Ask HN: What are the risks of radio frequency fields?

My friend's rooftop has a cell tower on it. The door leading to the roof has a sign that states: "Radio frequency fields beyond this point may exceed the FCC general public exposure limit. Obey all posted signs and site guidelines for working in radio frequency environments."

Rule(S) 47 CFR 1.1307(b)

Can anyone tell me the dangers associated? Should I not go on the roof at all?