Application of source term and dispersion models in developing Protective Action Distances in the Emergency Response Guidebook

This presentation discusses statistical risk assessment methods used to develop Protective Action Distances in the US Department of Transportation Emergency Response Guidebook (ERG). The ERG is developed jointly by the USDOT, Transport Canada, and the Secretariat of Communications and Transportation of Mexico (SCT). The purpose of the ERG is to assist first responders in identifying whether a shipment is potentially hazardous and what actions should be taken in the event of a transportation accident. Using proper shipping name and the 4-digit United Nation ID Number, the ERG provides information concerning fire fighting and mitigation strategies as well as potential health effects. Protective Action Distances are provided for a subset of these chemicals that are Toxic by Inhalation (TIH) or which react with water to produce TIH gases.

The Protective Action Distance (PAD) is the downwind distance to which persons should be either evacuated or sheltered-in-place depending on the nature of the population at risk and severity of the incident. The methodology we employ for specifying PADs balances the need to adequately protect the public from exposure to potentially harmful substances against the risk and expense of over reaction. We quantify this balance using a Level of Protection, defined as the percentage of time that the listed distance will be sufficient for the chemical, time of day and spill size involved. A 90% Level of Protection has been selected by Department of Transportation (DOT) for the ERG. Quantitative analysis of the Level of Protection requires a statistical approach. To these ends, we employ a statistical accident scenario analysis to develop a set of several million hypothetical incidents based on the best available statistical information. This set of incidents takes into account the variability in container type, incident type, accident severity (i.e., release amount), location, time of day, time of year, and meteorology. Each of these scenarios is analyzed using detailed emission rate and atmospheric dispersion models to calculate a chemical concentration footprint. The safe distance for that incident is defined as the furthest downwind distance that a threshold chemical concentration or health criteria is exceeded. The Acute Exposure Guideline Level 2 (AEGL-2) or equivalent defines the health criteria. The statistical sample of safe distance values for all incidents considered in the analysis is separated into day and night, and into small spills and large spills. The 90th percentile safe distance values for each of these groups become the PADs appearing in the Table. The presentation will discuss the general methodology for PAD estimation, as well as the data and transportation risk assessment models used including experimental studies on water reactive materials.