- Part 1 -- The Free-Radical Fallacy: "Just Living" Hurts DNA Much More than Ionizing Radiation
- Part 2 -- The Relative Frequency of DNA Damage-Events
- Part 3 -- Reality-Check for the "Same-Nature" Assumption
- Part 4 -- The Unique Power of Ionizing Radiation
- References
Part 1 
The
Free-Radical Fallacy: "Just Living"
Hurts DNA Much More than Ionizing Radiation
1a 
In
some peer-review journals and various interviews in the media,
what we call the
Free-Radical[1]
Fallacy has been employed in an effort to deny the menace of
low-dose ionizing radiation. Here, we will demonstrate why
it is a fallacy.
1b There
is no doubt that routine metabolic chemistry
in each cell produces legions of free-radicals every hour, in
the process of "just living." And there is no doubt that
exposure to ionizing radiation produces some extra
free-radicals in irradiated cells. And so radiation scientists
such as Dr. Daniel Billen (1990) have
tried to argue that low-doses of ionizing radiation must be
inconsequential because they add so few free-radicals to a
cell, by comparison with the number of free-radicals naturally
in each cell anyway.
1c The
hidden (and false) assumptions in such
reasoning are (1) that the nature of damage done by ionizing
radiation is the same as the nature of damage done by routine
metabolic free-radicals, and (2) that damage therefore can be
compared by comparing the relative numbers of free-radicals.
The erroneous "same-nature" assumption is at the heart of the
Free-Radical Refrain, and has been disseminated by statements
like "We are irradiating ourselves by living" (Dr. Bruce
Ames 1994, p.18; Ames
1995, p.5259).
1d Using
numbers from Dr. Billen's own presentation,
plus a reality-check with actual observations, we can
demonstrate that the nature of damage from ionizing radiation
cannot possibly be the same as the nature of the damage from
routine metabolic free-radicals. We have not seen this
demonstration elsewhere, but perhaps someone else has put it
forth, too. We use nothing but simple multiplication and
division (Part 3).
Part 2 The
Relative Frequency of DNA Damage-Events
2a Billen
(1990, p.242) cites various mainstream
sources for two estimates in his free-radical argument: (1)
"Approximately 10,000 measurable DNA modification events occur
per hour in each mammalian cell due to intrinsic causes," and
(2) "About 100 (or fewer) measurable DNA alterations occur per
centi-Gray of low-LET radiation per mammalian cell." These
two values are made comparable in Part 2d, below.
2b The
goodness of both estimates, above, will surely
improve a great deal with future methods of measurement, but
neither Billen's argument nor its refutation depends on
precision in these two values. Billen's reference to
"low-LET" radiation includes x-ray, beta, and gamma radiations.
2c Billen
states his conclusion (p.242): "Therefore,
every hour, human and other mammalian cells undergo at least
50-100 times as much spontaneous or natural DNA damage as
would result from exposure to 1 centi-Gray of ionizing
radiation." Centi-Gray and "rad" are two names for the same
amount of radiation exposure. How much is one rad of exposure?
2d On
the average, it takes about 10 years for a
person to accumulate one rad of whole-body exposure from
natural background radiation. So Billen's numbers mean that
the ratio of damage-events per unit of time (per hour, or per
day, or per year) may be as large as 8.8 million endogenous
damage-events for each damage-event due to natural background
radiation. A very large difference ... but is it meaningful?
2e The
estimates presented by Billen of "DNA
modifications" and "DNA alterations" are estimated numbers
prior to repair-work by the cell. Here (and elsewhere in the
literature), the term "damage-events" is preferred, to signal
that the event is not necessarily an unrepairable permanent
mutation of the DNA.
2f Billen's
arithmetic is correct, but a
reality-check is needed for his assumption that the nature of
DNA damage-events is the same from routine cellular metabolism
and from ionizing radiation.
Part 3 Reality-Check
for the "Same-Nature" Assumption
3a According
to Billen, a rad (centi-Gray) causes
about 100 or fewer measurable DNA damage-events per cell.
3b According
to Billen, the number of comparable
damage-events from intrinsic causes per cell, every hour,
is 50 to 100 times higher, which means 5,000 to 10,000
damage-events every hour from intrinsic causes, per cell. (Bruce
Ames 1995, p.5259, provides an estimate per
day, not per hour: "The number of oxidative hits to
DNA per cell per day is estimated to be about 100,000 in the rat
and roughly ten times fewer in the human." We will include this
estimate in Point 3e.)
