7:10 Rule of Thumb
 

Taking some free FEMA college credit courses

Found this gem in one of the early courses on radiation
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<!-- #BeginEditable "PageContent" --> From the exposure rate determined by a survey instrument, future exposure rates may be predicted from a basic rule known as the "7:10 Rule of Thumb."
The 7:10 Rule of Thumb states that for every 7-fold increase in time after detonation, there is a 10-fold decrease in the exposure rate. In other words, when the amount of time is multiplied by 7, the exposure rate is divided by 10. For example, let's say that 2 hours after detonation the exposure rate is 400 R/hr. After 14 hours, the exposure rate will be 1/10 as much, or 40 R/hr.
The exposure rate must be expressed in the same unit as the time increase. For example, if the time increase is expressed in hours, the exposure rate must be expressed as the radiation exposure per hour.