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RECENT INQUIRY REGARDING HORMESIS
RESPONSE TO EARLIER INQUIRY ON
RADSAFE BULLETIN BOARD,
"DO YOU BELIEVE IN RADIATION HORMESIS?"
David S. Gooden
Like several others who have responded to this inquiry,
I suggest T.L. Luckey's book. Also the 1994 UNSCEAR
Report to the General Assembly, with Scientific Annex B,
"Adaptive Responses to Radiation in Cells and Organisms," is
very informative. The entire May, 1987, issue of the Health
Physics Journal is devoted to hormesis. The report from the
Proceedings of the International Conference on Low Dose
Irradiation and Biological Defense Mechanisms, Kyoto, Japan,
is recommended and the Journal of Biological Effects of Low
Level Exposure (BELLE) from the University of Massachusetts
(School of Public Health) is valuable. There is a lot of good
literature on hormesis. Like all scientific writing, these
materials represent observations and opinions. Presentation of
data may include personal biases. All materials must be
interpreted and assimilated based on one's personal experience
and knowledge, and a knowledge of the authors agenda.
PERSONAL OBSERVATIONS REGARDING HORMESIS
AND RELATED ISSUES:
In my more than 30 years of experience in medical
physics and health physics I have seen much that suggests
hormesis may be true and much that suggests the linear no-
threshold model may be improper. Conversely, I personally
have not seen anything in practice or in the literature that
indicates that the linear no-threshold model accurately describes
late toxic injury (including ionizing radiation) at low level
exposures or that hormesis is unlikely. As a scientist, it is
important for me to have the courage to describe my
observations and to state my conclusions even though they may
differ from observations and conclusions of other respective
colleagues. Similarly, it is important for me and all of us, as
scientists, to remain open-minded regarding this issue. My
responsibility to my profession is to share my observations with
fellow scientists and to attentively listen to the opinions and
interpretations of others.
I perceive that controversy within our community
regarding issues like hormesis (and the linear no-threshold
model of radiation injury) may result from our inability to
separate "science policy" from science. "Science policy" is a
value judgement system, not science. Science policy, in its
most wholesome form, allows reasonable activities and
reasonable health and safety regulation in areas where scientific
certainty is lacking. Value judgements are not inherently less
good than science, but it is very wrong to present science policy
as science fact. Today, we, as a profession, are questioning
science policy based on what we know to be science fact (see
Gen. Roessler's editorial, HP Newsletter, April, 1995). I think
this is important and proper. The science policy that has
evolved regarding late radiation injury in humans appears to
include the following:
1. If radiation causes hereditary injury in any species
at any dose level, it causes hereditary injury in
humans, even if no hereditary injury has ever been
identified in humans;
2. Differences among species in mechanisms of injury
and repair are not taken into account when
extrapolating data from one species to another,
even if great differences are scientifically identified
between the responses in different species and the
response in one species may not even occur in
other species;
3. If radiogenic cancer exits at any dose level, failure
to demonstrate radiogenic cancer at much lower
doses is not indicative of safety at low levels and
negative findings cannot be used to quantify risk,
even if this results in ignoring valid scientific data;
4. A carcinogen by one route of exposure is a
carcinogen by any other route of exposure, even if
this is known not to be the case;
5. There is no threshold dose below which an
increased risk of carcinogenic or genetic injury
occur, even if significant data indicate a threshold
for cancer induction and hereditary injury in
humans has never been identified; and
6. The dose-response model is linear at low doses,
even if no toxin has ever been shown to have a
linear response as to biological effect at low level
exposure.
The primary problem with the concept of hormesis is that
it suggests to some an intentional exposure to radiation (at some
yet to be identified level) may be good for optimal health.
Even Luckey suggests this. One of the early respondees to this
Radsafe inquiry facetiously suggested that, if hormesis is fact,
health physicists of the future may be required to go around and
make sure that employees have received their monthly quota of
radiation exposure. Based on my personal experience and what
I perceive to be the present state of scientific knowledge, I think
this concept distracts from meaningful scientific discussion
regarding this important issue. I encourage each of us not to
promote the notion that there may be an optimal radiation
exposure for health. We must "crawl" regarding our knowledge
of the effects of low level radiation exposure in humans before
we attempt to "walk" or "run."
Exposures to low levels of radiation of humans must have
one of three effects: 1) no effect, 2) detrimental effects, or 3)
positive effects. The same is true of exposure to all toxins.
Can you think of any toxin or trauma (be it radiation or
whatever) that causes an identifiable injury (or observable
effects) in an organism at low level exposure? Some may point
to chromosome aberrations or mutagenesis (DNA events) as
observable effects in humans (observable at about 100-250 mSv
acute exposure; not very low level by some regulatory
standards). As health physicists we are interested in injury to
organisms not cells. Do chromosome aberrations or DNA
events cause identifiable injury in humans? The science appears
to point toward a "no" answer. D. Billen (Rad. Research 124,
242-245; 1990) says that there are about 70,000,000 naturally
occurring DNA events per cell per year in humans. An annual
exposure of 5 mSv adds about another 50 events/cell/year.
Identifiable injury in the organism must be related more to
"failures to repair" than to DNA events themselves.
Hormesis is not a difficult concept for me. I don't
understand why it is difficult for other scientists. Our present
state of knowledge allows humans to harness some beneficial
effects of exposure to low level biological toxins such as
measles, polio and chicken pox viruses through immunization.
Some seek the positive effects of injecting small amounts of
snake toxins into their body to protect against snake bite. We
know that many of our "infections" become resistant (through
adaptive responses) to antibiotics. Insects develop enhanced
tolerances to pesticides. The mechanism causing the
"beneficial" effects of exposure to low levels of these toxins
may be different from those of radiation hormesis, but these
type of effects, never-the-less, speaks to the plausibility and
reasonableness of hormesis.
Today I believe that no scientific knowledge suggests (or
even points toward concepts of) intentional exposure of humans
to chemical and/or radiation toxins based on known, positive
adaptive behavior in cells and organisms. However, it is
important to use today's evidence regarding hormesis to assist
in evaluating and developing proper radiation injury models and
in formulating reasonable "science policy." Understanding
mechanism of radiation hormesis may be very valuable in
assessing human responses to exposure to all toxins.