Assume that a one Megaton Bomb blast occurs and you are sheltered or far enough away to survive the blast. If fallout of one thousand Rads arrives at your location 1 hour after the blast, you have to be sheltered. A four hundred and fifty Rad accumulative dose can kill you, so one-half hour of unsheltered exposure can be fatal. If you have shelter and are using the seven – ten rule you will know that after seven hours the outside rate will drop to 100 Rads per hour. In another seven times seven hours or forty-nine hours, it will have decayed down to 10 Rads per hour. Then using the rule of seven times forty-nine hours which equals approximately two weeks it will be down to one Rad per hour. You need to stay sheltered until it drops to one-half Rad per hour, and that takes about twenty-five days total.
If you are lucky and the Rate is only 10 Rads per hour in your area, then seven hours after the blast it is down to one rad. Forty-nine hours after the rate is down to one tenth of a Rad and you can leave the shelter.
At this point you may want to review my post http://bit.ly/JI67kV on Protection Factors in Nuclear Emergencies. This will explain how shelter reduce your rates of exposure.
A definition of a Rad (radiation absorbed dose)
The rad is a unit used to measure a quantity called absorbed dose. This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material. One rad is defined as the absorption of 100 ergs per gram of material. The unit rad can be used for any type of radiation, but it does not describe the biological effects of the different radiations.
I will post more on this subject in the future.