High-energy radiation is radiation of any sort that has enough energy to damage molecules, for example, by removing electrons from them. X-rays were the first form of high-energy radiation to be discovered. The quantum of x-rays from an x-ray tube operated at 50,000 volts has an energy of 50,000 electron volts, and the amount of energy required to remove an electron from a molecule is only about 10 to 25 electron volts. Similarly, the amount of energy required to break a chemical bond is in general between one and eight electron volts. Accordingly a quantum of ordinary x-radiation has far more energy than is necessary to break a chemical bond in a molecule or to remove electrons from a molecule.

Radioactivity was discovered in 1896, one year after the discovery of x-rays, and it was at once recognized that radioactive substances are a source of energy. Within a few years it was found that the rays emitted by natural radioactive substances are of three kinds - alpha rays, beta rays, and gamma rays. All of these rays are high-energy rays. The alpha rays are alpha particles, the nuclei of helium atoms, moving with high velocity; the beta rays are electrons moving with high velocity; and the gamma rays are electromagnetic rays similar to x-rays but in general with far greater energy than ordinary x-rays have, of the order of millions of electron volts per quantum.

Another sort of high-energy radiation, neutron rays, was discovered in 1932. Neutrons moving with high velocity have a great amount of energy, and are able to damage molecules by collision with the nuclei, causing chemical bonds to be broken and resulting in ionization.

The common unit of high-energy radiation is the roentgen, the amount of radiation that causes the formation of 2.08 x 10⁹ ions when it passes through one cubic centimeter of dry air at 0˚ C and one atmosphere pressure. Sometimes other units (rad, rem, rap) are used. These units do not differ much from the roentgen, and in this article I shall use roentgen, abbreviated small r, as the unit.

It has been found in many experiments that equal amounts of high-energy radiation of various kinds, measured in roentgens, produce essentially equal biological effects in organisms exposed to them.

The biological effects produced by high-energy radiation are of many kinds. They may be divided into the genetic effects, principally the production of gene mutations that in human beings lead later on to the birth of defective children, and somatic effects. The somatic effects include production of cancer, including leukemia and bone cancer and also many other kinds of cancer, the damage of fetuses through irradiation in utero such as to cause congenital deformities, production of cataracts, and a general aging effect on human beings increasing the incidence of diseases of all sorts and leading to decreased longevity

There are several ways in which high-energy radiation may cause these biological effects. One way is that of direct damage to certain molecules, such as the molecules of deoxyribonucleic acids that constitute the genes. This damage might result from the ionization of the molecule itself and subsequent chemical reaction leading to the mutated form of the molecule, or from the expulsion of the nucleus of an atom from the molecule, with similar result, or from the production through ionization of active molecules which could then attack the DNA molecule, causing mutation, or in some other way. There is evidence that high-energy radiation produces peroxides that can attack DNA molecules that about one third of the mutations produced by a heavy dose of high-energy radiation result from the direct damage to the genes and two thirds from indirect damage by way of peroxides or other oxidizing agents.

I shall first discuss genetic damage by high-energy radiation, and then somatic damage, including especially cancer.

Professor H. J. Muller, who was then in the University of Texas and is now Professor of Genetics in Indiana University, discovered in 1927 that exposure of plants and animals to x-rays causes an increase in the number of mutations. Professor Muller was one of the original group of workers under the leadership of Professor Thomas Hunt Morgan who discovered the gene, was given the Nobel Prize in 1946 for his discovery of the mutagenic action of x-rays. This discovery made it possible for breeders of plants and animals to improve the strains more rapidly than was possible before, because by use of x-rays large numbers of mutants can be produced and the favorable ones selected, the unfavorable ones rejected and not allowed to have progeny. All kinds of high-energy radiation produce mutations, in amounts approximately proportional to the ionizing power of the radiations, measured in roentgens. Cosmic rays, gamma rays, beta rays, alpha rays, and neutrons are all effective if they reach the reproductive organs. Every living organism that has been investigated in this way - every animal, every plant, and every micro-organism, including even viruses - has been found to undergo mutation when subjected to the action of high-energy radiation. There is also evidence for man; for example, the sex-ratio of offspring of Hiroshima-Nagasaki survivors has been shown to have been changed in a statistically significant way, and it is generally believed that this change was the result of the exposure of these human beings to high-energy radiation at the time of the explosions.

The genetic damage to man by exposure to high-energy radiation has been discussed in detail by many groups of scientists, especially geneticists. Among the principal reports on this subject are the report "The Hazards to Man of Nuclear and Allied Radiations", presented to Parliament by the Chairman of the British Medical Research Council in 1956, the report "The Biological Effects of Atomic Radiation", formulated in 1946 by a National Academy of Sciences-National Research Council Committee of American scientists (the section on genetic effects of atomic radiation was prepared by a committee of sixteen, including most of the leading American geneticists), and the report of the United Nations Scientific Committee on The Effects of Atomic Radiation, 1958, which was prepared by eighty five scientists from fifteen countries, including thirteen from the United States, among whom were Professors G. W. Beadle of the California Institute of Technology, James Crow of the University of Wisconsin, Theodore Sobzhansky of Columbia University, Sterling Emerson of the California Institute of Technology, and J. V. Neel of the University of Michigan. From these reports it is evident that geneticists are in general agreement about the genetic effects of high-energy radiation on man. Among the conclusions reached by the geneticists of these three committees are that genetic mutations caused by radiation are similar to those arising from other causes, that the mutational effects of small doses of radiation delivered to the cells of the human germ line over long periods of time are cumulative, that there is no threshold of radiation exposure below which genetic damage does not occur, and that at the lower dose levels the relationship between the number of induced mutations and the dose is a linear one. Following are some quotations from the report of the U.S. National Academy of Sciences-National Research Council Committee:

1. "Radiations cause mutations."

