What to do in the first 24 hours
This page summarizes public guidance from US agencies. It is not advice from this site or its map, and it is not a substitute for the official sources: Ready.gov and the CDC radiation emergencies pages.
1. Get inside
After a nuclear explosion, fallout takes time to arrive. Ready.gov advises that there are typically 10 minutes or more, and longer the further away you are. Use that time to get into the best shelter you can reach quickly.
- Go into the nearest solid building. Brick and concrete are much better than wood. A vehicle offers very little protection.
- Put as much heavy material as possible between you and the outside: go to a basement, or to the center of a large building, away from windows and the roof.
- Do not try to drive away from an approaching fallout cloud unless officials tell you to. Being caught in the open or in traffic is far worse than sheltering in a solid building.
2. Stay inside
Fallout radiation is at its worst in the first hours and falls off quickly. After 7 hours it is down to about a tenth of what it was at 1 hour; after two days, about a hundredth.
- Plan to stay sheltered for at least 24 hours unless authorities give other instructions.
- Families who are apart should stay where they are rather than travel to reunite. Going outside exposes you to the most dangerous radiation.
- Close windows and doors, and turn off fans and ventilation that draw in outside air.
3. Stay tuned
Phone networks and the internet may not work. A battery-powered or hand-crank radio is the most reliable way to receive official instructions about when it is safe to leave and which direction to go.
If you were outside when fallout arrived
- Take off your outer layer of clothing as soon as you are inside. This can remove most of the radioactive material on you. Seal the clothing in a bag and put it away from people and pets.
- Shower or wash with soap and water if you can. Wash your hair with shampoo, but do not use conditioner, which can bind radioactive material to hair.
- If you cannot wash, wipe exposed skin with a clean wet cloth.
Food, water and medicine
- Food in sealed containers and food that was indoors is safe to eat. Bottled water and water in sealed containers is safe to drink. Wipe containers before opening them.
- Potassium iodide (KI) protects only the thyroid gland, and only against radioactive iodine. It does not protect against other radiation. Take it only if public health officials tell you to.
How this site fits in
Fallout Weather is for learning beforehand how wind shapes fallout. It will not tell you that something has happened, it may not be reachable if it does, and its map should not be used to decide what to do in an emergency. The guidance above comes from the agencies below, not from this map.
Sources
- Ready.gov: Radiation Emergencies (get inside, stay inside for 24 hours, time before fallout arrives).
- CDC: Get Inside (choosing shelter).
- CDC: Self-decontamination (removing clothing and washing).
- CDC: Potassium Iodide (what KI does and does not protect against).
- CDC: Food, drinking water and medicine safety.
- FEMA: Planning Guidance for Response to a Nuclear Detonation (sheltering versus evacuating).
- Glasstone and Dolan, The Effects of Nuclear Weapons (1977) (how fallout radiation decays).
Last reviewed November 13, 2026.