642 RADIATION PROTECTION and the RBE of the source of radiation. There are no quantitative dose-effect data relating the incidence of cataracts late in life in humans or animals to the acceleration of aging proc esses. Summary 1. Acute doses of radiation may produce immediate or delayed effects, or both. 2. As acute whole body doses increase above approximately 25 rems (units of radiation dose), immediately observable effects increase in severity with dose, beginning from barely detectable changes, to biological signs clearly indicating damage, to death at levels of a few hundred rems. 3. Delayed effects produced either by acute irradiation or by chronic irradiation are similar in kind but the ability of the body to repair radiation damage is usually more effective in case of chronic than acute irradiation. 4. The delayed effects from radiation are in general indistinguishable from familiar pathological conditions usually present in the population. 5. Delayed effects include genetic effects (effects transmitted to succeeding generations), increased incidence of tumors, life span shortening, and growth and development changes. 6. The child, the infant, and the unborn infant appear to be more sensitive to radiation than the adult. 7. The various organs of the body differ in their sensitivity to radiation. 8. Although ionizing radiation can induce genetic and somatic effects (effects on the individual during his lifetime other than genetic effects), the evidence at the present time is insufficient to justify precise conclusions on the nature of the dose-effect relationship especially at low doses and dose rates. Moreover, the evidence is insufficient to prove either the hypothesis of a "damage threshold" (a point below which no damage occurs) or the hypothesis of "no threshold" is man at low doses. 9. If one assumes a direct linear relation between biological effect and the amount of dose, it then becomes possible to relate very low dose to an assumed biological effect even though it. is not detectable. It is generally agreed that the effect that may actually occur will not exceed the amount predicted by this assumption. RADIATION PROTECTION 643 III. SOURCES OF RADIATION EXPOSURE 3.1 For convenience, the exposure of persons to radiation will be divided into three classes: (a) exposures from natural sources; (b) exposures from man-made sources other than environmental sources; and (c) exposures from environmental contamination. Where data are available, the exposures of various critical portions of the body are indicated separately. Of special interest are the gonadal dose because of its genetic significance and the bone marrow dose because of possible leukemogenesis. Therefore, the following discussions center their attention on the genetically significant and bone marrow doses as examples of the general problem. Natural Sources 3.2 Table 3.1 lists the doses received by persons in the United States from natural sources. The principal exposures from radiation sources outside of the body (external sources) and from sources inside of the body (internal sources) are listed separately. 3.3 The dose from cosmic rays for 38 principal cities in the United States was determined from data on the variation of cosmic ray dose with altitude1 (Solon et al-1959). As most of the large centers of population are near sea level, the mean dose to the population of the United states from cosmic rays is nearer the lower than the upper limit. 3.4 The dose from terrestrial external gamma rays was estimated by subtracting the cosmic ray component from measurements of the sum of the two components (Solon et al, 1959) and applying an approximate correction (0.6) for the average shielding of the outer tissues of the body. The resulting range of values includes mean values for 38 of the principal cities of the United States. However, it should be noted that doses obtained at different locations within a city varied in several cases by a factor of 2 or 3 for the limited data available. In part, this may be due to shielding of heavy structures or the proximity of structures whose building materials contained small quantities of gamma emitting nuclides. 3.5 When doses from internal sources are added, it appears (Table 3.1) from the limited data available that the radiation dose to soft tissue from all natural sources varies by at least a factor of 2 in the United States. Man-Made Sources Other Than Environmental Contamination 3.6 Exposure of persons to man-made radiation other than environmental contamination arises principally from (1) exposures received during medical procedures, (2) exposures received by radiation workers during their working hours, (3) exposures to persons in the vicinity of medical and industrial radiation sources (environs), and (4) exposure produced by other sources, such as radium dialed watches, television sets, etc. Table 3.2 summarizes the estimated per capita mean marrow doses and genetically significant doses to the population from man-made sources other than environmental contamination. The per capita dose is the sum of all of the doses received by the population divided by the number of individuals in the population. The annual genetically significant dose to the population is the average of the gonadal doses received by the individuals each weighted for the expected number of children to be conceived subsequent to the exposure. 