median population within the vulnerable zone of 1,500.2 Damage to infrastructure and subsequent economic loss are more readily caused by the flammable and explosive properties of chemicals.

Some chemicals are significantly more hazardous than others. For example, it is possible to rank chemicals with toxic inhalation properties according to toxicity, by using, for example, the Department of Transportation Hazmat Tables, which list toxicities according to LD50.3 A compound such as methyl isocyanate has a much lower LD50 (i.e. is significantly more toxic) than, for example, chlorine. Properties of the specific chemicals involved are important to first responders attempting to mitigate a specific incident to prevent a catastrophic result. However, this report is concerned with identifying research and development that will help prevent any incident from crossing the threshold of catastrophic impact. Depending on actual event circumstances, which include many factors other than toxicity, chlorine can be of equal or of more concern than methyl isocyanate. As a result, above a certain threshold, the relative toxicity (or flammability or explosivity) of two chemical species becomes less relevant.

Whatever the specific chemical involved, all releases progress through a similar series of stages (e.g., release, transport, diffusion, exposure). The release consequences will be affected by the source (e.g., release rate, release duration, and toxicity), meteorology (wind speed, wind direction, atmospheric stability, precipitation), and population (e.g., population distribution and structural protection; response action) factors. Similar opportunities for emergency response and similar consequences (casualties, property damage, and environmental insult) are possible. This provides the opportunity for investments in science and technology development that mitigate


speed and direction. The exposed area is likely to be only 25 percent (one quadrant) of the total vulnerable zone at most, and probably less. The number of people exposed in such an event depends on the size of the impact area and the size of the population remaining in that area. This might be only a small fraction of the total number of people living in the vulnerable zone, especially if they are able to take prompt protective action such as evacuation or sheltering in-place.


Belke, J. 2001. Chemical accident risks in US Industry—A preliminary analysis of accident risk data from US hazardous chemical facilities. Proceedings of the 10th International Symposium on Loss Prevention and Safety Promotion in the Process Industries. Stockholm, Sweden: Elsevier Sciences.


Lethal dose 50 (LD50) is the dose at which 50 percent of an exposed animal model population dies.

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