Chapter II: Part 2

This is a hopeful aspect of the situation for it means that, if the background radiation is doubled or tripled for mankind as a whole, only that small portion of the spontaneous mutation rate that is due to the background radiation will be doubled or tripled.

Let us suppose, for instance, that fully 1% of the spontaneous mutations occurring in mankind is due to background radiation. In that case, the tripling of the background radiation produced in the United States by man-made causes (see Table) would triple that 1%. In place of 99 non-radiational mutations plus 1 radiational, we would have 99 plus 3. The total number of mutations would increase from 100 to 102—an increase of 2%, not an increase of 200% that one would expect if all spontaneous mutations were caused by background radiation.

RADIATION EXPOSURES IN THE UNITED STATES[7]
Millirems[8]

Natural Sources
A. External to the body
1. From cosmic radiation 50.0
2. From the earth 47.0
3. From building materials 3.0
B. Inside the body
1. Inhalation of air 5.0
2. Elements found naturally in human tissues 21.0
Total, Natural sources 126.0
Man-made Sources
A. Medical Procedures
1. Diagnostic X rays 50.0
2. Radiotherapy X ray, radioisotopes 10.0
3. Internal diagnosis, therapy 1.0
Subtotal 61.0
B. Atomic energy industry, laboratories 0.2
C. Luminous watch dials, television tubes, 2.0
radioactive industrial wastes, etc.
D. Radioactive fallout 4.0
Subtotal 6.2
Total, man-made sources 67.2
Overall total 193.2

Dosage Rates

Another difference between the genetic and somatic effects of radiation rests in the response to changes in the rate at which radiation is absorbed. It makes a considerable difference to the body whether a large dose of radiation is absorbed over the space of a few minutes or a few years.

When a large dose is absorbed over a short interval of time, so many of the growing tissues lose the capacity for cell division that death may follow. If the same dose is delivered over years, only a small bit of radiation is absorbed on any given day and only small proportions of growing cells lose the capacity for division at any one time. The unaffected cells will continually make up for this and will replace the affected ones. The body is, so to speak, continually repairing the radiation damage and no serious symptoms will develop.

Then, too, if a moderate dose is delivered, the body may show visible symptoms of radiation sickness but can recover. It will then be capable of withstanding another moderate dose, and so on.

The situation is quite different with respect to the genetic effects, at least as far as experiments with _Drosophila_ and bacteria seem to show. Even the smallest doses will produce a few mutations in the chromosomes of those cells in the gonads that eventually develop into sex cells. The affected gonad cells will continue to produce sex cells with those mutations for the rest of the life of the organism. Every tiny bit of radiation adds to the number of mutated sex cells being constantly produced. There is no recovery, because the sex cells, after formation, do not work in cooperation, and affected cells are not replaced by those that are unaffected.

This means (judging by the experiments on lower creatures) that what counts, where genetic damage is in question, is not the rate at which radiation is absorbed but the total sum of radiation. Every exposure an organism experiences, however small, adds its bit of damage.

Accepting this hard view, it would seem important to make every effort to minimize radiation exposure for the population generally.

Since most of the man-made increase in background radiation is the result of the use of X rays in medical diagnosis and therapy, many geneticists are looking at this with suspicion and concern. No one suggests that their use be abandoned, for certainly such techniques are important in the saving of life and the mitigation of suffering. Still, X rays ought not to be used lightly, or routinely as a matter of course.

It might seem that X rays applied to the jaw or the chest would not affect the gonads, and this might be so if all the X rays could indeed be confined to the portion of the body at which they are aimed. Unfortunately, X rays do not uniformly travel a straight line in passing through matter. They are scattered to a certain extent; if a stream of X rays passes through the body anywhere, or even through objects near the body, some X rays will be scattered through the gonads.

It is for this reason that some geneticists suggest that the history of exposure to X rays be kept carefully for each person. A decision on a new exposure would then be determined not only by the current situation but by the individual’s past history.

Such considerations were also an important part of the driving force behind the movement to end atmospheric testing of nuclear bombs. While the total addition to the background radiation resulting from such tests is small, the prospect of continued accumulation is unpleasant.

