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Responders to spill accidents need to have at their fingertips a method of rapid prediction of chemical cloud dispersion behavior and other relative information to protect the public from death and injury during evacuation of the affected area. A survey of 123 chemical spill accidents in the US where evacuations occurred revealed that available mathematical models were not used to predict cloud dispersion behavior while the event was under way, although modeling was sometimes done after the fact. The evacuation distances based on a given downwind concentration (level of concern) vary considerably, depending upon the chemical released and upon atmospheric turbulence resulting from solar insolation and wind blowing across surface features. Examples are presented. The problem is complicated because available mathematical models and look-up tales often do not give the same answer under the same conditions, nor do authorities agree on the concentration levels representing a level of concern on which to base evacuations. The responders to chemical spills need to be aware of these limitations, recognizing that modeling is a tool and does not represent ultimate truth. Yet modeling is useful to responders and is required by law in the US in developing possible accident scenarios under risk managementmore » plans as part of the Clean Air Act. Western Research Institute, in surveying the problems, has determined a need to communicate the benefits of modeling in a format designed for responders and others in the decision-making processes. A system for estimating evacuation distances has been developed for a hand-held computer for field use. The major features are that the user inputs the chemical spilled, spill location, time of day, cloud cover, and source spill information; evacuation distances based on a level of concern are calculated in seconds. The system is designed for planners and emergency response personnel without special modeling or computer skills.« less

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