risk must be appraised also in comparison
with the risks acquired by natural aging, by genetic constitution, and by various elec-tions of individual behavior that determine the presence or absence of environmental hazards. Our society has already elected an
average life-span reduction of about 2 years per person in one conspicuous matter. This is the hazard accepted in exchange for the convenience of transportation. The figure I have cited is the biological cost of the maimed and tile dead from transport acci-dents. Similarly, the election to smoke ciga-rettes is estimated on the average to
short-en life by a decade and to reduce health during all of the shorter life lived. Thus, a person concerned about fallout effects,
which at the most might cause an average life-shortening of 1 or 2 weeks, should be more than 300 times as concerned about life-shortening disease associated with smoking one package of cigarettes per day over a lifetime. Tile matter of judgment
about relative hazards is the basis of ac-ceptable levels of risk-taking or exposure in a variety of conditions. More generally, however, regardless of the level of any par-ticular exposure to harm, the hazard should
not be increased except under the
compul-sion of having to accept that added risk for some equally worthwhile or more desired benefit. Radiation hazards and the corre-sponding benefits from the activities that create them are continuously evaluated by
our health agencies and others involved in these matters. Comparison to all other envi-ronmental factors which affect health and life span should afford one basis of judg-ment for acceptance of radiation exposure, in ranges that can be defined as acceptable,
because they are “small.” An additional
im-portant reference point for gauging the ac-ceptability of radiation risk has always been the natural radiation level. This guide seems useful both because its effects are small in comparison to other natural haz-ards and because the various species, in-cluding man, have evolved to their present forms in the face of whatever hazards the natural radiation levels impose. In the true sense, however, there is no “lifetime
toler-ance to radiation,” only an acceptability of a risk that buys some benefits-a risk that we must continually work to reduce so that
tile price of these benefits is as low as
science can make it.
DISCUSSION
DR. POWELL: I wish to comment first on
some of the data of Lorenz’s original
long-term, exposure work. It has been of great interest to me that there was an increased incidence of tumors in the 0.10 r per day
animals. This was obvious in re-reading the data and was implied by Lorenz himself, though it has been only infrequently re-ferred to.
I disagree that many people actually re-ceived a tenth of a roentgen a day when that was the maximum permissible dose. In relation to tile total population of the United States, or indeed in relationship to the total number of individuals actually
en-gaged in atomic energy work and related enterprises, there were relatively few
peo-ple who received exposures as great as one tentll of a roentgen per clay.
Many of us wish that we could have the immense body of data about potential human hazard from other environmental sources that we have for radiation. It is true that those of us deeply interested in this field wish we had much more quantitative data about radiation effects. We are dissat-isfied with the current state of knowledge. However, we must be careful that our rela-tively greater knowledge in the field of radiation hazards does not lead us to irra-tional conclusions.
Many of us were quite struck by Dr. Russell’s discussion. I think that those of us
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genetic effects and for those somatic effects
that have a genetic basis, then the most im-portant thing we need to establish is the re-lationship of this threshold to background
radiation levels. Of course there are two al-ternatives :
(
1)
that the threshold is abovenatural background levels and (2) that the
threshold is below natural background. If
tile threshold is below natural background, then from a practical, operational
stand-point there might as well not be a thresh-old. If the threshold is at such level that we already exceed it by overexposure to natural background, then this threshold is of great interest to radiation biologists but of little practical import to those who are
concerned primarily with radiation protec-tion standards. On tile other hand, if tile threshold turns out to be above back-ground, then we must he concerned about
exceeding it. The Ad Hoc Committee on
Strontium proposed that a multiple of
background exposure, and by implication a low multiple of natural background, would present a reasonable starting point for
be-ginning radiation protection standards.
However, those of us who have taken that
approach will have a serious problem if tile
threshold is above natural background. Under tilese conditions, natural back-ground does not give us a starting point and we cannot assume that one merely doubles the effect by doubling the dose.
Dn. SAENGER: Dr. Jones started out by getting rid of all the thresholds and Dr. Powell just put them back. What should we think?
DR. POWELL: Let me comment that this
reflects a change in my own thinking in light of the discussion we just heard on genetic effects from Dr. Russell.
DR. CHADWICK: I am sorry Dr. Russell
could not stay and be here to comment on that point. Perhaps he will comment for the record.
DR. JONES: I don’t think that Russell’s
work put aside the no threshold concept. I did not get that impression from listening to him; although, if the Russell work had come 10 years ago, we would never have
shifted from the threshold concept to tile no
threshold concept. However, in the genetic work done in Japan following up the atom-ic bomb survivors, there was a change in the ratio of male to female offspring born to
irradiated mothers. This shows there is a carry-over from high exposure rates and high doses directly for genetic effects and
there hasn’t been recovery from this. In the low dose range, who knows what we might
see? In the cancer range, I point out there is a lot of information already on man that is interpreted as showing that low exposure
rates cause more cancer
(
e.g., in American radiologists and in the experimental animal studies of Lorenz). At the present time, the work that is more likely to establish andfortify a concept of no threshold for public risk purposes is the cancer risk.
DR. SAENGER: Where did this low dosage
to radiologists come in? These men weren’t measured.
DR. JoNEs: They weren’t low doses but
they were at low dose rates.
DR. SAENGER: You could argue that they
were at very high dose rates at the time
they were actually exposed because tile dose rates were quite high in fact. In NCRP Handbook 411 the maximum permissible
fluoroscopic dose rate was 20 r per minute. In Handbook 762 this value was reduced to 10 r per minute. Prior to 1949 higher dose
rates were not unusual. Dose rates up to 100 r per minute were reported by Braestrup in 1942.