2. "Practically all radiation-induced mutations which have effects large enough to be detected are harmful."

3. "Any radiation dose, however small, can induce some mutations. There is no minimum amount of radiation dose which must be exceeded before any harmful mutations occur."

4. "The total dose of radiation is what counts, this statement being based on the fact that the genetic damage done by radiation is cumulative. A larger amount of radiation produces a larger number of mutations. The additional mutants would be of the same sort as those produced by the natural background radiation. That is to say, larger doses of radiation would produce more, but not worse, mutants."

"The concept of a safe rate of radiation simply does not make sense if one is concerned with genetic damage to future generations. What counts, from the point of view of genetic damage, is not the rate: it is the total accumulated dose to the reproductive cells of the individual from the beginning of his life up to the time the child is conceived."

"Any radiation is genetically undesirable. ... The genetic harm is proportional to the total dose."

"As geneticists we say: keep the dose as low as you can."

It is not easy to make a reliable estimate of the number of mutations that will be caused by a certain amount of high-energy radiation passing through the gonads of the population of human beings, and of the number of defective children that will be caused to be born as a result. It probably can be said that everyone suffers to some extent from his inheritance of some defective genes. A survey by R. Mcintosh, K. K. Merritt, M. R. Richards, M. R. Samuels, and M. T. Bellows, Pediatrics 14, 505 (1954) gave 7.4 percent as the incidence of malformations in live-born infants based upon examinations at birth, six months, and twelve months of age. About 43 percent of the malformations were observed at birth. The incidence was 7.0 percent based upon infants surviving the neonatal period. Some congenital defects, of course, would not be observed - an example is Huntington's chorea, which makes itself evident only later in life. Professor James F. Crow, a member of both the United States Committee and the United Nations Committee, stated in his testimony before the Special Subcommittee on Radiation of the Joint Committee on Atomic Energy of the U. S. Congress in June 1957 that the number of children born with gross physical or mental defects because of simple genetic origin is about two percent of children born. Professor Taage Kemp of the University of Copenhagen in his book "Genetics and Disease" says that two to three percent of the population suffer from severe hereditary affections. There is a steady state in the pool of human germ plasm, such that the defective genes producing the children with gross physical or mental defects are being removed from the pool of human germ plasm at the same rate as that which they are being produced by new mutations - that is, I should say that there was a steady state a century or two ago, but that we probably are now in the process of approaching a new steady state, representing an increased concentration of the defective genes and an increased incidence of children born with hereditary defects, as a result of the recent increase in the mutation rate.

We may ask how much of an increase in the mutation rate would be caused by the exposure of the gonads of all human beings to a certain amount of x-radiation and other high-energy radiation. This question may be put in the form of asking what dose of radiation would cause a doubling of the mutation rate. The Committee of the British Medical Research Council reached the conclusion that the doubling dose probably lies between 30 r and 80 r, but that it might be as low as 15 r. Values for the animals and plants for which there exists the most reliable data lie for the most part between 25 r and 60 r. The estimated values for man reported by the United States Committee lie between 5 r and 150 r, and the Committee said that the doubling dose is almost surely between 5 r and 150 r, and may very well lie between 50 r and 80 r. The United Nations Committee reported that it is reasonably probably that the doubling dose for human genes lies in the range 10 r to 100 r, and the Committee accepted the value 30 r for its calculations.

The average 30-year dose to the gonads of human beings from natural radiation (cosmic rays and natural radioactivity) is given by the United Nations Committee as 3 r, by the British Committee as 3 r, and by the United States Committee as 4.3 r. Thirty years is used here because it is the average age of parents and hence is the average period of exposure of the genes. Accordingly the background radiation has been estimated to cause about ten percent of all mutations in man.

