X-Message-Number: 558
From:	Ralph Merkle <>
Date:   Sat, 30 Nov 1991 10:11:48 PST

Background radiation is approximately 0.1 rads per year.  A lethal
dose of radiation is about 600 rads.  Therefore, someone held in
cryonic suspension and exposed to normal background radiation levels
will accumulate a lethal dose in about 6000 years.

Most damage is "latent" until the cell divides ("cycles" or "renews").
Therefore, cells that do not divide are more resistant to damage.
The central nervous system is more resistant to radiation damage
than other tissues.  "There are no rapid renewal systems in
the CNS, but the glial cells and the endothelial cells cycle
slowly and can show injury.  The neurons are not injured except secondarily
or by doses of 50 Gy [5,000 rads] or more.  Very large exposures of
15 to 50 Gy [1,500 to 5,000 rads] can produce acute functional changes
and, at the highest doses, immediate death.  After 15 Gy [1,500 rads],
changes begin in four to five months and develop further over one to
two years."

If we assume that survival of the central nervous system is the important
consideration in cryonic suspension, and further assume that any dose
of radiation which is not immediately lethal can be repaired by future
technology, then a person in cryonic suspension would accumulate a
lethal dose in 50,000 years.  Even this estimate is conservative.  The
repair capabilities of future technology are likely to allow repair even
after an "instantly lethal" dose of radiation by current standards
has been received.

In other words, radiation just ISN'T a problem.

Reference:  Cecil Textbook of Medicine, 17th edition, James B. Wyngaarden,
Lloyd H. Smith, Saunders 1985, pages 2298-2297.

1 Gy (gray) equals 100 rads.  1 rad, or Radiation Absorbed Dose, corresponds
to 100 ergs per gram of absorbed radiation.  1 rem is roughly equivalent
to 1 rad (I won't bore you with the precise definition of a rem).

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