There are fudge factors, and then there are safety factors...
Basically, there are three different kinds of effects of radioactivity: Acute, accumulated, and genetic. Or "somatic, teratogenic and genetic", depending on which report you read.
All that says is that there is a certain dose which is very likely to make you sick, which has unfortunately been determined rather precisely through the nuclear bombs of Hirosima and Nagasaki, and the firefighters of Chernobyl.
Then there is a lower dose which will eventually make you sick, as in increasing the risk of cancer at a future time.
Then there's the dose that might might make your children sick, or their children again, or...
As I wrote we have a fair idea of the acute dose. But 60-odd years is far too short a time to have any idea about the "safe" dose levels for the other effects - only that there might be one.
Interestingly there is a study that seems to indicate there might actually be a minimum safe exposure too, which makes people receiving at least some radiation healthier than those who live an extremely "screened" life.