The United States Committee estimated that medical x-radiation to the gonads of people in the United States is on the average three roentgens per thirty years. This exposure should increase the mutation rate by about ten percent, if 30 r is the doubling dose, and accordingly increase the number of children born with gross physical or mental defects by hereditary causes by about ten percent. There is much difficulty in determining, by experiment or statistical methods, the amount of genetic damage that may be expected to be done by a certain amount of exposure of the gonads of human beings to high-energy radiation. With the incidence of defective births rather small, on the order of, say, four percent, a population of a million children whose parents had been exposed to an extra amount of high-energy radiation might be compared with a control population of a million children whose parents had not been exposed. There would then be about 40,000 defective children in each group, and the statistical uncertainty, which is determined by the square root of the number (200), would be less than one percent. A study with two such populations might well give highly significant results, but a study of a smaller population, say of 10,000 children, could hardly be expected to give significant results, because the statistical uncertainty of five percent, together with other sources of error, might well prevent the effect of the added radiation from being detected. It is gratifying that one statistically significant study has now been made, under conditions such as to permit us to have considerable confidence in the conclusions reached by the investigators. The investigation was an epidemiological study of congenital malformations in New York State, carried out by John T. Gentry, Elizabeth Parkhurst, and George V. Bulin, Jr., American Journal of Public Health 49, 497 (1959). Dr. Gentry, of the New York State Department of Health, and his associates carried out in the period 1955 to 1959 an analysis of birth records and other records about 1,242,744 children born in upstate New York (the state exclusive of New York City) during the eight-year period 1948 to 1955. It had been called to their attention that a larger number of children with cleft palate seemed to be born in certain regions in the state than in other regions, and they soon noticed that the townships with a high reported malformation rate were located in the areas with igneous bedrock or with outcrop of black shale having relatively high levels of radioactivity. They divided the state up into the high-radiation regions and the low-radiation regions on the basis of geological information; the high-radiation regions were the igneous bedrock areas, the areas with outcrops of black shale, and these over glacial material of igneous origin. The low-radioactivity regions are those over sedimentary material. From various studies, including that of Dr. Willard F. Libby, it is likely that the level of background radiation is between fifty percent and one hundred percent greater in the high-radiation regions than in the low-radiation regions. The two regions are closely interpenetrating, and the communities in the two regions seem to be closely similar in their nature.

It was found that the incidence of malformations was eighteen percent greater in the high-radiation regions than in the low-radiation regions. It was thirty-one percent greater for igneous bedrock areas not covered with glacial deposits, the regions with probably the highest radiation, and sixteen percent greater for the river valleys and glacial drift border areas than for the low-radiation areas. The two urban populations and the two rural populations were compared separately: the increase for high-radiation areas over low-radiation areas was found to be eleven percent for urban populations and twenty-three percent for rural populations (the populations are not stable - only about sixteen percent of families have had a history of residence of some member of the family in the same area for over 75 years, and there is probably less stability for the urban population than the rural population).

Nine classes of malformations were considered separately, and for eight of these classes (malformations of central nervous system, circulatory system, bones and joints, etc.) a greater incidence was found in the high-radiation areas than in the low-radiation areas. The only exception was mongolism, for which the same incidence, 0.31 per 1,000 births during the eight-year period, was found. Mongolism is, however, to be known to have little familial factor, and not to be the result of a gene mutation. It is instead the result of non-disjunction - 47 chromosomes.

From these statistical studies we may conclude that it is likely that the doubling of the background radiation causes an 18-percent increase in mutation rate, and perhaps more. Some of the malformations are probably not the result of mutations, although they may be the result of damage done by the radiation to the developing fetus. It hence seems likely from the study reported by Dr. Gentry and his associates that the doubling dose for mutations for human beings is as small as 15 r, and perhaps as small as 10 r, and that background radiation in general accounts for twenty percent or thirty percent of all mutations. I believe that this is the most significant and reliable evidence that we have at the present time about the effects of small amounts of radiation in causing mutations in human beings.

Now let us discuss the somatic effects of high-energy radiation, beginning with cancer.

There is no doubt that large amounts of high-energy radiation cause cancer in human beings. As a first example we may consider the incidence of leukemia among the survivors of the atomic bombings at Hiroshima and Nagasaki. The incidence of leukemia among the population of Japan in general is only about twenty cases per million per year. Among the survivors of the atomic bombings who were closest to the hypocenters of the bombs it has been found to be nearly a thousand times greater. For example, during the period 1948 to 1955 there were 18 deaths by leukemia among 1,870 people within one kilometer of the hypocenters, whereas no cases would have been expected for a normal population, and among the 13,730 people between one and 1.5 kilometers from the hypocenters there were 41 cases, with only two normally expected. It is interesting that the high incidents of leukemia has remained almost constant for this population year after year, from 1948 to the present time. Apparently the damage done to the survivors was such as to cause a certain probability of occurrence of leukemia each year, year after year. (E. B. Lewis, Science, 17 May 1957)

It has also been found that there is a high incidence of leukemia among patients treated with x-rays for ankylosing spondylitis, a hereditary disease of the spine (W. M. Court Brown and R. Doll, in the report of the British Medical Research Council quoted above, and other investigators). Among 11,287 male patients irradiated during the period 1935 to 1954 thirty seven cases of leukemia were discovered, during an average period of follow-up of each patient of five years. The patients were estimated to have between 500 and 2,750 r during the treatment. The number of cases of leukemia that would have been expected of this population normally is about one, so that there is no doubt that the incidence of leukemia is abnormal.

Many infants, in past years, were irradiated as treatment of enlargement of the thymus. A comparison has been made (C. L. Simpson, L. H. Hemplemann, and L. M. Fuller, Radiology, 64, 840 (1955)) of 1,400 individuals who were thus irradiated, with 1,795 unirradiated siblings taken as control. The average period of follow-up was about fifteen years. In the irradiated group there were seven confirmed leukemia (and one unconfirmed case), while there were no cases in the control group.

