Should I be worried about radon while I live on the 9th floor?
Threats to health can be scary.  But in this case, you'll want to keep on learning about radon.  Learn what it is, exactly; learn where it's found in terms of what part of the building, and how it's mitigated.  

The upshot is I've never heard of a radon mitigation system on the second floor, least of all the 9th!  You have more to fear from wildfire.