What’s more, whereas X rays used in diagnosis and therapy have a humane purpose and chiefly affect the patient who hopes to be helped in the process, nuclear fallout affects all of humanity without distinction and seems, to many people, to have as its end only the promise of a totally destructive nuclear war.

It is not to be expected that the large majority of humanity that makes up the populations outside the United States, Great Britain, France, China, and the Soviet Union can be expected to accept stoically the risk of even limited quantities of genetic damage, out of any feeling of loyalty to nations not their own. Even within the populations of the three major nuclear powers there are strong feelings that the possible benefits of nuclear testing do not balance the certain dangers.

Public opinion throughout the world is a key factor, then, in enforcing the Nuclear Test Ban Treaty, signed by the governments of the United States, Great Britain, and the Soviet Union on October 10, 1963.

Effects on Mammals

Although genetic findings on such comparatively simple creatures as fruit flies and bacteria seem to apply generally to all forms of life, it seems unsafe to rely on these findings completely in anything as important as possible genetic damage to man through radiation. During the 1950s and 1960s, therefore, there have been important studies on mice, particularly by W. L. Russell at Oak Ridge National Laboratory, Oak Ridge, Tennessee.

While not as short-lived or as fecund as fruit flies, mice can nevertheless produce enough young over a reasonable period of time to yield statistically useful results. Experimenters have worked with hundreds of thousands of offspring born of mice that have been irradiated with gamma rays and X rays in different amounts and at different intensities, as well as with additional hundreds of thousands born to mice that were not irradiated.

Since mice, like men, are mammals, results gained by such experiments are particularly significant. Mice are far closer to man in the scheme of life than is any other creature that has been studied genetically on a large scale, and their reactions (one might cautiously assume) are likely to be closer to those that would be found in man.

Almost at once, when the studies began, it turned out that mice were more susceptible to genetic damage than fruit flies were. The induced mutation rate per gene seems to be about fifteen times that found in _Drosophila_ for comparable X ray doses. The only safe course for mankind then is to err, if it must, strongly on the side of conservatism. Once we have decided what might be safe on the basis of _Drosophila_ studies, we ought then to tighten precautions several notches by remembering that we are very likely more vulnerable than fruit flies are.

Counteracting the depressing nature of this finding was that of a later, quite unexpected discovery. It was well established that in fruit flies and other simple organisms, it was the total dosage of absorbed radiation that counted and that whether this was delivered quickly or slowly did not matter.

This proved to be _not_ so in the case of mice. In male mice, a radiation dose delivered at the rate of 0.009 rad per minute produced only from one-quarter to one-third as many mutations as did the same total dose delivered at 90 rads per minute.

In the male, cells in the gonads are constantly dividing to produce sex cells. The latter are produced by the billions. It might be, then, that at low radiation dose rates, a few of the gonad cells are damaged but that the undamaged ones produce a flood of sperm cells, “drowning out” the few produced by the damaged gonad cells. The same radiation dose delivered in a short time might, however, damage so many of the gonad cells as to make the damaged sex cells much more difficult to “flood out”.

A second possible explanation is that there is present within the cells themselves some process that tends to repair damage to the genes and to counteract mutations. It might be a slow-working, laborious process that could keep up with the damage inflicted at low dosage rates but not at high ones. High dosage rates might even damage the repair mechanism itself. That, too, would account for the fewer mutations at low dosage rates than at high ones.

To check which of the two possible explanations was nearer the truth, Russell performed similar tests on female mice. In the female mouse (or the female human being, for that matter) the egg cells have completed almost all their divisions before the female is born. There are only so many cells in the female gonads that can give rise to egg cells, and each one gives rise to only a single egg cell. There is no possibility of damaged egg cells being drowned out by floods of undamaged ones because there are no floods.

Yet it was found that in the female mouse the mutation rate also dropped when the radiation dose rate was decreased. In fact, it dropped even more drastically than was the case in the male mouse.

