I've seen this in several of Jerry's stories and didn't know what it meant so I finally looked it up. In a nutshell: The "seven-ten" rule - For every sevenfold increase in time after the initial blast, there is a tenfold decrease in the radiation rate. For example, a 500 rad level can drop to 50R in 7 hours and down to 5R after 2 days (49 hours). In other words, if you have shelter with good shielding and stay put for even just 7 hours ... you've really increased your chances of survival. Your detection devices, emergency radio or cell phone [if the last 2 are working, that is] can assist you in knowing when it’s safe to come out.