photon emission computed tomography (SPECT) functional images, compared with other forms of chronic obstructive pulmonary disease (COPD).

METHODS

Dynamic 133Xe SPECT was performed in 34 patients with emphysema and 15 patients with other forms of COPD. Three-dimensional voxel-based functional images of the half-clearance time (T1/2) mainly reflecting the initial rapid washout of 133Xe gas from the large airways, and of the mean transit time (MTT) reflecting 133Xe gas washout from the entire lungs, including the small airways and alveoli, were created based on an area-over-height method. T1/2 and MTT values were compared with the regional extent of low attenuation areas (%LAA) on density-mask computed tomography images and the diffusing capacity of the lungs for carbon monoxide (DLCO).

RESULTS

The MTT/T1/2 ratio in each lung in emphysema was significantly higher than that in other forms of COPD (1.60+/-0.74 vs. 1.21+/-0.26; P<0.01). In the selected unilateral lungs with similar T1/2 values, MTT values were also significantly higher in emphysema. MTT values in each lung showed a significantly closer correlation with the corresponding %LAA values compared with T1/2 values in emphysema (R=0.698, P<0.0001 vs. R=0.338, P<0.01; P<0.05); while only the T1/2 values showed a significant correlation in other forms of COPD (P<0.0001). In correlation with DLCO, MTT values showed a significantly closer correlation compared with T1/2 values in emphysema (R=0.909, P<0.0001 vs. R=0.555, P<0.001; P<0.05); while either value did not show a significant correlation in other forms of COPD.

CONCLUSION

MTT values are more critically affected in emphysema compared with other forms of COPD without significant alveolar destruction, and MTT and T1/2 values appear to be differently correlated with the regional extent of LAA between these two disorders. Direct comparison of regional T1/2 and MTT values on functional images may contribute to the demarcation of lung pathology of these two disorders.

Removal of high concentration of NH3 and coexistent H2S by biological activated carbon (BAC) biotrickling filter

High efficiency of NH3 and H2S removal from waste gases was achieved by the biotrickling filter. Granular activated carbon (GAC), inoculated with Arthrobacter oxydans CH8 for NH3 removal and Pseudomonas putida CH11 for H2S removal, was used as packing material. Under conditions in which 100% H2S was removed, extensive tests to eliminate high concentrations of NH3 emission-including removal characteristics, removal efficiency, and removal capacity of the system-were performed. The results of the Bed Depth Service Time (BDST) experiment suggested that physical adsorption of NH3 gas by GAC was responsible for the first 10 days, after which NH3 gas was biodegraded by inoculated microorganisms. The dynamic steady state between physical adsorption and biodegradation was about two weeks. After the system achieved equilibrium, the BAC biotrickling filter exhibited high adaptation to shock loading, elevated temperature, and flow rate. Greater than 96% removal efficiency for NH3 was achieved during the 140-day operating period when inlet H2S loading was maintained at 6.25 g-S/m3/h. During the operating period, the pH varied between 6.5 and 8.0 after the physical adsorption stage, and no acidification or alkalinity was observed. The results also demonstrated that NH3 removal was not affected by the coexistence of H2S while gas retention time was the key factor in system performance. The retention time of at least 65 s is required to obtain a greater than 95% NH3 removal efficiency. The critical loading of NH3 for the system was 4.2 g-N/m3/h, and the maximal loading was 16.2 g-N/m3/h. The results of this study could be used as a guide for further design and operation of industrial-scale systems.

The Two-Phase Water/Silicon Oil Bioreactor Prospects in Off-Gas Treatment

Research was carried out to develop a biphasic biologic reactor able to clean the gas effluents polluted by volatile organic compounds. Initially, Rhodococcus erythropolis T 902.1 was selected on the basis of its capacity to degrade isopropylbenzene (IPB). The effect of gas flow and IPB concentration on the biodegradation of IPB was evaluated. The results show that the use of silicon oil allows large quantities of IPB to be absorbed within the medium of biologic abatement. On the other hand, the biodegradation rate was directly correlated to the inlet flow of IPB. Thus, the reactor presents interesting opportunities for the biologic treatment of gas effluents.

A modified ICRP 66 iodine gas uptake model and its parametric uncertainty

Intakes via inhalation may occur from radionuclides released in the form of a gas. The chemical characteristics pertaining to the release influence the intake and subsequent dose to an exposed individual. Gases are taken up or absorbed in the entire respiratory tract and the associated uptake mechanisms are quite different from deposition of particulates. Gaseous iodine can exist in various chemical forms, e.g., elemental iodine, inorganic, and organic iodine compounds. These different chemical species play an integral role in the gaseous uptake o f iodine in t he respiratory tract. Gas uptake in the various regions of the respiratory tract results in the intake of iodinated material into the body. The radioactive iodine taken up in the gas-exchange tissues is absorbed into the bloodstream of an individual and subsequently transferred to other organs. Iodine in the circulatory system can then be taken up by the thyroid gland, with resulting dose to the thyroid. The magnitude and uncertainty in regional gas uptake is important in the assessment of individuals exposed to airborne releases of radioiodine. The current ICRP 66 model is rudimentary and estimates regional gas uptake based on solubility and reactivity of the different radionuclides entering the respiratory tract. The modified model proposed here employs methodology and a mathematical structure to determine estimates of fractional gas uptake rather than defaulting to literature values, as in the current ICRP model. Model parameters have been assigned input distributions and estimates of uncertainty have been determined. A sensitivity analysis of these parameters has been performed to demonstrate the importance of each of these parameters. The sensitivity analysis ranks the model-input parameters by their importance to estimates of regional gas uptake. The model developed herein may be used for improved estimation of gas uptake in the respiratory tract and subsequent dose estimates from the different chemical forms of radioiodine.