DR. JONES: I would maintain that they were probably at a reasonably low dose
rate because tlley weren’t the same as the patients they were working on.
DR. SAENGER: What is a low dose rate?
These men had their fingers ill tile
fluoro-scopes and their heads under the beam. Most of these men died from aplastic anemia, leukemia, and Hodgkin’s disease.
DR. CHADWICK: The protracted episodes
of high dose rate, I guess, is what von are calling this.
DR. BENGELSDORF: I just returned from a
vi-ruses. Evidence was presented which
mdi-cated that ill mouse cells infected with the
SV-40 virus tile malignant transformation does not occur until the cell divides. Appar-ently at tile moment when the DNA is undergoing replication it becomes
vulnera-ble to tile foreign DNA of the infectious
virus. It intrigues me that Dr. Jones mdi-cates that children are more sensitive to ion-izing ra(liatiOll than adults, infants more
than children, embryos more than infants. This is precisely the range which indicates more and more DNA replication. I would like to Ilear if anyone has anything to say about the mechanism of damage of ionizing
radiation.
DR. BRENT: Although there is no doubt
that the fetus and infant have an increased
sensitivity to high doses of acute
irradia-tion, this is primarily due to the fact that these orgailisms are involved in integrated processes of proliferation and differ-entiation. Although most people are con-cerned with the embryo’s great scnsitiv-ity to irradiation, one should also be amazed at the embryo’s recuperative pow-ers. In our own laboratory, we have irradi-ated 12-day-old rat embryos with 300 r and
observed no thyroid on the fifteenth day; yet, all had normal thyroids at term. L. B. Rus-sell administered 200 r to pregnant mice protracted over the entire period of mouse gestation and observed no effects in the
embryos. We actually have little quantita-tive information about the recovery powers of the irradiated embryo. Before we extend the generalization that the embryo is more sensitive than the adult organism into the low dose range, we had better produce some data. We may find that at very low dose rates the differences between the em-bryo and adult are not as great as when higher doses and dose rates are
adminis-tered.
DR. HAYNIE: We had a symposium at the
#{149}University of Texas M. D. Anderson Hos-pital last week on carcinogenesis, where virologists, immullOlogistS, and geneticists all presented their views on what causes
cancer. There are many conditions and fac-tors in carcinogenesis. In fact, there was some work presented there which suggest-ed that it is not always possible for radiolo-gists to associate radiation and
carcinogen-esis in a given experiment. The confusion in regard to carcinogenesis may be tilat,
de-pending on the presence of other condition-ing factors, radiation may or may not cause cancer. On the business of threshold and no
threshold theories, these concepts must be replaced by a benefit versus risk concept. Namely, if there is no benefit to be derived from exposure to radiation, then there is no reason to accept any radiation; but, were a certain benefit to be derived, a certain level of radiation might be acceptable.
DR. BRus: A philosopher some 50 years
ago said that philosophy would have been better off if all philosophers had been blind. I wonder whether we wouldn’t be better off if we had never thought of a threshold. We
were lured into this by persons who wanted to have a number and who wanted to ra-tionalize this number after it was produced as something that was a secure number representing safety. I think we would be better off if during this period we had looked at the phenomena and forgotten about simplification of the phenomena, par-ticularly on the question of the effects at the cellular level where it is clear, as very few things are on the level of the higher or-ganisms, that the effect starts out very slow-ly with dose. I am talking now about the killing of cells by gamma rays or x-rays. The first 50 r of exposure does very little and a great deal is done thereafter. This is not within the threshold concept; also, it is not within the way with which the no threshold concept has been very commonly viewed by people who like to think of a two value world. There are many
pro-SUPPLEMENT 277
trolls, this effect may disappear and you
may get an effect that starts from the first
roentgen equivalent.
Dii. EISENBUD: I think that we have come
to a point where one needs to distinguish between tile implications of this discussion
to the practicing pediatrician and to public health officials. While it has not been
clear-ly brought out, I think there is general agreement in the room that if you take all the epidemiological data and look at them, generally speaking, the response or effects we are talking about seem to be of the order of one-in-a-million per-rad-per-year.
This seems to be the order of magnitude for
bone cancer, leukemia, or genetic effects. Since in most cases these types of exposures are in the range of 100 mr, we are talking
about risks of the order of one in ten mil-lion per child per year. The public health
implications of this are readily calculable
on this basis. There are three billion peo-ple in the world and if you multiply ten to
the minus six or ten to the minus seven by three billion you come up with a number that is big enough (300 cases per year) to warrant doing something about the
expo-sures. There are many other problems in public health that are like this-where risk
that involves a few hundred or a few thou-sand deaths throughout the world in a year
would be worth a good deal of official
at-tention. However, at the level of the single child and at the level of the pediatrician-parent relationship
(
pediatricians do call me occasionally and ask me what they should tell a parent who asks if a child should be taken off milk or if the childshould be x-rayed) I have found it extreme-ly persuasive to emphasize to the parents that the risk to their individual child is of the order of one-in-a-million to one-in-ten-million per year. This is an extremely im-portant point. We are dealing with a social problem and at the same time with a public health problem. The risk to the individual is
so minute against the background of things that a mother has to worry about, that if
she is going to be interested in the problem it should be in the context of the broad so-cial implications of the problem rather than
a danger to her individual child.
REFERENCES
1. National Committee on Radiation Protection:
Handbook 41. Washington, D.C.:
Depart-ment of Commerce, National Bureau of
Standards.
2. National Committee on Radiation Protection: Medical X-ray Protection up to Three Million
Volts. Washington, D.C.: Department of
Commerce, National Bureau of Standards,
