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Time, Distance & Shielding
There are four factors that determine the amount of radiation reaching your body from fallout:
- The time that has passed since the explosion.
- The length of time you are exposed to fallout.
- The distance you are from ground zero.
- The shielding between you and the fallout.
Time
The radioactivity in fallout weakens rapidly in the first hours after an explosion. This weakening is called "decay". After seven hours, fallout has lost about 90% of the strength it had one hour after the explosion. After two days it has lost 99%; in two weeks 99.9% of its strength is
gone. Nevertheless, if the radiation at the beginning were high enough, the remaining 0.1% could be dangerous.
Radiation must be measured by special instruments handled by speciallytrained personnel. If you remain in a shelter during the first days following an explosion, you escape the strongest radiation. If possible, you should stay in the shelter until radiation has been measured and you have been notifi ed that it is safe to come out.
Distance
The strength of radiation reaching your body is reduced the farther you are from the fallout. Here are some illustrations of the safest place to be when you are in various kinds of buildings.
Shielding
The most effective protection is to place some heavy material between yourself and the fallout. The heavier the material, the better the protection. Many common materials give excellent protection. The following materials will stop 99% of radiation:
- 16 inches of solid brick
- 16 inches of hollow concrete blocks filled with mortar or sand
- 2 feet of packed earth; 3 feet if loose
- 5 inches of steel
- 3 inches of lead
- 3 feet of water
