How long to stay inside after nuclear fallout
Stay inside for at least 24 hours after nuclear fallout, then keep sheltering until officials say it is safe or direct you to evacuate. You do not need to hide for months: fallout radiation falls to about 10% of its one-hour level after 7 hours, and to about 1% after two days.
This page is general information, not official advice; instructions from your local authorities always come first.
How long do officials say to stay inside?
Ready.gov and the CDC both say to remain sheltered for at least 24 hours, unless there is an immediate danger or authorities tell you otherwise.
The right length of time depends on where the fallout lands, which you cannot reliably judge from the ground. FEMA's planning guidance expects people in less-affected areas to shelter for 12 to 24 hours. People in the most heavily affected areas may be told to wait longer before using protected escape routes.
So 24 hours is a minimum, not a countdown. After the first day, the next step is whatever official broadcasts tell you.
What is the 7:10 rule?
The 7:10 rule is a rule of thumb for how fast fallout weakens. Every time the time since the detonation multiplies by 7, the radiation level drops by a factor of 10.
| Time after detonation | Radiation level compared with 1 hour after |
|---|---|
| 1 hour | 100% |
| 7 hours | about 10% |
| 2 days (49 hours) | about 1% |
| 2 weeks | about 0.1% |
Fallout also gives off more than half of its total radiation energy in the first hour. That is why the first hours matter so much, and why sheltering works.
Why can't I leave when it looks safe outside?
Radiation is invisible. Fallout particles can look like sand, grit, dust or ash, but a street that looks normal may not be safe.
Short trips inside the building, to a bathroom or kitchen, are fine. Spend most of your time in the best-protected spot, especially during the first several hours when radiation is highest. See the best place to shelter for where that is.
When is leaving early justified?
Leave before the 24 hours are up only if:
- Officials tell you it is safe, or tell you to evacuate along a specific route.
- An immediate danger forces you out. That means fire, a gas leak, structural collapse, or a medical emergency that cannot wait.
Collecting family is not on that list. Schools and workplaces have shelter plans, and going outside exposes you and them.
If you must leave because of an immediate danger:
- Move to the nearest other solid building, not into the open and not to your car.
- Cover your nose and mouth and as much skin as you can.
- Move quickly and keep your time outside short.
- Take off your outer clothing and wash when you arrive.
What if my shelter is poor?
Official guidance is still to stay where you are. Research by Lawrence Livermore National Laboratory does note one exception: if you are in a very poor shelter, such as a car, a mobile home or a single-storey wooden building with no basement, and a much better one is only a few minutes away, moving can reduce your total dose.
The best time to make that move is before fallout arrives, in the first 10 minutes or so. Once fallout has arrived, move only if officials advise it or there is an immediate hazard.
What happens after the first 24 hours?
Authorities may ask some areas to keep sheltering and others to evacuate in an organised way. If you are told to go:
- Listen for the route. Officials direct people away from the detonation and the heaviest fallout.
- Cover up with long sleeves, long trousers, a hat, gloves and a mask.
- Take your go-bag, medication, documents and pets.
- Follow the official route even if it seems longer.
- Don't return until authorities say it is safe.
Power, water and shops may be disrupted for much longer than the shelter period, so keep supplies at home. The full sequence from the first minute is in how to survive a nuclear attack.
Sources
- Ready.gov: Radiation Emergencies
- CDC: What to Do - Get Inside
- FEMA: Planning Guidance for Response to a Nuclear Detonation, 3rd edition
- Lawrence Livermore National Laboratory: Key Response Planning Factors for the Aftermath of Nuclear Terrorism
- Glasstone and Dolan, The Effects of Nuclear Weapons, 3rd edition, 1977, chapter 9 (the 7:10 rule)
Questions
How long should you stay inside after a nuclear bomb?
At least 24 hours, according to Ready.gov and the CDC, unless there is an immediate danger such as fire, a gas leak or building collapse, or officials tell you otherwise. After that, keep sheltering until authorities say it is safe or give you an evacuation route.
How long does nuclear fallout last?
Fallout is most dangerous in the first hours and weakens quickly. Under the 7:10 rule of thumb, the radiation level is about 10% of its one-hour level after 7 hours, about 1% after two days and about 0.1% after two weeks. When a particular area is safe is a decision for the authorities.
Do you need to stay in a shelter for two weeks?
Not necessarily. Official guidance is to shelter for at least 24 hours and then follow instructions, which may mean staying longer in some areas or an organised evacuation in others. Supplies for two weeks are still useful because power, water and shops can be disrupted long after the shelter period.
