Details: It's a 3 story building. Each apartment has its own HVAC system. There is no shared ventilation system. The foundation is a slab. The radon levels on all three floors were measured simultaneously. The first floor was 7.3 pCi/L. The 2nd floor was 6.7, and the 3rd was 9.2. Each measurement was in the same horizontal place, near the center of the building. It was done when the weather was cold enough to be sure all windows were likely to be closed.
Other tests have had similar results, but were not done as rigorously. E.g., not simultaneously, not in the same part of the building, etc. The results vary a lot with the weather and probably other factors. But it doesn't seem logical that the 3rd floor should ever have higher levels than the other two floors.
Tester101 asked if there is a radon mitigation system in place. How would I find out? What does a radon mitigation system look like in a 3 story building? In other words, even if I saw one, how would I recognize it? Can I recognize it by something coming out on the roof? What would that look like? What would the difference be between that and ordinary vents such as drain vents?
Is it likely that a radon mitigation system could be malfunctioning and dumping radon onto the 3rd floor?
I read somewhere that people don't have to worry about radon on 3rd floors and higher because by the time it gets that high its levels are drastically reduced. But that contradicts another answer to this question, which implied it would be normal for the levels to be higher depending on ventilation etc.
I also read somewhere that if the levels aren't lower on higher floors, the radon could be from building materials instead of coming up from the ground. How could I tell if that's a factor here?