All of these occurrences correspond to about the same effectiveness of high-energy radiation, per roentgen year, in causing leukemia during each year after exposure to radiation us 2x10^-6 per roentgen full-body exposure. This means, for example, that exposure to 1,000 roentgens increases the chance of leukemia to two per thousand for each succeeding year, or to one percent for a twenty-year period after the exposure. This value is stated by Professor Lewis to be such that about 10 percent of natural leukemia can be attributed to the background radiation.

Other well-documented cases of induction of cancer by high-energy radiation include cancer of the lung in the uranium mines of Schneeberg and Joachimsthal, cancer of the bones and joints in workers making use of radium paint for dials of watches and clocks, bone cancer occurring after the use of x-rays for the treatment of non-malignant bone tumors and infections, cancer of the skin in radiologists and radiological technicians, and cancer of the thyroid gland, the pharynx, and the larynx among children who have been given x-ray treatment for suspected enlargement of the thymus gland, bronchitis, infected tonsils and adenoids, and enlarged glands in the neck.

A very important question is that as to whether or not there is a certain exposure to high-energy radiation that has no chance of causing cancer - that is, whether or not there is a threshold. Opinion has been divided on this point. My own opinion is that there is strong evidence that even small amounts of high-energy radiation have a corresponding chance of causing cancer; that is, that there is no threshold.

One important argument is based upon the approximate equality in cancerogenic action of radiation exposures over a long period of time and of a Single large exposure of equal amount. For example, the radium painters who got radium in their bones were exposed to a very small amount of high-energy radiation during any one period of one day, say, but the exposure continued day after day throughout their lives. Ordinarily one expects that an effect would not be cumulative. For example, a person who receives between 400 and 500 r of full-body exposure to radiation in a short time, within one hour, has a good chance of dying of acute radiation sickness - about fifty percent survival, and 1,000 r is almost certain to kill a person. Nevertheless, he can receive 100 r per day for ten days or twenty days without being killed. The threshold here is about 450 r. It is evident that the damage done by the radium in causing cancer is quite different in nature from that done by the single large dose of radiation in causing death by acute radiation sickness, for which there is a threshold. Similarly it is found that death by leukemia among radiologists occurs at the rate of 600 per million per year, about ten times that for the population at large and for other physicians. The increased incidence of leukemia per roentgen exposure (estimated) is about the same for the chronic exposure that the radiologists have received as for the single large doses received by the Nagasaki and Hiroshima survivors and the patients with ankylosing spondylitis and other diseases who were subjected to x-ray therapy. There is accordingly no doubt that the cancerogenic action of high-energy radiation is essentially cumulative, so that whatever damage is done by a small dose of radiation continues to affect the human being.

A simple explanation of this cumulative activity is that the damage is, at least in part, similar to that done to the genetic cells, in the gonads, giving rise to gene mutations. Most cells in the human body contain nuclei, similar to those in the reproductive organs. It seems highly probable that some cases of cancer are the result of damage to the molecules in the nuclei, especially the molecules of deoxyribonucleic acid that govern the functioning of all of the cells in the body.

Indication that the threshold for production of cancer, if there is a threshold, is a low one is provided by the incidence of leukemia among Hiroshima and Nagasaki survivors, as reported by Professor Lewis. During the period from January 1948 to September 1955 there were eighteen cases of leukemia among the 1,870 survivors less than one kilometer from the hypocenter, with none normally expected. The average radiation dose was 650 r, corresponding to an incidence of 15 per million per roentgen. For the 13,730 survivors in the range from one to 1.5 kilometer there were 41 cases, with two normally expected, leaving 39 attributed to radiation, and the average radiation dose was 250 r. For this population the incidence was eleven per million per r. From the average of these two values, thirteen per million per r, one would predict seven cases attributed to radiation for the 23,060 survivors in the range 1.5 to 2 kilometers from the hypocenter, with average radiation dose 25 r. The number of cases attributed to radiation found for this population was six, indicating strongly that the assumption of proportionality of the induction of leukemia to the amount of exposure is still valid for exposures of 25 r.

Much publicity was given to an investigation carried out two years ago by Dr. Miriam T. Finkel of Argonne National Laboratory. On 19 Sept. 1958 she published in Science a paper about her studies of the effects of strontium-90 injected into mice on life expectancy an on incidence of tumors of bone and blood-forming tissues (M. P. Finkel, Science 128, 637, (1958)). She discussed the question of whether or not the effects are proportional to the amount of injected strontium-90 at low doses, and reached the conclusion that it is likely that there is a threshold with value for man between five and fifteen microcuries, as compared with the present average value from fallout, about 0.0004 microcuries, and the predicted steady-state value from fallout for testing of nuclear weapons at the average rate for the period 1954 to 1958, about 0.02 microcuries. Her paper ended with the sentence "In any case, the present contamination with strontium-90 from fallout is so very much lower than any of these levels that it is extremely unlikely to induce even one bone tumor or one case of leukemia." On the same day, 19 September 1958, newspapers throughout the United States published accounts of this work. For example, the Pasadena (Calif.) Star-News contained an article with the headline "Tests on Mice Show Fallout Safe" and the first sentence "A woman researcher says tests on mice show that the present fallout from nuclear weapons tests will not produce a single case of bone cancer or leukemia in humans." The New York Times published accounts of the work in both 19 September and 28 September 1958.

