Toxic gas dispersion models: Can they predict protective action distances in case of a chemical spill?

Abstract

Emergency responders often use a gas dispersion model to estimate downwind airborne concentrations of a toxic chemical in case of a chemical spill accident. For protecting the public, a protective action distance from the spill source is established based on the distance where the toxic concentration drops below some level of concern. This distance is used as a basis for evacuation of the public from the area or for instructions to shelter-in-place. However, in real-world accidents, the responders neither know the amount of chemicals released into the air nor the duration of the release, and moreover, the concentrations of chemicals at any location will vary over time. Depending on what input information is put into the model, different results will be obtained. The problem of what input parameters to use for gas dispersion modeling is illustrated for a hypothetical 90-ton chlorine railcar accident, where the railcar is breached. Different answers for a protective action distance are obtained depending on whether the tables in the Emergency Response Guidebook or any of the popular gas dispersion models are used. Very different answers are obtained from any model depending on whether whole of the chemical is released at once as a gas or aerosol or whether the liquefied chlorine evaporates slowly inside a ruptured 90-ton railcar tank, and also the weather conditions. To avoid misunderstandings, people who use models to establish a protective action distance must also communicate the circumstances in which the models are used, eg, “worst possible what-if scenario,” etc, or “nighttime stable conditions,” or other situations.