Chapter 15 · Nuclear and radiological emergency
15.5 How long to stay: the 7-10 rule
Fallout "burns itself out." FEMA notes that it gives off more than half its energy in the first hour and then keeps decaying rapidly. The 7-10 rule is a classic rule of thumb (Glasstone and Dolan, The Effects of Nuclear Weapons; Health Physics Society): each time the time since the explosion is multiplied by 7, the radiation rate is divided by 10.
| Time since the explosion | Approximate radiation rate (1 hour = 100%) | What it means |
|---|---|---|
| 1 hour | 100% | Reference point. Maximum danger in fallout areas. |
| 2 hours | ≈ 44% | Already dropped to less than half. |
| 3 hours | ≈ 27% | |
| 7 hours | ≈ 10% | Dropped 90%. |
| 12 hours | ≈ 5% | |
| 24 hours | ≈ 2% | That is why the minimum recommendation is 24 hours under cover. |
| 49 hours (2 days) | ≈ 1% | Dropped 99%. |
| 343 hours (2 weeks) | ≈ 0.1% | Dropped 99.9%. |
The intermediate values (2, 3, 12, and 24 hours) were calculated with the same decay law the rule is based on. It is an approximation for planning: the actual level in your area depends on the size of the weapon, the wind, and rain, and only response teams can measure it. That is why "stay tuned" is part of the instruction.