Apparently, then, there must be actual repair within the cell. There must be some chemical mechanism inside the cell capable of counteracting radiation damage to some extent. In the female mouse, the mutation rate drops very low as the radiation dose rate drops, so that it would seem that almost all mutations might be repaired, given enough time. In the male, the mutation rate drops only so far and no farther, so that some mutations (about one-third is the best estimate so far) cannot be repaired.

If this is also true in the human being (and it is at least reasonably likely that it is), then the greater vulnerability of our genes as compared with those of fruit flies is at least partially made up for by our greater ability to repair the damage.

This opens a door for the future, too. The workings of the gene-repair mechanism ought (it is to be hoped) eventually to be puzzled out. When it is, methods may be discovered for reinforcing that mechanism, speeding it, and increasing its effectiveness. We may then find ourselves no longer completely helpless in the face of genetic damage, or even of radiation sickness.

On the other hand, it is only fair to point out that the foregoing appraisal may be an over-optimistic view. Russell’s experiments involved just 7 genes and it is possible that these are not representative of the thousands that exist altogether. While the work done so far is most suggestive and interesting, much research remains to be carried out.

If, then, we cannot help hoping that natural devices for counteracting radiation damage may be developed in the future, we must, for the present, remain rigidly cautious.

Conclusion

It is unrealistic to suppose that all sources of man-made radiation should be abolished. The good they do now, the greater good they will do in the future, cannot be abandoned. It is, however, reasonable to expect that the present Nuclear Test Ban Treaty will continue and that nations, such as France and China, which have nuclear capabilities but are not signatories of the Treaty will eventually sign. It is also reasonable to expect that X ray diagnosis and therapy will be carried on with the greatest circumspection, and that the use of radiation in industry and research will be carried on with great care and with the use of ample shielding.

As long as man-made radiation exists, there will be some absorption of it by human beings. The advantages of its use in our modern society are such that we must be prepared to pay some price. This is not a matter of callousness. We have come to depend a great deal for comfort and even for extended life, upon the achievements of our technology, and any serious crippling of that technology will cost us lives. An attempt must be made to balance the values of radiation against its dangers; we must balance lives against lives. This involves hard judgments.

Those working under conditions of greatest radiation risk—in atomic research, in industrial plants using isotopes, and so on—can be allowed to set relatively high limits for total radiation dosages and dose rates that they may absorb (with time) with reasonable safety, but such rates will never do for the population generally. A relative few can voluntarily endure risks, both somatic and genetic, that we cannot sanely expect of mankind as a whole.[9]

From fruit fly experiments it would seem that a total exposure of 30 to 100 rads of radiation will double the spontaneous mutation rate. So much radiation and such a doubling of the rate would be considered intolerable for humanity.

Some geneticists have recommended that the average total exposure of human beings in the first 30 years of life be set at 10 rads. Note that this figure is set as a _maximum_. Every reasonable method, it is expected, will be used to allow mankind to fall as far short of this figure as possible. Note also that the 10-rad figure is an _average_ maximum. The exposure of some individuals to a greater total dose would be viewed as tolerable for society if it were balanced by the exposure of other individuals to a lesser total dose.

A total exposure of 10 rads might increase the overall mutation rate, it is roughly estimated, by 10%. This is serious enough, but is bearable if we can convince ourselves that the alternative of abandoning radiation technology altogether will cause still greater suffering.

A 10% increase in mutation rate, whatever it might mean in personal suffering and public expense, is not likely to threaten the human race with extinction, or even with serious degeneration.

The human race as a whole may be thought of as somewhat analogous to a population of dividing cells in a growing tissue. Those affected by genetic damage drop out and the slack is taken up by those not affected.

If the number of those affected is increased, there would come a crucial point, or threshold, where the slack could no longer be taken up. The genetic load might increase to the point where the species as a whole would degenerate and fade toward extinction—a sort of “racial radiation sickness”.

We are not near this threshold now, however, and can, therefore, as a species, absorb a moderate increase in mutation rate without danger of extinction.

On the other hand, it is _not_ correct to argue, as some do, that an increase in mutation rate might be actually beneficial. The argument runs that a higher mutation rate might broaden the gene pool and make it more flexible, thus speeding up the course of evolution and hastening the advent of “supermen”—brainier, stronger, healthier than we ourselves are.

