The intensity of radiation decreases logaritmically after a nuclear detioation (first it halves, then the remaining half halves, and so on). The 7:10 rule of thumb is the easiest way to approximate this type of calculation, and the numbers were chosen to match real world data.

The rule of thumb is saying that when 7 times as much time has elapsed, the radiation has gone down 10 times. So if it is 10x the deadly dose after 1 hour, it’s 1x the deadly dose after 7 hours, and 0.1x after 14 hours. Deadly doses are more complicated than this, but this is the basic idea.

Are you talking about the rule of thumb for nuclear radiation exposure?

If so, it lets you estimate how much exposure there will be in the future if you know the current exposure rate and how long since the explosion. Multiply however long it’s been since the explosion by 7 and you can divide exposure by 10.

>**The 7:10 Rule of Thumb**

>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.

https://cdp.dhs.gov/shared/se/courses/default/AWR-923-W%2005122021%201.2-20210512144644/groups/394.html

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