3.7 For the occupational exposure it is assumed that as much as a half of one per cent of the population might be exposed in the future to as much as an average annual dose of 4 rems. Both estimated figures are high because the fraction of the population occupationally exposed to 1Variation of the dose from cosmic rays with latitude is small compared to that with alti tude. 644 RADIATION PROTECTION radiation and the annual dose they receive at the present time is considerably less than that assumed in Table 3.2. There are presently only about 66,000 radiation workers out of a total employment approximating 120,000 in the Atomic Energy Commission and its contractors (see Table 5.1) and perhaps 250,000 persons occupationally exposed to x-rays in medical applications. Persons in these two areas plus the industrial radiography field probably do not constitute more than 0.2 per cent of the population at the present time. Morgan (1959) indicates that the average annual exposure of radiation workers at Oak Ridge National Laboratory is 0.4 г, and at Hanford, 0.2 r (see Table 5.1). In the fields of medical applications and industrial radiography, the annual doses received by most radiation workers falls within the range of 0.5 to 5 rems. Most of them probably receive doses in the lower half of this range but a few possibly receive more than 5 and some less than 0.5 rems. Thus, the average annual dose for all radiation workers is probably much less than the 4 rems assumed for the calculation at the present time. 3.8 For exposure of persons in the environs it is assumed that one per cent of the population might be involved and they would have an annual dose of as much as 0.5 rems. This assumption concerning per capita dose from the exposure of environs is probably larger than will be obtained in the foreseeable future. The fraction of the population assumed is quite large and it is unlikely that the average individual will receive as much as 0.5 rem per year. 3.9 Unfortunately, there are no data on the mean marrow dose from medical therapy, but it is obvious that diagnostic x-rays contribute considerably to the total exposure from manmade sources other than environmental contamination. While diagnostic x-rays are an important clinical tool, the practitioner of the healing arts should always attempt to balance the risk against the gain for each exposure. He should also assure himself that the most modern techniques are being used in order that the dose is reduced as much as practicable. Current recommendations of the NCRP (H54, 1954 and H60, 1955) indicate methods by which the gonadal dose can be minimized. If these recommendations are observed the bone marrow dose will also be minimized. Man-Made Environmental Contamination 3.10 Sources of environmental contamination may result from fallout after the explosion of nuclear devices and during the use and processing of fuels for reactors. There are other sources which contribute relatively smaller amounts to environmental contamination. 3.11 Environmental contamination from fallout has received considerable attention over the past decade. When there is a nuclear explosion in the megaton range, the gases cool so slowly that a major portion of the fission products enter the stratosphere where they are distributed widely. Some fission products drift back into the troposphere before losing their radioactivity and are deposited in patterns which depend at least in part upon meteorological conditions. This final fallout, however, takes a long time to drift back to earth so that the fission products from this stratospheric source consist mainly of the long-lived nuclides. For nuclear explosions in the kiloton range, the heat of the fireball is considerably less so that the fission products do not reach the stratosphere but stay in the troposphere. About half of the radioactive material from the troposphere comes back to the earth in about three weeks and most of the fallout reaches the earth in about three months (UNSCEAR p. 99, 1958). From such a fallout, many of the nuclides are of short half-life. 3.12 According to reported estimates,2 the genetically significant per capita dose in the United States from both external and internal radiation from fallout of cesium-137 will be about 53 millirem in 30 years providing nuclear weapons testing in the atmosphere is not resumed after the cessation at the end of 1958. It was also reported that the per capita mean marrow dose in the United States would be, under the same conditions, about 331 millirem in 70 years from cesium-137 and strontium-90. For continued testing at the same rate as in the previous 5 years, it was estimated that the above numbers should be multiplied by a factor of 8. Other estimates (UNSCEAR 1958 and Feeley 1960) are somewhat lower. 