The truth seems to be that the gene pool, as it exists now, supplies us with all the variability we need for the effective working of the evolutionary mechanism. That mechanism is functioning with such efficiency that broadening the gene pool cannot very well add to it, and if the hope of increased evolutionary efficiency were the only reason to tolerate man-made radiation, it would be insufficient.

The situation is rather analogous to that of a man who owns a good house that is heavily mortgaged. If he were offered a second house with a similar mortgage, he would have to refuse. To be sure, he would have twice the number of houses, but he would not need a second house since he has all the comfort he can reasonably use in his first house—and he would not be able to afford a second mortgage.

What humanity must do, if additional radiation damage is absolutely necessary, is to take on as little of that added damage as possible, and not pretend that any direct benefits will be involved. Any pretense of that sort may well lure us into assuming still greater damage—damage we may not be able to afford under any circumstances and for any reason.

Actually, as the situation appears right now, it is not likely that the use of radiation in modern medicine, research, and industry will overstep the maximum bounds set by scientists who have weighed the problem carefully. Only nuclear warfare is likely to do so, and apparently those governments with large capacities in this direction are thoroughly aware of the danger and (so far, at least) have guided their foreign policies accordingly.

SUGGESTED REFERENCES

Books

_Radiation, Genes, and Man_, Bruce Wallace and Theodosius Dobzhansky,
Holt, Rinehart and Winston, Inc., New York 10017, 1963, 205 pp., $5.00
(hardback); $1.28 (paperback).

_Genetics in the Atomic Age_ (second edition), Charlotte Auerbach,
Oxford University Press, Inc., Fair Lawn, New Jersey 07410, 1965, 111
pp., $2.50.

_Atomic Radiation and Life_ (revised edition), Peter Alexander, Penguin
Books, Inc., Baltimore, Maryland 21211, 1966, 288 pp., $1.65.

_The Genetic Code_, Isaac Asimov, Grossman Publishers, Inc., The Orion
Press, New York 10003, 1963, 187 pp., $3.95 (hardback); $0.60
(paperback) from the New American Library of World Literature, Inc.,
New York 10022.

_Radiation: What It Is and How It Affects You._ Ralph E. Lapp and Jack
Schubert, The Viking Press, New York 10022, 1957, 314 pp., $4.50
(hardback); $1.45 (paperback).

_Report of the United Nations Scientific Committee on the Effects of
Atomic Radiation_, General Assembly, 19th Session, Supplement No. 14
(A/5814), United Nations, International Documents Service, Columbia
University Press, New York 10027, 1964, 120 pp., $1.50.

_The Effects of Nuclear Weapons_, Samuel Glasstone (Ed.), U. S. Atomic
Energy Commission, 1962, 730 pp., $3.00. Available from the
Superintendent of Documents, U. S. Government Printing Office,
Washington, D. C. 20402.

_Effect of Radiation on Human Heredity_, World Health Organization,
International Documents Service, Columbia University Press, New York
10027, 1957, 168 pp., $4.00.

_The Nature of Radioactive Fallout and Its Effects on Man_, Hearings
before the Special Subcommittee on Radiation of the Joint Committee on
Atomic Energy, Congress of the United States, 85th Congress, 1st
Session, U. S. Government Printing Office, 1957, Volume I, 1008 pp.,
$3.75; Volume II, 1057 pp., $3.50. Available from the Office of the
Joint Committee on Atomic Energy, Congress of the United States,
Senate Post Office, Washington, D. C. 20510.

_Genetics, Radiobiology, and Radiology_, Proceedings of the Midwestern
Conference, Wendell G. Scott and Evans Titus, Charles C. Thomas
Publisher, Springfield, Illinois 62703, 1959, 166 pp., $5.50.

Articles

Genetic Hazards of Nuclear Radiations, Bentley Glass, _Science_, 126:
241 (August 9, 1957).

Genetic Loads in Natural Populations, Theodosius Dobzhansky, _Science_,
126: 191 (August 2, 1957).

Radiation Dose Rate and Mutation Frequency, W. L. Russell and others,
_Science_, 128: 1546 (December 19, 1958).

