2.3 Evacuation and Shelter-in-Place Actions

2.3 Evacuation and Shelter-in-Place Actions

Introduction to Protective Actions

When hazardous materials are released into the environment, initial responders must decide how to protect the surrounding population from exposure. The two primary protective action options are evacuation and shelter-in-place (also known as in-place protection). Selecting the correct strategy requires a rapid assessment of the chemical's properties, the rate of release, weather conditions, the population at risk, and the time required to implement the action. Choosing the wrong strategy can increase exposure and casualties. Awareness-level personnel must understand the differences between these options, the criteria for selecting each, and how to apply the principles of time, distance, and shielding to protect both the public and themselves.

Evacuation Operations and Safe Routing

Evacuation is the systematic movement of people from a threatened area to a safe place. It is the most comprehensive method for protecting the public, but it requires significant time, resources, and coordination.

1. When Evacuation is Indicated

Evacuation is typically the preferred strategy under the following conditions:

• Long-Duration Incidents: If a leaking container cannot be easily patched, or if a fire is burning out of control and will release toxic products for hours or days, the public must be moved away.
• Fire or Explosion Hazard: If there is a high probability that a container will explode (such as a threatened LPG railcar) or if a toxic plume is accompanied by a massive fire.
• Available Lead Time: If the release is far enough away or the wind speed is low enough that there is sufficient time to warn and move the population before the toxic cloud arrives.

2. Evacuation Routing Rules

When directing an evacuation, routing is a matter of life and death. The public must be directed to move along routes that avoid the hazardous plume.

• Avoid the Plume: Evacuation routes must lead people upwind or crosswind. Responders must never route evacuees downwind of the spill.
• Crosswind Evacuation: In many urban environments, moving directly upwind is not possible due to road layouts. Moving crosswind (perpendicular to the wind direction) allows evacuees to quickly step out of the path of the narrow downwind plume.
• Command Post Coordination: Evacuees must be directed to a specific, designated assembly area in the Cold Zone where they can be registered, monitored for contamination, and housed.

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Shelter-in-Place (In-Place Protection)

Shelter-in-place is the practice of keeping people inside a building or structure until the danger passes. This strategy relies on the building's walls and sealed air volume to act as a barrier against toxic vapors or gases.

1. Key Implementation Steps

For shelter-in-place to be effective, residents must be instructed to:

1. Close and lock all windows and exterior doors (locking creates a tighter seal).
2. Turn off all heating, ventilation, and air conditioning (HVAC) systems, window fans, and fireplace dampers to prevent drawing contaminated air inside.
3. Move to an interior room with the fewest windows and doors, preferably on an upper floor if the toxic gas is heavier than air.
4. Seal cracks around doors, windows, and vents using plastic sheeting and duct tape if available.
5. Remain inside and monitor emergency broadcasts (radio, TV, or mobile alerts) for instructions on when it is safe to exit.

2. When Shelter-in-Place is Safer Than Evacuation

Sheltering in place is selected when evacuating would actually expose people to higher concentrations of the hazardous material.

• Short-Duration Releases: If a gas cylinder releases its contents in a few minutes, the plume will pass quickly. Evacuating people would force them outdoors directly into the moving cloud, whereas remaining inside keeps them protected.
• High-Toxicity Gases with Rapid Onset: If a highly toxic gas (like chlorine or anhydrous ammonia) is released suddenly, there is no time to organize an evacuation. The air outdoors is lethal, making the indoors the only survivable option.
• Vulnerable Populations: Evacuating a hospital, nursing home, or prison is extremely difficult and dangerous. The physical toll of moving intensive care patients or frail elderly residents may cause more deaths than keeping them sheltered indoors with the HVAC systems shut off.
• Vapor Plume is Already Present: If the toxic cloud has already enveloped the area, residents attempting to leave their homes would walk directly into a high-concentration chemical vapor.

Evacuation vs. Shelter-in-Place Decision Matrix

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Time, Distance, and Shielding for Self-Protection

While initial responders work to protect the public, they must also ensure their own safety. The classic radiation protection principles of Time, Distance, and Shielding apply to all hazardous materials responses.

1. Time

Minimizing the time spent near the hazard area reduces the cumulative dose or exposure to toxic chemicals, heat, or radiation. Awareness-level responders must complete their initial size-up and perimeter setup as quickly as possible and then withdraw to the outer edge of the Cold Zone.

2. Distance

Maximizing the distance between the responder and the hazard is the most effective safety measure. As distance increases, the concentration of toxic gases decreases due to atmospheric dispersion, and the intensity of thermal radiation or blast pressures drops exponentially. Awareness-level personnel must stay as far away as possible while maintaining sight of the scene.

3. Shielding

Shielding involves placing a physical barrier between the responder and the hazard. Examples of shielding include:

• Physical structures: Concrete walls, buildings, or large earthen berms that block blast waves, flying shrapnel, or thermal radiation.
• Vehicles: Using an emergency vehicle as a shield, though vehicles do not provide protection against toxic vapors or heavy explosions.
• Terrain: Positioning behind a hill or depression (if the vapors are heavier than air and will not settle there) to block exposure.