Professor Barclay Kamb and I made a statistical analysis of Dr. Finkel's data. This analysis led us to the conclusion that she had no justification whatever for her concluding statement. All of her data were found to be compatible with a zero threshold for strontium-90. Moreover, the statistical analysis showed that in order for Dr. Finkel to have been justified with 90-percent confidence (10% type-II error) is making her concluding statement on the basis of her data she would have to have used over 1,000,000,000,000 mice in each of her experimental groups, instead of the 150 or less that were used (B. Kamb and L. Pauling, Proc. U. S. Nat. Acad. Sci., 45, 54 (1959)).

It is easy to see that Dr. Finkel could not have been justified in making her statement that not even one bone tumor or one case of leukemia would occur in the population of the world on the basis of experiments carried out with only a few hundred mice. Even without the use of statistical methods, anyone can see that to make a statement about one case in the nearly three billion people on earth one would have to carry out experiments with at least the same number of mice. Our analysis showed that to make a really confident statement that not a single case would occur among the worlds population would require that a study be made involving a much larger number of mice than the number of people in the world.

At the present time no one can honestly say that even the small amount of high-energy radiation from radioactive fallout from the bomb tests carried out up to the present time will not damage human beings in such a way as to cause leukemia and bone cancer, and probably also cancer of other sorts.

The best evidence on this point is in my opinion that provided by the statistical survey of the death of children by cancer carried out by Alice Stewart, Josefine Webb, and David Hewitt, and reported In the British Medical Journal, June 28, 1958, pp. 1495 to 1508. These authors made an effort to trace all children in England and Wales who had died of leukemia or cancer before their tenth birthday during the years 1953 to 1955 and to compare their pre-natal and post-natal experiences with those of a corresponding group of healthy children, selected in such a way as to match the dead children for age, sex, and locality but otherwise picked at random from the local birth register. The total number of deaths by childhood cancer for this period was 1,694, of which 792 were ascribed to leukemia and 902 to other cancers. By May 1957 the mothers of 1,416 of these children had been interviewed, and the mothers of an equal number of children in the control group. It was found that only one factor in the history of the two populations of the children was significantly different. This factor was whether or not the mother had been given a pelvimetric roentgenographic investigation during the months before the child was born. Of the children who had died of cancer, 13.7 percent had been subjected to direct x-ray exposure in the fetal stage, whereas of the control children, who had not died, only 7.2 percent had been so exposed. The estimated amount of exposure of the fetus to x-radiation (a little over an average of two films per child exposed) is 2 r. Accordingly we reach the conclusion that as small an amount of radiation as 2 r is enough to cause cancer. This is of the order of magnitude of background radiation and fallout radioactivity, and it provides strong evidence that there is no threshold for the production of cancer by high-energy radiation.

From the above values it can be calculated that exposure of the fetus to x-radiation during an x-ray investigation of the mother doubles the chance that the child will die of cancer before he has passed the tenth year of his life. Cancer is the principal cause of death of children from age one to ten. The chance that the child will die of cancer is one in twelve hundred for unirradiated children and one in six hundred for irradiated children. The statistical evidence obtained by Stewart, Webb, and Hewitt indicates that lymphatic leukemia, blast-cell leukemia, malignant tumors of the central nervous system, the kidney, the suprarenals, and other sites are caused by irradiation of the fetus, whereas myeloblastic leukemia and lymphosarcoma and other reticuloses seem not to be caused by radiation.

These results show that x-radiation must be used with caution, and should not be misused - it should not be used unless there is sound medical reason. In England, where the state of medical practice is excellent, about six percent of pregnant women are subjected to x-ray examination of the abdominal region. This percentage may be a reasonable estimate of the number of cases in which the x-radiation is justified. In the United States there are obstetricians who have a rule that every pregnant woman coming under their care should be subjected to this examination. For example, my daughter, when she was pregnant, was told by her obstetrician that she should have an x-ray picture made of the pelvic region. He had already told her that examination showed her to be in excellent health and very well suited to child-bearing. When she objected to having the x-ray picture made, she was told by the obstetrician that she would not be admitted to the hospital unless it was made. When she continued to object, and said that she would find another obstetrician, the obstetrician then brought in a legal document, absolving him from all responsibility for anything that might happen during her pregnancy and confinement. She signed this document, and was admitted to the hospital without the x-ray photograph.

It is my opinion that a physician who causes his patients to be given exposure to x-radiation when there is no medical justification for it is guilty of immoral behavior, and I believe that this practice illustrates one way in which physicians are not fulfilling their duty.

I have been told by physicians that obstetricians and other physicians often find it necessary to have x-ray examinations made of patients in order to protect themselves against malpractice suits. The action of my daughter's obstetrician, described above, supports this statement. The physicians who told me this were saying that they order x-ray exposures of their patients to be made for purely financial reasons, when there is no medical justification for them, but only a financial one, namely, protection against financial loss as a result of a malpractice charge. This action seems to me to represent a height of immorality that I would not have thought possible among physicians, who often prescribe themselves as the servants of mankind.

Dentists should not have a routine habit of taking x-ray photographs of the teeth and jaws of children on a regular schedule, such as every six months. These dental x-ray photographs should be made only when there is sound reason for making them.