Ionizing Radiation and the Living Cell, Alexander Hollaender and George
E. Stapleton, _Scientific American_, 201: 95 (September 1959).

Radiation and Human Mutation, H. J. Muller, _Scientific American_, 193:
58 (November 1955).

Ionizing Radiation and Evolution, James F. Crow, _Scientific American_,
201: 138 (September 1959).

Motion Pictures

_Radiation and the Population_, 29 minutes, sound, black and white,
1962. Produced by the Argonne National Laboratory. This film explains
how radiation causes mutations and how these mutations are passed on
to succeeding generations. Mutation research is illustrated with
results of experimentation on generations of mice. A discussion of
work with fruit flies and induced mutations is also included. This
film is available for loan without charge from the AEC Headquarters
Film Library, Division of Public Information, U. S. Atomic Energy
Commission, Washington, D. C. 20545 and from other AEC film libraries.

The following films were produced by the American Institute of
Biological Sciences and may be rented from the Text-Film Division,
McGraw-Hill Book Company, 330 West 42nd Street, New York 10036.

_Mutation_, 28 minutes, sound, color, 1962. This film discusses
chromosomal and genetic mutations as applied to man. Muller’s work in
inducing mutations by X rays is described.

These three films are 30 minutes long, have sound, are in black and
white, and were released in 1960. They are part of a 48-film series
that is correlated with the textbook, _Principles of Genetics_, (fifth
edition), Edmund W. Sinnott, L. C. Dunn, and Theodosius Dobzhansky,
McGraw-Hill Book Company, 1958, 459 pp., $8.50.

_Mutagen-Induced Gene Mutation._ The narrator of this film is Hermann J.
Muller, who won a Nobel Prize in 1946 for his work in the field of
genetics. The measurement of X-ray dose in roentgens and the dose
required to double the spontaneous mutation rate in _Drosophila_ and
mice are discussed. The magnitude and meaning of permissible doses of
high-energy radiation are discussed. Other mutagenic agents
(ultraviolet light and chemical substances) are discussed, concluding
with comments on the importance of gene mutation in the present and
future.

_Selection, Genetic Death and Genetic Radiation Damage._ The narrator of
this film is Theodosius Dobzhansky, the coauthor of this booklet.
Genetic death is discussed in detail, as are examples of how genetic
loads are changed subsequent to radiation exposure. While it is
generally agreed that the great majority of mutants are harmful when
homozygous, more evidence is needed about the beneficial and
detrimental effects of mutants when heterozygous. In the case of
sickle cell anemia, heterozygotes are adaptively superior to normal
homozygotes. This makes for balanced polymorphism, by which a gene is
retained in the population despite its lethality when homozygous
because of the advantage it confers when heterozygous.

_Gene Structure and Gene Action._ The lecturer of this film is G. W.
Beadle of Cornell University. The Watson-Crick structure of DNA is
discussed in terms of mutation. Several tests of the chain separation
hypothesis for DNA replication are described (experiments with heavy
DNA, radioactive chromosomes, and the replication of DNA in vitro).
This working hypothesis is presented: The coded information in DNA is
transferred to RNA, which serves as a template for polypeptide
synthesis.

PHOTO CREDITS

Dr. Asimov’s photograph by David R. Phillips, courtesy _Chemical and
Engineering News_

Page

4 James German, M.D.
6 Bausch & Lomb, Inc.
12 James German, M.D.
20 Indiana University
24 Robert C. Filz, Air Force Cambridge Research Laboratories
25 J. K. Boggild, Niels Bohr Institute, Copenhagen University
26 Brookhaven National University
28, 31 Herman Yagoda, Air Force Cambridge Research Laboratories
41 Oak Ridge National Laboratory

Footnotes

[1]For more detail about cell division, see _Radioisotopes and Life
Processes_, another booklet in this series.

[2]This is more commonly known as “Mongolism” or “Mongolian idiocy”
though it has nothing to do with the Mongolian people.

[3]Actually, all waves have some of the characteristics of particles and
all particles have some of the characteristics of waves. Usually,
however, the radiation is predominantly one or the other and little
confusion arises under ordinary circumstances in speaking of waves
and particles as though they were separate phenomena.

