Permissible Emergency Levels of Radioactivity in Water & Food is an interesting topic, they actually used to put little tins containing a radioactive source and if the water you wanted to drink tested lower than the test item then it was considered safe to drink.
I have one of those old tins somewhere in a storage container, but it has half-lifed to where it is just a novelty.
This PDF is similar to yesterdays post on Dosimeters as it is actually from an old civil defense Technical Bulletin TB 11-8 of December 1952.
This bulletin has some good charts and information that, while old, is still very much relevant to those worried about radiation exposure.
It is the EPA that regulates the permissible dose. They call them PAGs or Protective Action Guides. When I was in Emergency Management, we lived and died the PAGs during radiological drills. It is those PAGs that determine when people have to evacuate and when they can go back home. The Utility would make recommendations on evacuation based upon the situation at the plant, and the State would make the decision.
I remember a particular drill for our Nuclear Power Plant when the FEMA grader dinged us for evacuating earlier than the Utility recommended (he sited cost and this was the only issue he could find)
General Bassham (the State Emergency Management Director at the time) took the podium like Sherman took Georgia and said he didn’t care about cost when it came to safety and he would evacuate anytime he thought it was in the best interest of the citizens. I shouldn’t talk out of school, but the State really lost out when he left the Agency.Permissible Emergency Levels of Radioactivity in Water & Food (1952)