Originally Posted by Av8tr1

Another poorly understood topic (and I am in no way a expert at any of this). My understanding is that radio activity of the fallout degrades by a significant amount shortly after the nuclear event. Within a few days it is safe to leave an area of contamination and not risk some of the worst side effects. The danger of radiation from fallout also decreases rapidly with time due in large part to the exponential decay of the individual radionuclides. This is known as the seven-ten rule. Cresson H. Kearny who was a well-known researcher on the topic said that for the first few days after the explosion, the radiation dose rate is reduced by a factor of ten for every seven-fold increase in the number of hours since the explosion. Supposedly "it takes about seven times as long for the dose rate to decay from 1000 roentgens per hour (1000 R/hr) to 10 R/hr (48 hours) as to decay from 1000 R/hr to 100 R/hr (7 hours)."

The idea behind the shelter in place for a few days after a nuclear event make the fallout survivable. It's the people out and exposed during the event or the people who try to evac shortly after the event who are at the greatest risk. Staying put in a sheltered area for a few days to as much as a week will greatly increase survivability. While it seems ridiculous to survive a nuclear bomb using plastic sheeting on windows and doors with duct tape there is science behind it.

But I am in no way an expert on this type of stuff.