3c It
follows from Billen that per day, the DNA
damage-events per cell from endogenous causes are either:
( 5,000 events/hr) x (24 hr/day) = 120,000 events/day, or:
(10,000 events/hr) x (24 hr/day) = 240,000 events/day ...
in each cell.
3d And
something else follows from Billen's
assumption that there is no important difference between the
endogenous and the radiation-induced damage-events. If
correct, then the DNA-based consequences from a radiation dose
which delivers 120,000 or 240,000 damage-events each day, per
cell, should be the same as from 120,000 or 240,000 such
events per cell each day, from endogenous sources.
3e The
whole-body radiation dose per day required (by
Billen's numbers) to deliver 120,000 to 240,000 such DNA
damage-events per cell, each day, would be either:
(120,000 events) x (1 rad/100 events) = 1,200 rads, or:
(240,000 events) x (1 rad/100 events) = 2,400 rads. And:
If we substitute Ames' figure (from Point 3b),
we would calculate (10,000 events) x (1 rad/100 events) = 100
whole-body rads per day to deliver DNA damage equivalent to
daily damage from intrinsic causes.
Bottom Line of the Reality-Check
3f If
there were equivalance between DNA damage by
free radicals from normal, intrinsic processes and DNA damage
from ionizing radiation, then whole-body doses of 100 rads to
2,400 rads per day every day would be easily tolerated.
Instead, such doses are promptly lethal.
3g For
half the humans exposed, promptly-lethal doses
are estimated by the radiation community at 300 whole-body
internal-organ rads accumulated in one week or less (or 420
such rads within a month).
3h There
is an additional observation worth noting.
The background rate of cancer (a disease widely acknowledged
to be DNA-related) is doubled by extra radiation doses of a
few hundred whole-body rads, or fewer, of accumulated
exposure. According to Billen (Point 3a), 300
rads cause about 30,000 or fewer DNA damage-events per cell ---
a number far exceeded in a single day by intrinsic processes
(Point 3b). If DNA damage from intrinsic
causes and from low-LET ionizing radiation were equivalent, it
is hard to see how anyone could escape having multiple
clinical cancers from intrinsic processes.
3i From
these two reality-based observations (acute
lethal doses and doubling-doses for radiation-induced cancer),
we have demonstrated that the nature of damage caused by
ionizing radiation cannot possibly be the same as it is from
normal metabolic processes and oxidative damage. Without an
equivalence, the Billen argument and its variations collapse.
The Free-Radical Refrain is just a Free-Radical Fallacy.
Part 4 The
Unique Power of Ionizing Radiation
4a The
difference between free-radical damage from
routine metabolism and from ionizing radiation almost surely
lies in repairability. If DNA damage is perfectly repaired
by a cell, such damage has no health consequences. It is
inconsequential. The consequences arise only from injuries
which are non-repairable or mis-repaired.
4b The
demonstration in Part 3 supports other
evidence (and vice versa) that ionizing radiation can induce
the special kinds of complex DNA damage which cannot be
perfectly repaired. A leading figure in this research is John
F. Ward; see Reference List.
4c The
power of ionizing radiation to induce the
complex injuries is not in dispute. Billen himself appears to
acknowledge it, but then to ignore it (Billen
1991, p.388).
4d The
power of ionizing radiation to induce
particularly complex and unrepairable genetic injuries is
surely related to a unique property of this agent. Ionizing
radiation instantly unloads biologically abnormal amounts of
energy at random in an irradiated cell. Biochemical reactions
in a cell generally involve net energy-transfers in the ballpark
of 10 electron-volts and below. By contrast, Ward reports
(1988, p.103) that the average energy-deposit
from low-LET ionizing radiation is thought to be about 60
electron-volts, all within an area having a diameter of only 4
nanometers. (The diameter of the DNA double-helix is 2
nanometers). In other words, ionizing radiation produces
violent energy-transfers of a type simply absent in a cell's
natural biochemistry.
4e Because
of its unique property, ionizing radiation
is a unique menace to our DNA and chromosomes. This fact
needs wide recognition, as mankind learns that far more health
problems are mutation-based than anyone could prove 15 years
ago.
# # # # #