Dispersion of toxic substances

Acute Exposure Guideline Levels (AEGLs - time of exposition 10, 30 minutes 1, 4 and 8 hours). Effect of toxic substances dispersed in the air on population for different exposure times and for different severity levels of toxic effects (three severity levels of toxic effects AEGL 1,2, 3). (ERA 2001)... 

The evaluation was concentrated on the release of toxic substance followed by dispersion into the atmosphere. The possibility of formation of toxic product from combustion was not considered. The limits that are used as decision criteria (which evaluate only impact ranges) - as in selective method IAEA TECDOC-727 (IAEA 1996) and in other methods of risk analysis - were not considered as well. If the limits were found in the safety documentation, the following criteria were taken into account, too ... 

TXDS Acute Toxic Concentration. Some states have their own exposure guidelines. For example, the New Jersey Department of Environmental Protection (NJ-DEP) uses the Toxic Dispersion (TXDS) method of consequence analysis for the estimation of potentially catastrophic quantities of toxic substances as required by the New Jersey Toxic Catastrophe Prevention Act (TCPA) (Baldini and Komosinsky, 1988). An acute toxic concentration (ATC) is defined as the concentration of a gas or vapor of a toxic substance that will result in acute health effects in the affected population and one fatality out of 20 or less (5% or more) during a 1 hr exposure. ATC values as proposed by the NJ-DEP arc estimated for 103 extraordinarily hazardous substances, and are based on the lowest value of one of the following ... 

Loss of containment. The hazards belonging to this category are concerned with the release and dispersion of dangerous substances. Two examples of these hazards are release of flammables and release of toxic material. 

This recommended practice is intended to apply to faciUties that (/) handle or store flammable or explosive substances in such a manner that a release of ca 5 t of gas or vapor could occur in a few minutes and (2) handle toxic substances. The threshold quantity for the toxic materials would be determined using engineering judgment and dispersion modeling, based on a potential for serious danger as a result of exposures of <1 h. 

S Dispersion of a toxic substance +++ +++ + Pt this depends on the molecular structure... 

Dispersion models describe the airborne transport of toxic materials away from the accident site and into the plant and community. After a release the airborne toxic material is carried away by the wind in a characteristic plume, as shown in Figure 5-1, or a puff, as shown in Figure 5-2. The maximum concentration of toxic material occurs at the release point (which may not be at ground level). Concentrations downwind are less, because of turbulent mixing and dispersion of the toxic substance with air. 

Design and operation of sewers affect sewer processes, and, what is considered important in this context, knowledge of sewer processes can be actively considered in the design and operation of a sewer network. The type of sewer determines, to a great extent, if aerobic or anaerobic processes proceed. Furthermore, ventilation of sewers may affect the buildup and dispersion of odorous and toxic substances produced by microbiological processes. 

The 1990 CAAA required EPA to promulgate regulations to prevent the accidental release of substances that could cause death, injury, or serious adverse effects to human health or the environment. Congress directed EPA to regulate at least 100 substances and to take into account several factors when developing a chemical list, including toxicity, reactivity, volatility, dispersibility, combustibility, or flammability of the substance, and amount of the substance. ... 

This acclimation phase may be of considerable public health or ecological significance because the compound is not destroyed. Hence, the period of exposure of humans, animals, or plants is prolonged, and the possibility of an undesirable effect increased. Furthermore, if the pollutant is present in flowing waters above or below ground, it may be widely disseminated laterally or vertically because of the lag of detectable biodegradation. In the case of toxicants, such increased dispersal may result in the exposure of susceptible species at distant sites before the harmful substance is destroyed. 

It is operationally difficult to distinguish between dissolved and colloidally dispersed substances. For example, colloidal metal-ion precipitates occasionally have particle sizes smaller than 100 A, sufficiently small to pass through a membrane filter, and organic substances can exist as a stable colloidal suspension. Information on the types of species encountered under different chemical conditions (type of complexes, their stabilities, rate of formation) is a prerequisite to better understanding of the transformation in properties of toxic chemicals in a water body. 

Botulinum toxin is the most toxic substance known. One gram of crystalline toxin adequately dispersed can kill a population of a million people, so its use in bioterrorism is a possibility. The toxin can be introduced through inhalation or ingestion but not through dermal... 

Generally a toxic substance is in solution or mixed with other substances. A substance with which the toxicant is associated (the solvent in which it is dissolved or the solid medium in which it is dispersed) is called the matrix. The matrix may have a strong effect on the toxicity of the toxicant. 

Among several analytical methods for the prediction of movement of dissolved substances in soils, one model (Leij et al., 1993) was developed for three-dimensional nonequilibrium transport with one-dimensional steady flow in a semi-infinite soil system. In this model, the solute movement was treated as one-dimensional downward flow with three-dimensional dispersion to simplify the analytical solution. Another model (Rudakov and Rudakov, 1999) analyzed the risk of groundwater pollution caused by leaks from surface depositories containing water-soluble toxic substances. In this analytical model, the pollutant migration was also simplified into two stages predominantly vertical (one-dimensional) advection and three-dimensional dispersion of the pollutants in the groundwater. Typically, analytical methods have many restrictions when dealing with three-dimensional models and do not include complicated boundary conditions. 

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