Science
How long to stay in a shelter after a nuclear detonation: the 7:10 rule
What fallout is, how quickly it arrives, how fast it decays under the 7:10 rule, and why 48 hours is the minimum and two weeks is the planning figure.
What fallout is
When a nuclear weapon bursts at or near the ground, it pulls soil and debris into the fireball. That material becomes radioactive, rises in the cloud and falls back to earth as fine particles, sometimes visible as dust or grit. This is fallout. It is the main radiation hazard for people outside the immediate blast area, and the wind can carry it tens to hundreds of kilometres.
Fallout gives off gamma radiation, which penetrates walls and roofs. That is why shelter is about mass between you and the particles settling outside, and why sealing the air matters: you do not want particles inside with you.
How quickly it arrives
Close to a detonation, fallout can begin arriving within 15 to 30 minutes. Further away it can take hours. You may not be able to see or smell it, so official alerts and radiation meters matter more than your senses. The practical rule is simple: if an alert comes, go inside immediately and stay inside.
The 7:10 rule
Fallout is a mixture of many radioactive substances. Most decay very quickly, so the radiation level drops fast at first and then more slowly. Civil defence planners summarise this with the 7:10 rule of thumb: for every sevenfold increase in time after the detonation, the radiation rate falls about tenfold.
| Time after detonation | Radiation rate (vs 1 hour) | What it means |
|---|---|---|
| 1 hour | 100% | The most dangerous period. Be sealed inside. |
| 7 hours | 10% | Still dangerous in heavy fallout areas. |
| 49 hours (about 2 days) | 1% | The usual minimum time to stay sealed. |
| 2 weeks | 0.1% | Short, measured trips outside may become possible. |
| About 14 weeks | 0.01% | Recovery in most areas. |
The rule is an approximation that works best in the first weeks. It is a planning tool, not a replacement for measurement.
Why two weeks is the planning figure
Most of the total radiation dose a person outside would receive arrives in the first days. Staying sheltered for at least 48 hours avoids most of it. By two weeks, levels have typically fallen by about 99.9% from the one-hour level, and short trips outside to fetch water or help others may be possible, guided by measurements and official advice. That is why our smallest shelter class, Haven, is designed for 14 days sealed. Longer autonomy covers repeated events, disrupted supplies and slow official guidance. See shelter classes.
What changes the picture
- Burst height. A high air burst produces much less local fallout than a ground burst.
- Weather. Wind direction sets where fallout lands. Rain can wash it out in concentrated spots.
- Distance. Further away, fallout arrives later and is usually thinner, but it is not zero.
- Shielding. The protection factor of your shelter decides what fraction of the outside dose reaches you. See radiation shielding.
Deciding when to come out
Leave based on readings and official guidance, not on the calendar alone. Every Qilyx shelter has radiation meters inside and outside, and the written protocol sets out how to read them, how to plan short trips and how to decontaminate on return. See the emergency protocol.
If you have no shelter
Go inside the most solid building you can reach, to a basement or the centre of the building away from windows and the roof. Stay there for at least 24 hours unless told otherwise, and listen for official instructions. Remove and bag outer clothing if you were outside after the event, and wash skin and hair. Even ordinary buildings cut exposure substantially, and the first hours matter most.
Further reading
- Cresson H. Kearny, Nuclear War Survival Skills (Oak Ridge National Laboratory, updated 1987), chapter on fallout decay.
- NDMA, National Disaster Management Guidelines: Management of Nuclear and Radiological Emergencies (2009).
This guide is general information, not engineering or safety advice for a specific site. Follow official instructions from the Government of India and NDMA in any emergency.