Atmospheric dispersion, effect exposure

Many sophisticated models and correlations have been developed for consequence analysis. Millions of dollars have been spent researching the effects of exposure to toxic materials on the health of animals the effects are extrapolated to predict effects on human health. A considerable empirical database exists on the effects of fires and explosions on structures and equipment. And large, sophisticated experiments are sometimes performed to validate computer algorithms for predicting the atmospheric dispersion of toxic materials. All of these resources can be used to help predict the consequences of accidents. But, you should only perform those consequence analysis steps needed to provide the information required for decision making. 

If linear (dose) models without thresholds are to be used for carcinogen (or other) risk assessment, estimation of exposure at specified levels becomes irrelevant to risk assessment or, at least, its use is nonintuitive. For example, a carcinogen risk analysis may be based on a linear, nonthreshold health effects model. The total health risk would thus be proportional to the long-term exposure summed for all affected people for the identified period, and exposure of many people at low concentrations would be equivalent to exposure of a few to high concentrations. The atmospheric dispersion that reduces concentrations would also lead to exposure of more people therefore, increments... 

Occupational and toxicological studies have demonstrated adverse health effects from exposure to toxic contaminants. Emissions data from stationary and mobile sources are used in an atmospheric dispersion model to estimate outdoor concentrations of 148 toxic contaminants for each of the 60,803 census tracts in the contiguous United States for 1990. Approximately 10% of all census tracts had estimated concentrations of one or more carcinogenic HAPs at a greater than l-in-10,000 risk level. Twenty-two pollutants with chronic toxicity benchmark concentrations had modeled concentrations in excess of these benchmarks, and approximately 200 census tracts had a modeled concentration 100 times the benchmark for at least one of these pollutants. This comprehensive assessment of air toxics concentrations across the United States indicates hazardous air pollutants may pose a potential public health problem (Woodruff et al., 1998). 

PCSI materials used in the context of atmospherically dispersed materials are sometimes classified as stemutators if their main action is on the upper respiratory tract, and lacrimators if the principal action is on the eye. For most currently used PCSIs, this is not a useful classification descriptor, since both effects are frequently present. Thus, exposure to an airborne PCSI RCA will cause effects in skin, eye and respiratory tract. When dispersed in solution, the effects are generally limited to the area of contact, and the reflexes elicited are a function of the afferent nerve involvement. In general, PCSI effects appear within seconds of contact, and subside within 10-60 min, depending on exposure concentration and site affected. 

The consequences of accidents are expressed in terms of dose, which can be calculated using standard atmospheric dispersion and dose calculation software, such as those based on the Canadian standard (CSA, 1991). Stochastic doses are usually expressed in terms of the effective and thyroid doses for emergencies that involve the release of radioactive iodine. For emergencies that do not involve iodine, the critical organ dose would be based on the main radionuclide most likely to be released. For the calculation of deterministic health effects, it is important to use organ-specific equivalent doses because the effective dose concept is not applicable to doses this high. It is also important to consider the rate at which the dose is received because the thresholds for deterministic effects vary with the exposure rate. 

The damage caused by the atmospheric dispersion of radioactive material may be the result of ln<]iury of people or the contamination of property. The Injury type of damage may be further categorized as those Injuries which result from direct exposure to the cloud of contaminants (direct effects) and those which result from material deposited on the ground (Indirect or residual effects). 

Exposures to chemicals, resulting in toxic effects or oxygen-deficient atmospheres, may arise in a variety of industrial situations. A summary of common sources is given in Table 5.18 clearly this is not exhaustive since exposure may result whenever materials are mixed, machined, heated, dispersed or otherwise processed or used. 

Many measurements were made throughout the world during the period of atmospheric nuclear testing, and much is known of the release, dispersion, and deposition of radionuclides and the doses resulting from this practice. Exposures of the world population have been evaluated by the United Nations Scientific Committee on the Effects of Atomic Radiation. From this information the deposition and doses from individual tests or from a specific test series may be inferred. 

Gas dispersion in atmosphere boundary layer is strongly dependent on weather conditions, it means on atmosphere stability and on wind speed and direction. Bandwidth exposure assessment is compUcated by large difference in concentration value, because the reach of effects may vary about more than one place value. This is why the occurrence frequency of single situations must be respected by bandwidth assessment. 

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