Development and operation of a trickling biofilter system for continuous treatment of gas-phase trichloroethylene

A parallel trickling biofilter (TBF) system that consists of two TBFs units in parallel, one for biodegradation of trichloroethylene (TCE) and the other for reactivation of an inactivated biofilm, was developed and operated for continuous treatment of gas-phase TCE by Burkholderia cepacia G4. For inlet loadings below 8.6 mg TCE l(-1) d(-1) complete removal of TCE was achieved. The maximal TCE elimination capacity was 17 mg l(-1) d(-1).

Shallow layer simulation of heavy gas released on a slope in a calm ambient. Part I. Continuous releases

Although much research considers heavy gas dispersion over flat ground, less is known about the physics of dense gas dispersion on a slope. Here, the appropriateness of shallow layer models for the simple case of releases over a slope in a calm ambient is assessed. This two-part paper assesses the value of shallow layer modelling using the established shallow layer model TWODEE [J. Hazard. Mater. 66 (3) (1999) 211; J. Hazard. Mater. 66 (3) (1999) 227; J. Hazard. Mater. 66 (3) (1999) 239] and the experimental results of Schatzmann et al. [M. Schatzmann, K. Marotzke, J. Donat, Research on continuous and instantaneous heavy gas clouds, contribution of sub-project EV 4T-0021-D to the final report of the joint CEC project, Technical Report, Meteorological Institute, University of Hamburg, February 1991]. Part I considers continuous releases, and part II considers instantaneous releases; both use the same model with the same entrainment coefficients. For continuous releases, cloud arrival times are generally well predicted, and cloud concentrations are generally correct to within a factor of two. Shallow layer models thus appear to be capable of physically accurate simulation of continuous releases over a slope in a calm ambient.

Shallow layer simulation of heavy gas released on a slope in a calm ambient. Part II. Instantaneous releases

This paper assesses the value of shallow layer modelling for instantaneous releases of heavy gas over a slope using the established computer model TWODEE [R.K.S. Hankin, Heavy gas dispersion over complex terrain, Ph.D. thesis, Cambridge University, 1997; J. Hazard. Mater. 66 (1999) 211; J. Hazard. Mater. 66 (1999) 227; J. Hazard. Mater. 66 (1999) 239] and the experimental results of Schatzmann et al. [M. Schatzmann, K. Marotzke, J. Donat, Research on continuous and instantaneous heavy gas clouds, Contribution of sub-project EV 4T-0021-D to the final report of the joint CEC project, Technical report, Meteorological Institute, University of Hamburg, February 1991]. This is the second of a two-part paper; part I considered continuous releases using the same model, using the same entrainment parameters. Schatzmann et al. carried out instantaneous releases of heavy gas over three slopes; each experiment was repeated five times under nominally identical conditions. The goodness-of-fit measures (GFMs) of Hanna et al. [Atmos. Environ. 27A (15) (1993) 2265] are generalized to account for the multiple releases carried out by Schatzmann et al. Using these statistical GFMs, predicted peak concentrations are generally correct to within a factor of two; and cloud arrival times are generally late.

Heavy gas dispersion: integral models and shallow layer models

Integral models for heavy gas dispersion approximate a dispersing cloud in terms of a small number of variables; each of these is ultimately a function of an independent variable which is usually time (instantaneous releases) or downwind distance (continuous releases). This type of model is used almost exclusively in risk assessment [HSE's risk assessment tool, RISKAT, in: Major Hazards: Onshore and Offshore, October 1992, pp. 607-638; Ann. Rev. Fluid Mech. 21 (1989) 317], but many distinct integral models exist. The code comparison exercise of Mercer et al. [CEA/AEA exchange agreement on external event. Comparison of heavy gas dispersion models for instantaneous releases: final report, Technical Report IR/L/HA/91/6, Health and Safety Laboratory, Sheffield, June 1991; J. Hazard. Mater. 36 (1994) 193] presented the results from a number of integral models in a common format; Mercer found that the range of predictions for some scenarios exceeded three orders of magnitude. Here, the TWODEE shallow layer model [J. Hazard. Mater. 66 (3) (1999) 211; J. Hazard. Mater. 66 (3) (1999) 227; J. Hazard. Mater. 66 (3) (1999) 239] is added to Mercer's code comparison exercise. The physical assumptions used in shallow layer models differ profoundly from those used in integral models and the implications of these differences for risk assessment are discussed. TWODEE was used to simulate four representative cases considered by Mercer. In terms of cloud averaged concentration (CAC) vs. centroid position, the present model gave predictions that were consistent with the integral models used by Mercer. As the model neglects horizontal diffusion for passive clouds, overprediction at large downwind distances was expected, but not generally observed.

Removal and destruction of high concentrations of gaseous toluene in a two-phase partitioning bioreactor by Alcaligenes xylosoxidans

A two-phase bioreactor consisting of hexadecane dispersed in an aqueous, cell-containing medium (organic fraction = 0.33) was used to trap toluene vapours from an air stream. The affinity for toluene by the solvent resulted in high efficiency of removal and transfer to the aqueous phase based on equilibrium transfer. The system was readily able to handle a loading capacity of 748 mg l(-1) h(-1) at a toluene degradation efficiency of greater than 98%.

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.