Shielding (sigh, again.) and radiation dose

From: Dr. Omni
> Yes, I remember that in the AMES study the thickness of the shield
was 4 meters or so (about 12 feet - close to your 10 feet figure).

Mike C. wrote:
> I was thinking that it was closer to 2 meters. And that was what
produced the "safer than Denver" conditions.

For some reason, O'Neill's book, and the study Al G. has on the web
uses feet; as in 6 feet of slag shielding, or 20 inches for solar
flare shielding.
Perhaps we're suffering from the Lockheed measurement syndrome, and
that accounts for the confusion even here, at the 'net center of
space colonization studies. (how many feet to a cubic pound? and what
is a dram, and who cares? Which Pharoh quantified the cubit, and how
big was he?)

Call it 2 meters of sand. The "Concrete Space Colonies" study by
Shepard uses slabs 1.6 meter thick. In "The Millennial Project"
Savage uses a water 'balloon' shield 5 meters thick.
It comes out to ~4.5 to 5.4 tonnes/square meter.

5T/m2 is about half of what sea level Earth atmosphere gives (not
pressure, but the mass of material over us).
I recall hearing a while back that though Denver ("Mile High city")
is at 1.6km (in Boulder, I'm at 5300ft) isn't at 1/2 atmopspheric
pressure, we are at an elevation where half of the atmosphere's mass
is below us.
All this comes out the same, and the figure accepted for space
colony passive shielding is indeed more than adequate.

A while ago, via the Yahoo orion project club, I came across some
interesting papers on radiation damage, protection, and effects.
What strikes me now, is one opinion about low level radiation damage.
Several sites said something about the commonly accepted figures for
radiation exposure being overly cautious. There's some reason to
believe that the commonly accepted theory on low level exposure is
wrong: cumulative lifetime doses don't add up to the same hazard as a
one-time equivalent dose.
The body has a lot of capability to repair low exposure damage, and
indeed, through hormesis, low levels even seem to stimulate the
body's self repair mechanisms, to make the body more generally
healthy and resistant to cancers.
This one person (somewhere?) speculated that low levels of radiation
may not in fact be the danger we believe. The body has the senses to
detect lots of other low levels of what may, at higher levels of
exposure, be dangerous: pressure, heat, cold, light, etc. (Birds
being used to detect gasses in mines & such, while the concentration
is still far below harmful levels. The skin producing melanin to
protect aginst invisible UV, before damage is done)
Why has no life form on Earth the ability to detect radiation, if
even low levels of exposure are dangerous? Radiation is everywhere.
Surely evolution would produce some mutation somewhere to warn the
creature to move away.