The Emergency Response Guidebook (Part 2): Evacuate or shelter
A few winters ago my county ran a full-scale drill at the industrial park on the east side of town. A simulated rail release, a simulated chlorine cloud, and a simulated question that turned out to be the hardest one of the day. A school sat 800 meters downwind. The drill controllers gave the emergency managers two options: evacuate the school, or seal it. The room split, and the argument went on for twenty minutes. Evacuating put two busloads of kids on roads that crossed the plume’s path. Sealing put them in a building with single-pane windows and an old ventilation system nobody could shut off quickly.
That argument, in miniature, is the subject of an entire section of the Emergency Response Guidebook 2024 (the ERG), the US DOT guidebook every fire truck in the country carries. Part 1 of this series covered identifying what you are looking at. This part covers the decision that responders and emergency managers wrestle with, and that civilians get wrong in both directions: when to leave, and when to shelter in place. The uncomfortable truth from the guidebook is that evacuating into a toxic vapor cloud is worse than staying inside a sealed building.
Disclaimer: The information provided in this article is for general informational and educational purposes only. It is not intended as, and should not be considered, professional or emergency-response advice. Protective action decisions in a hazmat incident belong to trained emergency officials. Always follow the instructions of local authorities during an active event.
The Readiness Audit
If officials ordered a shelter-in-place for your neighborhood right now, could you execute it correctly in ten minutes?
- Green: You know what shelter in place means and the steps it involves (close up, shut off ventilation, seal, stay away from windows), you know vehicles are poor shelters, and you know the all-clear comes over broadcast media.
- Yellow: You would “stay inside,” but you would not think to shut down the HVAC, and you do not know which windows matter most.
- Red: You would get in the car and drive, because leaving feels like doing something.
If you are Yellow or Red, execute Phase 1 immediately.
Phase 1: Isolate First, Then Choose
Goal: Understand the step that comes before everything else, and what the two protective actions actually are.
The guidebook defines protective actions as the steps taken to preserve the health and safety of responders and the public during a release. But before either action comes a step the book calls the first step for any protective action that may follow: isolate the hazard area and deny entry. Everybody who is not directly involved in emergency response stays out of the isolation zone, including unprotected responders. Isolation establishes control over the area of operations.
Only then come the two choices:
- Evacuate means to move all people from a threatened area to a safer place. To perform an evacuation there must be enough time for people to be warned, get ready, and leave. If there is enough time, the book is direct: evacuation is the best protective action.
- Shelter-in-place means people seek shelter inside a building and remain inside until the danger passes. The book uses it when evacuating the public would cause greater risk than staying, or when evacuation cannot be safely performed.
The book’s evacuation sequence starts small: begin with people nearby and anyone outdoors in direct view of the scene. When help arrives, expand the evacuated area downwind and crosswind to at least the recommended extent. And here is the detail that changes how I read every evacuation order: even at the recommended distances, people may not be completely safe. The book says evacuees should be sent to a definite place, by a specific route, far enough away that they will not have to relocate again if the wind shifts.
The Takeaway: Isolate, then choose. Evacuation wins when there is time, shelter wins when leaving is the greater risk.
Phase 2: The Decision Factors
Goal: Learn the three groups of factors officials weigh, so the instructions you receive make sense.
The guidebook is careful to say that in some cases evacuation is best, in others sheltering-in-place is best, and sometimes the two are used in combination. The decision comes from evaluating three groups of factors:
The hazardous material itself:
- Degree of health hazard
- Chemical and physical properties
- Amount involved
- Containment and control of the release
- Rate of vapor movement
The population threatened:
- Location
- Number of people
- Time available to evacuate or shelter-in-place
- Ability to control the evacuation or the sheltering
- Building types and availability
- Special institutions or populations, such as nursing homes, hospitals, and prisons
The weather:
- Effect on vapor and cloud movement
- Potential for change
- Effect on evacuation or shelter-in-place
And the book adds a note that captures the whole discipline: officials must continue to gather information and monitor the situation until the threat is removed. The instruction you get can change as conditions change, and that is not indecision. That is the process working.
The Takeaway: The instruction you receive is the output of these factors. Trust the process, and expect updates.
Phase 3: The Decision Table
Goal: Learn the six-line table that decides the answer, because it is the most transferable knowledge in the whole guidebook.
The book’s table compares the two options directly. Read it as paired rows:
| Consider evacuation when | Consider shelter-in-place when |
|---|---|
| The vapors are flammable | The vapors are toxic, and people are likely to be exposed by evacuating |
| Buildings cannot be closed tightly | Buildings can be quickly sealed by closing all windows and ventilation systems |
| The vapors are continuously generated and will hug the ground, or will take a long time to clear | The vapors will quickly rise in the air column or rapidly dissipate |
| People are outdoors | People are already indoors |
| There are few people to evacuate | There are too many people to evacuate for the resources available |
| The threat seems stable but long-lasting | Circumstances are changing too quickly to evacuate safely |
The pattern underneath the table is worth naming. Flammable threats favor leaving, because you are running from a fire risk that no building holds back. Toxic vapor threats favor sealing, because moving people through the cloud exposes them to exactly the substance they are escaping. Building quality and time tip both columns, which is why the school in my county’s drill was such a hard case: it was a toxic scenario (favoring shelter) in a poor building (favoring evacuation).