[4]For more about this subject, see _Radioisotopes in Industry_ and
_Radioisotopes in Medicine_, companion booklets in this series.

[5]For more about this subject, see _Fallout from Nuclear Tests_,
another booklet in this series.

[6]For details on _somatic_ effects of radiation, see _Your Body and
Radiation_, a companion booklet in this series.

[7]Estimated average exposures to the gonads, based on 1963 report of
Federal Radiation Council.

[8]One thousandth of a rem.

[9]Nevertheless, it should be pointed out that the precautions taken in
the atomic energy industry are such that absorption of radiation is
not as severe a problem as one might suspect. Fully 95% of those
engaged in this work receive less than 1 rem a year. Only 1% receive
more than 5 rems.

UNITED STATES ATOMIC ENERGY COMMISSION

_Dr. Glenn T. Seaborg, Chairman_
_James T. Ramey_
_Dr. Gerald F. Tape_
_Dr. Samuel M. Nabrit_
_Wilfrid E. Johnson_

_ONE OF A SERIES ON UNDERSTANDING THE ATOM_

Nuclear energy is playing a vital role in the life of every man, woman, and child in the United States today. In the years ahead it will affect increasingly all the peoples of the earth. It is essential that all Americans gain an understanding of this vital force if they are to discharge thoughtfully their responsibilities as citizens and if they are to realize fully the myriad benefits that nuclear energy offers them.

The United States Atomic Energy Commission provides this booklet to help you achieve such understanding.

Edward J. Brunenkant
Director
Division of Technical Information

This booklet is one of the “Understanding the Atom” Series. Comments are invited on this booklet and others in the series; please send them to the Division of Technical Information, U. S. Atomic Energy Commission, Washington, D. C. 20545.

Published as part of the AEC’s educational assistance program, the series includes these titles:

NUCLEAR POWER AND MERCHANT SHIPPING
PLUTONIUM
OUR ATOMIC WORLD
NUCLEAR ENERGY FOR DESALTING
CONTROLLED NUCLEAR FUSION
WHOLE BODY COUNTERS
PLOWSHARE
POPULAR BOOKS ON NUCLEAR SCIENCE
SNAP, NUCLEAR SPACE REACTORS
NUCLEAR REACTORS
ATOMS, NATURE, AND MAN
MICROSTRUCTURE OF MATTER
SYNTHETIC TRANSURANIUM ELEMENTS
COMPUTERS
RESEARCH REACTORS
GENETIC EFFECTS OF RADIATION
POWER FROM RADIOISOTOPES
NONDESTRUCTIVE TESTING
RARE EARTHS
FOOD PRESERVATION BY IRRADIATION
FALLOUT FROM NUCLEAR TESTS
RADIOACTIVE WASTES
RADIOISOTOPES IN INDUSTRY
ATOMS AT THE SCIENCE FAIR
RADIOISOTOPES AND LIFE PROCESSES
ATOMIC FUEL
ATOMIC POWER SAFETY
DIRECT CONVERSION OF ENERGY
CAREERS IN ATOMIC ENERGY
RADIOISOTOPES IN MEDICINE
ACCELERATORS
NUCLEAR TERMS, A BRIEF GLOSSARY
NEUTRON ACTIVATION ANALYSIS
ATOMS IN AGRICULTURE
POWER REACTORS IN SMALL PACKAGES

Single copies of any booklet may be obtained free by writing to:

USAEC, P. O. BOX 62, OAK RIDGE, TENNESSEE 37830

Requests for more than three titles generally can not be honored.

Complete sets of the series are available to school and public librarians, and to teachers who can make them available for reference or for use by groups. Requests should be made on school or library letterheads and indicate the proposed use.

Students and teachers who need publications on specific topics related to nuclear science, or references to other reading material, may also write to the Oak Ridge address. Requests should state the topic of interest exactly, and the use intended.

_IMPORTANT_: All requests should include the “Zip Code” in the address to which the material is to be mailed.

Printed in the United States of America

USAEC Division of Technical Information Extension, Oak Ridge, Tennessee
September 1966

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