Grunff wrote:

The Natural Philosopher wrote:

Perhaps I should have said more exactly that 'any radioactive element
with a sufficient half life to actually be present and represent a
risk to health will be very heavy'


That covers you for the many radioactive isotopes of common elements
then (C13, N15 etc).


Yup.

I realised that I had an internal disconnect between 'dangerous'
isotopes and 'normal naturally ocurring ones'

If you like the anthropic principle. We couldn't live on the earth if
the air were dangerously radioactive, ergo all dangerously* radioactive
compounds tend to accumulate towards the surface, or below it.

* by virtue of nastiness or concentration