Fallout shelter: where to shelter after a nuclear explosion

What fallout is, and why shelter works

Ready.gov explains that a nuclear device detonated on the ground can suck material up into the air and create dangerous, sand-like radioactive particles called fallout. Those particles drift downwind and settle. A fallout shelter is simply a place with enough mass between you and the outside, walls, earth and floors, to block most of that radiation while it decays.

Time is on your side. Ready.gov says radiation levels decrease rapidly, becoming significantly less dangerous, during the first 24 hours. That's why the guidance is to get into the best shelter you can reach quickly and stay there, rather than to travel through the fallout.

Get inside: where to shelter

Ready.gov says that after a detonation you will have 10 minutes or more to find an adequate shelter before fallout arrives. If a multi-story building or a basement can be safely reached within a few minutes, go there immediately. The safest buildings have brick or concrete walls.

Inside, put as much building between you and the outside as you can: Ready.gov names a basement or the center of a large building as the most protective locations. If you're warned that a radiation hazard is possible, it says to get inside the nearest building immediately and move away from windows.

• A basement
• The center of a large building
• A building with brick or concrete walls
• Away from windows

Stay inside: the first 24 hours

Ready.gov says to remain in the most protective location, the basement or center of a large building, for the first 24 hours unless you're threatened by an immediate hazard, such as fire. If possible, turn off fans, air conditioners and forced-air heating that bring in outside air, close windows and doors, and close fireplace dampers. Pets and service animals should come inside too.

That's why the supplies matter. Ready.gov's list for radiation emergencies includes a spare change of clothes for each family member, a trash bag for contaminated clothing, and sealed food, water and medicine. Our emergency kit builder and water storage pages cover the basics.

If you were outside: decontaminate

If you were outside during the release, Ready.gov says to remove your outer layer of contaminated clothing, which can remove up to 90% of radioactive material, carefully so radioactive dust doesn't shake loose, and to avoid touching your eyes, nose and mouth. Then shower or wash with soap and water to remove particles from skin or hair that wasn't covered.

If you can't wash, wipe uncovered skin and hair with a wipe or a clean wet cloth. Ready.gov says not to use household cleaning wipes on your skin, and that hand sanitizer does not protect against radioactive material.

Stay tuned, and plan now

Ready.gov's third step is to follow instructions from emergency response officials. If you're told to evacuate, listen for routes, shelters and procedures, and if you've left, don't return until local authorities say it's safe. A battery or crank weather radio keeps you informed if the power and cell networks are down.

Planning ahead is mostly ordinary preparedness: know the best shelter spots at home, at work and at school, identify an out-of-state contact, and keep a kit with water, food and medicine. Ready.gov also suggests learning the shelter and evacuation plans of the schools, workplaces and care facilities your family uses.