Re: Radiation hormesis, thresholds, etc

[Christoph_Hofmeyr/CNS1@cns.co.za]

Dear Radsafers,
0.  I wrote a draft response to Glenn Carlson in May (3.), but now a few
related thoughts have accrued.   I hope it is not too heavy - my own
musings.  Sorry about the delay.
1.  It might be useful to point out that from an RP perspective the LNT is
extremely convenient, allowing concepts like collective dose to be used
'constructively'.  The concept of a threshold would require a very
significant adaptation in thinking and doing.  Collective dose
(man-Sieverts) would become meaningless as an expression of risk, except
where individual doses are well in excess of the threshold.  Then, to a
dose accrued over what period would the threshold apply?  When are the
counters reset?  Does it apply to acute or chronic exposures?  If it is a
lifetime phenomenon, and incremental dose is deleterious once you have
passed the optimum?  These questions would have to be addressed
satisfactorily before a new RP strategy can be developed.  Health Physics
might well change to Health Math.
2.  Hormesis and a threshold are, to my mind, different concepts, as
hormesis is supposed to change the high dose response, while a threshold
concerns only the low dose response - or lack thereof.  Hormesis seems to
contain a time component and ideally one would have to pre-empt a
significant exposure by the right amount of exposure at the right time.
Not very practical, except possibly in medicine.  What about the chronic,
low-level exposure situation?  Would that be a threshold case?  For how
long?  Do we know enough to take a chance, as Al would like us do?
3.   It is misleading to just look at the definition of energy/mass for
absorbed dose (1Gray = 1J/kg) and compare it with other forms of energy.
If a person of 70 kg takes in 7000 kJ of nourishment per day, it is surely
not equivalent to a truly deadly 100 000 Gy.  In the same way, the human is
much more tolerant for energy deposition by e.g. immersion in a hot bath,
and elevating one's temperature by 1 C or more without suffering detriment.
Some measurable energy deposition by infra-red, microwaves,  etc., is also
tolerable, possibly with the exception of strong molecular resonance
energies.  I am therefore deeply sceptical about the perceived mutagenic/
carcinogenic danger from power lines, cell phones,  etc.  However, whole
body ionising radiation enough to kill (3 - 5 Gy) would raise one's
temperature by only 1E-3 C!  That is RBE or quality factor or radiation
weighting factor for you!  Ionising radiation remains a very potent poison.
At the ionisation potential of body constituents, the toxicity of energy
takes a veritable quantum leap by several orders of magnitude. Threshold
believers hold that there is an effective damage threshold at non-zero
fluence.  In view of the truly enormous change in toxicity of energy
deposition at the ionisation threshold, this would be a second order
effect. Comment?
4.  It is interesting to hear rumblings that the ICRP might start
de-emphasizing the concept of collective dose and concentrate more on dose
to the individual, thereby trying to defuse the LNT debate to some degree.
The concept of collective dose, when applied to low-level irradiation of
future generations ad infinitum, is apparently stretching things beyond the
reasonable.  Is it true that the cost of 'cleaning up' contaminated sites
and tailings dams in the USA on the grounds of collective dose arguments is
a prime mover in questioning the LNT? Are there any opinions?  I buy Dr B
Cohen's remark that a threshold would have PR value with the public.  In
fact, it is an old concept, which can be demonstrated with respect to
deterministic effects, and which was generally held even in ICRP circles in
the fifties, but was dropped in favour of the LNT with the emergence of
'stochastic effects'.  The latency period of stochastic effects would seem
to indicate conclusively that a single mutation event does not cause a
cancer, although it could represent a precursor which requires further
stochasic events at a cellular level to develop.  It could well be that
damage to the repair mechanism or the apoptosis mechanism is important.

Chris Hofmeyr
chofmeyr@cns.co.za

Original communication:

Subject:  Re: Radiation hormesis

In a message dated 5/6/99 1:06:16 PM EST, tmohaupt@wright.edu writes:

<< Glen presents an interesting point of view.  Since ionizing radiation
 is simply a form of energy, it can be readily compared with other
 forms of energy and their effects.  Keep in mind that there is no
 type of energy that is safe in high levels.  Radiation is the only
 one to which a LNT is applied.  The LNT is a political
 position and, most likely, not an accurate description of reality.
 Personally, I'd like to see radiation regulated to prevent real
 damage, much the way we do with other forms of energy, rather than
 treat it like the witchcraft trails of the late 17th century in New
 England. >>

My issue is not whether LNT, or LT, or NLT, or NLNT is the more accurate
model for calculating risks from radiation exposure, but, rather, how any
alternative to LNT would be implemented.

Let's ASSUME the LNT model for radiation exposure is invalid, and, say,
there
is a threshold exposure below which increasing exposure produces no
increasing or, even, a decreasing risk of excess cancer, and, above that
threshold, increasing exposure results in increasing risk.

Do we agree, then, that exposures above that threshold should only be
permitted if the increased risk is outweighed by some benefit?  And, do we
also agree that exposures below that threshold may be permitted without
regard to whether there is a corresponding benefit?

Now, assuming we agree up to this point, my question is:

How is one to determine for which situations the cost-benefit analysis must
be done (i.e, the NET benefit of the exposure MIGHT be negative) and for
which the cost-benefit analysis need not be done (i.e, the NET benefit of
the
exposure CANNOT be negative)?

Glenn
GACarlson@aol.com




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