Island 3 Question

I see reference to O'Neill saying that the Island 3 won;t need a
radiation shield because of its size.

Now, is it safe for me to infer from his statement that the
thickness of the structural shell will be siz feet of solid metal,
thus no need for additional shielding.

I've seen reference to the Habitat's atmosphere being used as an
effective shield, but that doesn't make sense to me. Yes, 4 miles of
atmosphere above your head will probably stop the radiation, but how
about the radiation that comes from the ground below you. If the
shell isn't six feet thick, then what's to stop the radiation from
entering there. Also, how about radiation leakage from the windows.
Any activity near those windows will have radiation leakage. How
about the poor sap who lives very close to a window - what's to stop
tangential radiation from striking him. Or lets say its just parks,
of offices in that location, they'll still have the same radiation
issues.

The proximity to windows is still a radiation issue even if the
shell is six feet of solid metal. How thick would the glass have to
be in order to provide the same protection as six feet of metal?

Thanks for your clarifications, TangoMan