> 150k people had to leave their homes because they were not safe.
The fact that they were not safe is disputed.
There are claim that there was some over-reaction and that the evacuation was actually counter-productive.
In retrospect there's no doubt the evacuation was an overreaction. Of course, in the moment, it's a harder decision since you have to act on incomplete information.
Unsafe based on standards made incredible extreme because of the anti nuclear movement in the 70s based on false science.
This was the great trick the environmental movement bulled, they made so much panic about radiation that acceptable amounts were lowered to absurd levels. This was all based on suds-science. If this science were even remotely true, people in Denver would by dying of cancer at incredible rates.
There are also lots of places that have more radiation because sand beaches (historically associated with healing sands). People living in that region for 100s of years should also suffer from higher cancer rates and don't.
There are still people in Germany flipping out about the fact that some mushrooms have 'unsafe levels of radiation' and yet lots of people eat them and it has no effect what so ever.
Yes, of course the limits are below "dangerous" levels. That, in itself, says nothing about what we should call "safe" levels.
Radiation has two types of dangers, chronic and acute. Our understanding of what levels of radiation cause acute damage is pretty limited and not very precise since we simply don't have that much data. However, I highly doubt that anything close to those levels of radiation were measured anywhere outside the reactor complex.
In terms of chronic danger, the official stance is that there is no safe amount of radiation exposure and the general principle in managing exposure is ALARA (As Low As Reasonably Achievable). Lifetime cancer risk is cumulative so higher levels just add to that cumulative risk faster.
Thus there isn't really a clear "level of acceptable impact" but rather, we set arbitrary levels to to to limit that cumulative addition of cancer risk. Like with many things, that risk is a price you pay for other things that are important to you, like flying, living in denver, or getting xrays.