The Committee of the United States National Academy of Sciences-National Research Council estimated that the average exposure of the gonads of human beings to diagnostic and therapeutic x-radiation is about 0.1 r per year. This is about equal to the background exposure. It might well be increasing the mutation rate by ten percent or twenty percent, and causing a corresponding increase in the number of defective children born. It might also be causing a similar increase of ten percent or twenty percent in the incidence of cancer. In particular, it might be responsible for the striking increase in the incidence of leukemia - about three fold over the last thirty years. An increase of twenty percent in the incidence of cancer means that about four people per one hundred would be caused to die of cancer resulting from the exposure to medical x-rays.

Fluoroscopy is a much less satisfactory method of use of x-radiation for diagnostic purposes than x-ray photography with large films, so far as the amount of exposure of the patient is concerned. Dr. Karl Z. Morgan, Director of The Health Physics Division of Oak Ridge National Laboratory, has summarized human exposures to radiation in an article in the Bulletin of the Atomic Scientists, 15, 384 (1959). He states that chest x-rays at Oak Ridge National Laboratory using full size x-ray film in direct contact with intensifying screen give an exposure of 0.02 r per photograph, and that the average chest x-ray with large film in the United States gives an exposure of 0.20 r. A chest x-ray made by taking a photograph of the fluorescent screen requires an exposure of between 0.1 r and 1 r. Gastrointestinal examinations involve from exposure of 27 r to 35 r, on the average, and fluoroscopic examinations from 5 r to 270 r. Fluoroscopy in the course of shoe fitting with x-ray machines gives an exposure of 7 to 14 r - this is an example of mis-use of x-radiation which does not involve physicians. It is illegal in many states.

I understand that some pediatricians are in the habit of fluoroscoping children who come under their care. Special effort should be made to avoid exposing children to high-energy radiation because there is evidence that the children, as well as the fetus, is especially sensitive to damage by radiation. Medical x-rays, when properly used for diagnostic and therapeutic purposes, are a great boon to man, and have saved the lives of very many people. I, of course, advocate the proper use of x-rays for these purposes; I object only to the misuse of x-rays by physicians.

During recent years there has been much discussion of the radioactive nuclei liberated into the atmosphere by the explosion of atomic bombs. The radioactive material that falls to the earth after such an explosion is called radioactive fallout. Local fallout is the fallout that occurs within a few hours after the explosion; it is largely restricted to the neighborhood of the explosion, usually to the region within about two hundred miles. Worldwide fallout is the fallout that occurs over the whole world.

Most of the radioactive nuclei in fallout are those of the products of the fission of heavy nuclei. Fission products are produced by all atomic bombs. The Hiroshima and Nagasaki bombs, each of which had explosive energy equal to about 20,000 tons of TNT (each was a 20-kiloton bomb), liberated fission products through the fission of uranium-235 and plutonium-239. These small atomic bombs, and all other small atomic bombs, are pure fission bombs. The hydrogen bomb has a first stage of fission, involving the fission of perhaps two or three times as much fissionable material (U-235 or Pu-259) as in a Hiroshima or Nagasaki bomb, and a second stage of fusion of the nuclei of the light elements, lithium and deuterium, in a few hundred pounds of lithium deuteride. A hydrogen bomb of this sort may have a thousand times as much explosive energy as a Hiroshima or Nagasaki bomb, but the amount of fission products produced is not proportional to the total explosive energy, but only to the fraction involving fission. The hydrogen bombs are unwieldy, and seem not to be manufactured by the great nuclear powers. Instead, superbombs are made and stored in the nuclear stockpiles of the great nuclear powers. A superbomb is a three-stage bomb: a small first stage of fission, as for a hydrogen bomb, followed by a fusion stage, involving a few hundred pounds of lithium deuteride, and then a fission stage of ordinary uranium metal, perhaps one thousand pounds (U-238). Both the first stage and the third stage produce radioactive fission products. These fission stages are responsible for about one half of the explosive energy, and the fusion stage for the other half. A representative superbomb was the Bravo bomb, exploded by the United States as at Bikini on 1 March 1954. This bomb had the explosive energy of about twenty million tons of TNT (20 megatons) seven times the explosive energy of all of the explosives used in the whole of the Second World War, and three times the explosive energy of all of the explosives used in all wars in the entire history of the world. Great super tombs of this sort have been exploded by the United States, the USSR, and Great Britain, and it is these great bombs that are responsible for most of the radioactive fallout that has been produced up to the present time.

When a superbomb is exploded on the surface of the earth or near it (within about one mile) thousands of tons of dirt are swept up into the fireball and vaporized. As this material condenses, it traps a large fraction of the fission products into the condensed particles, which fall to earth within a few hours after the explosion and within a couple of hundred miles of the site of the explosion. About eighty percent of the radioactive fission products is thus brought to earth in the local fallout. This radioactive material has very high radioactivity during the first few hours and days, and retains part of its radioactivity for centuries.

