An incident involving hazardous materials has a higher probability of causing an evacuation of an affected area than any other incident. By the very nature of the hazard, this type of evacuation often provides very little preparation time. Decisions will need to be made quickly, and citizens moved rapidly.
Levels of Evacuation:
Experience has reflected three levels of evacuation. Each requires a different resource commitment. They include: Site Evacuation, Intermediate Level Evacuation, and Large Scale Evacuation
Site evacuation involves a small number of citizens. This typically includes the workers at the site, and persons from adjacent occupancies or areas. The citizens are easily evacuated and collected upwind at the perimeter area. Evacuation holding times are typically short, generally less than an hour or two, and citizens are permitted to return to their businesses or homes.
Intermediate Level Evacuation:
The next level, or intermediate level, involves larger numbers of citizens and/or affects a larger area. This level affects off-site homes and businesses and normally affects fewer than 100 persons. Persons may remain out of the area for two to four hours or more. Evacuation completion times will be somewhat longer, but generally rapid. Collecting, documenting, and controlling the evacuees becomes more difficult. Off-site collection sites or shelter areas will need to be determined and managed. Some evacuees will leave the area on their own or be sent home by employers. Site perimeters become larger and perimeter security requires more resources. Close coordination with the Police Department and other agencies will be required.
Large Scale Evacuation:
A large or concentrated release of a hazardous substance may cause a large off-site evacuation. Thousands of citizens could be evacuated. Rapid initiation of the evacuation process may be required. Evacuees may be out of their homes and businesses for many hours if not days. Evacuation completion time frames will be extended. Evacuation shelters will need to be located, opened and managed. Documentation and tracking of evacuees becomes more important as well as more difficult to manage. Very close coordination with the police and multiple agencies will be required. Site and evacuation perimeters become extended and require much more resources to maintain. Security of the evacuated area becomes a concern. In most cases, the City's Emergency Operations Center (EOC) will be opened to support the evacuation and site operations.
Time Factor Considerations:
Time factors are also an important consideration in the evacuation decision. A rapidly developing moving toxic cloud will demand a more immediate size-up and quick decision making. Such forced speed of decision making often is made with less information than a slow moving event. Accuracy of information will also be limited. The speed of the developing hazard will dictate the speed of evacuation. Immediate evacuation will require more resources than a slower developing event. It will take time to complete the evacuation. The more people to be evacuated, and the distance between the occupancies to be evacuated, the more time required. The greater numbers needing evacuation will also require a greater resource commitment.
Decision to Evacuate:
The decision to evacuate needs to be considered quickly and early. Delays in initiating evacuations can expose greater numbers of the public to the hazardous product. An unnecessary evacuation should be avoided. However, once the hazard has been identified and verified, the process of deciding who, when, and how to evacuate should proceed quickly. In some cases, in-place sheltering (staying indoors) may provide adequate protection and should be a serious consideration in the decision making process.
Factors to consider when evaluating the evacuation need include:
- Product Toxicity (as a health hazard)
- Concentrations (before it becomes a health hazard)
- Length of Time Exposed
- Weather Conditions (temperature, humidity)
- Wind Direction (direction, speed)
- Wind Changes
- Predicted Weather Changes
- Distances From Site Requiring Evacuation
- Evacuation Risk to Public (bringing them outdoors)
- Infiltration Into Buildings
- Shelter Locations
- Transportation Needs and Availability
- Evacuation Time Factors
- Resources Required for Evacuation
- Concentrations of the population in the area