The Takeaway: Flammable means move. Toxic and sealable means seal. Everything else is judgment from trained officials.
Phase 4: Executing Shelter-in-Place
Goal: Know the physical steps, because the book’s list is specific and the order matters.
When sheltering-in-place, the guidebook directs people inside to:
- Close all doors and windows.
- Shut off all ventilating, heating, and cooling systems.
- Stay far from windows to avoid shattered glass and projectile metal fragments in the event of fire or explosion.
- Seal cracks around doors, windows, and vents with duct tape or wet cloths.
- Tune in to local media and remain inside until told it is safe to leave by first responders or emergency authorities.
- Breathe through a wet cloth until an all-clear has been communicated.
The book also notes something responders are tasked with: it is vital for responders to maintain communications with sheltered-in-place people so they are advised about changing conditions. In plain terms, you are expected to be reachable and to stay informed. A charged phone and a way to hear broadcasts are part of this kit.
Caution: A vehicle is not a shelter. The guidebook allows that vehicles can offer some protection for a short period with windows closed and ventilation shut off, and then states it flatly: vehicles are not nearly as effective as buildings for in-place protection.
The Takeaway: The seal is the protection. Shutting off the ventilation system is the step people skip and the step that matters most.
Phase 5: When Evacuation Is Ordered
Goal: Leave correctly, because the book has opinions about how it is done.
- Move upwind and crosswind of the incident, not merely away from it. Downwind is the direction the plume is traveling.
- Do not gather at the edge of the recommended distance. The book says people at the recommended distances may not be completely safe and should not be permitted to gather there. Go to a definite place, by a specific route, far enough away that a wind shift does not force a second move.
- Expect the evacuated area to expand downwind and crosswind as help arrives. Leaving early beats being relocated twice.
- Take what the time allows. The book’s definition of evacuation assumes warning time is the scarce resource: people must be warned, get ready, and leave.
The Takeaway: An evacuation is a route problem, not a speed problem. The right route outperforms the fast car.
The Essential Kit Checklist
- The seal kit: Duct tape and plastic sheeting, stored where you can reach them in five minutes, for covering windows, vents, and door cracks.
- The wet-cloth habit: Every family member should know to grab wet cloths for breathing if irritants get indoors.
- The HVAC knowledge: Know where the fresh-air intake and the system switch are for your heating and cooling, so it can be shut off in one move. Do it today, it takes five minutes.
- The media plan: A charged phone, alert configuration, and a battery radio, since instructions and the all-clear come over broadcast channels.
- The two-route habit: For your home and workplace, know two evacuation routes heading in different directions, one of them crosswind-capable.
- The car honesty: Know that your car is a temporary refuge at best, useful only to get you out of the area, never to wait in.
Scenario Planner (Contingencies)
“Evacuation always feels safer than sitting still.” The trap: equating motion with control. The fix: the decision table exists because moving people through a toxic cloud exposes them to exactly the substance they are escaping. When officials order shelter-in-place, they are not being passive. They are applying the guidebook’s own decision table, and the table exists because the wrong choice costs lives.
“I will seal the whole house perfectly.” The trap: perfectionism under time pressure. The fix: the book’s list is a priority order, not a pass/fail exam. Doors and windows, ventilation off, away from windows, seal cracks, monitor media, wet cloth if needed. Follow the order, do the best you can, and do not skip the ventilation shutdown.
“My car has a good filter, I will ride it out in the driveway.” The trap: overestimating what a vehicle can do. The fix: the guidebook tested this question for us. Vehicles help for a short period and are not nearly as effective as buildings. If you are home, go inside and seal. If you are driving away from the area, keep driving, upwind or crosswind, and do not stop inside the plume.
Next Steps
Do these four things this week, in order:
- Read Part 3: Toxic gas distances to learn how the guidebook’s isolation zones are calculated, and why night changes everything.
- Do the HVAC drill today. Find your heating and cooling system’s fresh-air intake and shutoff, and practice killing it in one move. Five minutes, once.
- Build the seal kit. Duct tape and plastic sheeting into a five-minute-reach spot, and a battery radio beside it if you do not have one.
- Stock what sheltering assumes. A sealed house only works if you can stay in it, so make sure your basics are covered with the emergency supply kit.
Adapted from US DOT/PHMSA, Emergency Response Guidebook 2024 (public domain). Read the original: Emergency Response Guidebook 2024.