The remaining twenty percent of the fission products, in the case of a ground burst, and all of the fission products, in the case of a high-altitude burst, enter the troposphere and stratosphere, and in the course of time become distributed more or less uniformly over the surface of the earth, but with a larger fraction in the general latitude of the site of explosion than in the other hemisphere. The radioactive material in the troposphere (the lower atmosphere, in which rain is produced) is brought to earth within a month or two by the rain, and largely in a belt around the earth at approximately the latitude of the site of the explosion. The material in the stratosphere comes to earth in a period of years, the time required for transfer from the stratosphere to the troposphere. During the period of residence in the stratosphere the short-lived radioactive nuclei decompose, and only the long-lived nuclides are present in considerable amount. Of these the most important are cesium-157 and strontium-90, which have half-lives of about thirty years.

Two years ago, on the basis of information provided by the United States, British, and United Nations Committees mentioned above and from other sources, I estimated, in my book No More War! (Dodd, Mead and Company, 1958), that the testing of nuclear weapons at the rate of ten megatons of fission per year would produce ultimately a steady state in which the reproductive organs of human beings were subjected to additional radiation of 0.01 r per year, which is ten percent of the background radiation. All of the bomb tests carried out so far amount to 180 megatons, with about 90 megatons of fission and 90 megatons of fusion; the five years of the great tests, 1954 to 1958, involved about 18 megatons of fission per year. The fission products from these great tests are still coming to earth from the stratosphere, and much of the radioactive fallout remains in the soil on the surface of the earth, so that the amount of exposure continues to increase, year after year. It is estimated by the Atomic Energy Commission that the maximum will be reached in 1965, and that there will then be a decrease, because of the radioactive decomposition of the fission products and the decrease in the amount that continues to fall out of the stratosphere. The reproductive organs of people all over the world will, for scores of years in the future, be subjected to some radiation from fallout from the bomb tests carried out during the past decade.

Last summer the Scientific Advisory Committee of the Atomic Energy Commission stated that the exposure of the reproductive organs of people all over the world from fallout is now less than five percent of the exposure by background radiation. The estimate that I made, which may be low, corresponds to an average of five percent for a period of twenty years, or somewhat less for a longer period. This is equivalent to the entire background radiation for one year.

We may use this estimate in order to calculate the effect of the bomb tests carried out so far on the incidence of the birth of defective children. There are about 75,000,000 children born in the world each year. The National Academy of Sciences-National Research Council Committee estimated, that two percent have gross physical or mental defect because of mutated genes; this is a total of 1,500,000 defective children per year. About ten percent or (as indicated by Dr. Gentry's results) twenty percent of the defective children may reasonably be attributed to mutations caused by background radiation; accordingly the background radiation is estimated to cause 150,000 to 300,000 children to be born in the world each year with gross physical or mental defect, and a very much larger number with minor genetic defects. The same number, for one year only, represents the estimated integrated effect of the mutations caused by radioactive fission products from the bomb tests carried out so far; that is, over a period of several generations about 150,000 to 300,000 children may be expected to be born in the world with gross physical or mental defect as the result of mutations caused by the radioactive fission products from the bomb tests carried out so far, plus a very large number of children born with minor defects, which, as stated by Professor Crow, may cause more human suffering than the major defects.

It is evident that the number of defective children caused by the radioactive fission products from the bomb tests is very small compared with the total number born in the world, or with the number caused by background radiation or by medical x-rays. However, the absolute number, amounting to hundreds of thousands of children altogether, is not so small as to make it justified for us to ignore it. Even one grossly defective child represents a large amount of suffering, for the child himself and for his family, and we may well consider that it is our duty to prevent actions from being taken unnecessarily that lead to an increase in the number of defective children that are born.

In addition to the fission products, the radioactive element carbon-14 is produced by the explosion of atomic bombs, hydrogen bombs, and superbombs. Carbon-14 is a radioactive form of carbon that is normally produced in the upper atmosphere by the neutrons in cosmic rays. Since 1954 the amount of carbon-14 in the atmosphere has been increasing steadily at the rate of two percent per year, as a result of action of the neutrons produced by the atomic bomb tests, and it is now fifteen percent greater than it was six years ago. The carbon-14 is built into the bodies of human beings, along with ordinary carbon. The carbon-14 atoms are radioactive, and they irradiate the tissues of the human body. Carbon-14 has a long life - its average life is 8,000 years, so that human beings will continue to be damaged century after century by the carbon-14 from the bomb tests that have been carried out so far. I have calculated (Science 128, 1183 (1958)) that if the human race survives, the probable effect of the carbon-14 produced by the bombs tested so far will be to cause in the world 330,000 children with gross physical or mental defect, 1,000,000 stillbirths and childhood deaths, and 2,500,000 embryonic and neonatal deaths, spread out over many generations. (There is some overlap between the first two categories.) The AEC scientists Totter, Zelle, and Hollister (Science 128, 1200 (1958)) have estimated twice as big an effect; that is, 660,000 with gross defects, 2,000,000 stillbirths and childhood deaths, and 5,000,000 embryonic and neonatal deaths. These estimates are based on the assumption that the world population will continue to increase until it reaches five times the present value.

Cesium-137, iodine-131, and especially strontium-90 are the principal radioactive substances from the bomb tests that cause cancer in human beings. It is not unlikely that ten percent of all cases of cancer are caused by the background radiation to which human beings are subjected, from cosmic rays and natural radioactivity. The effect in causing cancer of the fission products from the bomb tests carried out so far may be roughly equal to that of the background radiation for a single year, and the number of cases of cancer caused by the fission products may hence be estimated to be of the order of some hundreds of thousands. A still larger number may be caused by the carbon-14.

The strontium-90 from the bomb tests continues to come to earth from the stratosphere. It gets into the food we eat, and is then built into the bones of human beings. Every human being in the world now has strontium-90 in his bones, whereas fifteen years ago nobody in the world had this radioactive substance in his bones. The strontium-90 irradiates the bone marrow and bone tissue in such a way as to cause leukemia and bone cancer. The Iodine-131 irradiates the thyroid and causes cancer of the thyroid. The cesium-137 and carbon-14 irradiate all of the tissues in the body and cause all kinds of cancer.

Dr. Charles L. Dunham, Director of the Division of Biology and Medicine of the AEC, states last year that it is possible that the strontium-90 from the bomb tests carried out so far will cause between 10,500 and 21,000 Americans to die of leukemia and bone cancer. This number of Americans corresponds to between 160,000 and 320,000 people in the whole world (perhaps these numbers are a little high, because there is more fallout in the United States than the average for the world as a whole).

There is very little that can be done to protect human beings from the radioactivity caused by the bomb tests carried out so far. I have suggested, in a letter to the editor of the New York Times (13 September 1959) that the amount of strontium-90 in milk could be decreased by feeding the milk cows a little additional dicalcium phosphate free of strontium-90 (as it would be if mined from under the ground or made from subsurface limestone), and that the amount built into the bones of children could be further decreased by the addition of a calcium compound, such as dicalcium phosphate, free of strontium-90 to the diet of the children. For twenty years calcium carbonate has been added to all bread baked in Britain, and a similar addition of calcium compound free of strontium-90 might be worth while in regions in the United States where the fallout is high.

The one step of greatest value that the world could take as a contribution to this problem is that of stopping the tests of nuclear weapons. This would also be a valuable step toward general disarmament and the abolition of war in the world. We may be grateful that the negotiators in Geneva, Ambassador Wadsworth for the United States, Ambassador Wright for Great Britain, and Ambassador Tsarapkin for the U.S.S.R., have during the last seventeen months been making great progress toward the formulation of an acceptable international agreement to stop the testing of all nuclear weapons, with a satisfactory system of inspection stations. An agreement of this sort can never be perfectly safe, but we are not safe in the world now, and such an agreement could increase our safety significantly.

One other great problem relating to high-energy radiation in the modern world is the existence and continued production of immense amounts of radioactive fission products in the plutonium plants and fission power plants. There is no safe way of disposing of these radioactive materials, which are immensely larger in quantity than the products from the bomb tests. At present some of these fission products are being liberated directly into the ocean, some are being placed in barrels, which are then encased in concrete and sunk into the depths of the ocean, and some, representing the major part being stored underground in tanks, where they will be a great hazard, which might be the cause of a great catastrophe. Nobody has been able to propose a satisfactory solution to the problem of disposing of these fission products in a thoroughly safe manner. Probably the best outlook for the world is that the fission of radioactive substances may before long be abandoned. There is a real possibility that nuclear fusion, with deuterium as the fuel, can be used as a source of power, and we may hope that in a few decades we shall begin to construct fusion power plants and that the fission power plants and plutonium plants will be abandoned. The fusion power plants will not produce radioactive fission products, and the major problem associated with them is that of preventing radioactive substances, such as carbon-14, from being produced by the released neutrons. This problem may be a serious one, but not comparable in its seriousness with that of the fission products from the fission power plants.

During the last fifty years scientists have learned that genetic and somatic damage is done to the human race by high-energy radiation. Part of the damage is caused by the background radiation due to cosmic rays and natural radioactivity. There is little that we can do to protect ourselves against this damage. At the present time about an equal amount of damage is being done to Americans by medical x-rays. The diagnostic and therapeutic use of medical x-rays is of great value to humanity, and this use must be continued; we may, however, protest against the misuse of medical x-rays by unthinking, careless, or unscrupulous physicians. A smaller amount of damage is being done by the radioactive substances produced by the testing of nuclear weapons. There is little that we can do about the fallout and carbon-14 from the bomb tests that have been carried out in the past, but we can encourage the national governments of the world to agree not to carry out any tests in the future. The fission products from plutonium plants and nuclear power plants represent a great hazard to the human race; efforts should be made to control this hazard.

The greatest of all dangers to the human race is that of nuclear war; the local fallout from bombs exploded in a nuclear war might well cause the majority of the people on earth to die, and might well change the pool of human germ plasm in such a way as to destroy mankind. The existence of tens of thousands of great nuclear weapons in the nuclear stockpiles of the world, with each one of the largest bombs powerful enough to destroy any city in the world and to kill as many as ten million people, makes it essential now that war be abandoned, and that the nations of the world from now on agree to solve their problems in accordance with the principles of justice and morality, and with use of a far more effective system of international law than exists at the present time. I believe that we now have reason to hope that we are entering upon a lasting period of peace in the world, that the terrible nuclear weapons that now exist will not be used, and that there will never again be a great war.