2. Langham and E. C. Anderson, Fallout from Nuclear Weapons Tests, Hearings of the Joint Committee on Atomic Energy, Congress of the United States, May 1959, p. 1061 ff. RADIATION PROTECTION 645 3.13 Under normal operating conditions, most industries in the nuclear engineering field, including the use of reactors, do not now release activity which will give significant contributions to the population dose. 3.14 It is usually considered very unlikely that the core of a reactor would melt down accidentally and release fission products. This possibility, however remote, is considered in designing a reactor. Modern reactors are designed with a containment shell which would permit only a very small portion of the fission products, from a melt-down, to contaminate the environment. However, according to the best engineering estimates, this and other containment provisions will not trap all of the activity. An additional major reduction in the activity released by the shell would substantially increase the cost of the reactor. 3.15 Plants used for the processing of spent fuel elements have a larger potential for contaminating the environment. Here the fuel element is dissolved and the radioactive material is liberated from the fuel element. However, the amount of material treated at any one time is much less than the material present in a reactor. In this process, fission product gases, such as radioactive iodine, bromine, xenon, and krypton are released from the fuel element. Most of the other radionuclides remain in the solutions. Some nuclides, such as cesium-137 and strontium-90, may be separated out for other uses. The remainder of the radionuclides are now stored in huge tanks. Such storage is, of course, expensive. Summary 3.16 From a limited survey it appears that the human annual gonadal, soft tissue, and bone marrow doses from natural sources may be from 80 to 170 millirem (see Table 3.1). 3.17 The estimated annual genetically significant dose from all man-made sources except environmental contamination probably is about 80 to 280 millirem. The per capita annual mean marrow dose is probably greater than 100 millirem, although no data are available on the contribution from medical radiation therapy. The genetically significant dose and the mean marrow dose are each of the order of the dose received from natural sources. Diagnostic x-rays provide a substantial contribution to these totals (see Table 3.2). 3.18 It has been estimated3 that fallout will contribute about 53 millirem to the genetically significant per capita dose of the population in 30 years if nuclear weapons testing in the atmosphere is not resumed after the cessation at the end of 1958. If testing were to continue at the same rate as in the previous 5 years, it was estimated that the above number should be multiplied by a factor of 8. The estimated corresponding per capita mean marrow doses for 70 years are 331 millirem and 2648 millirem respectively. 3.19 Under normal operating conditions, most industries in the nuclear engineering field, including the use of nuclear power plants do not now release activity which will give a significant contribution to the population dose. 3. Langham and B. C. Anderson, Fallout From Nuclear Weapons Tests, Hearings of the Joint Committee on Atomic Energy, Congress of the United States, May 1959, p. 1061 11. 646 RADIATION PROTECTION TABLE 3.1 ANNUAL RADIATION DOSES1 FROM NATURAL SOURCES 1Doses to the gonads and other soft tissue including bone marrow. 2Report of United National Scientific Commission on the Effects of Atomic Radiation (UNSCEAR, p. 58, 1958). 3Unconfirmed research of Muth et al, Brit. J. of Radiol. Suppl. No. 7, 1957, indicates that the dose may be of the order of 2 millirem per year to the gonads and 5 to 15 millirem per year to other soft tissue. 4The lungs may receive an additional dose of from 125 to 1570 millirem per year from radon given off by building structures. The spread is caused by variations in ventilation and differences in building materials (UNSCEAR, p. 58, 1958), TABLE 3.2 ESTIMATED EXPOSURE FROM MAN-MADE SOURCES (OTHER THAN 1Fallout from tests of nuclear weapons is not included (see sub-section on environmental contamination). 2International Commission on Radiological Protection (ICRP) and International Commission on Radiological Units and Measurements (ICRU) Joint Study Group Report. Physics in Med. and Biology, 2 107 (1957). 3These are probable values. 4Report UNSCEAR, p. 66. 5Clark, S. H., Bull. of the Atomic Scientists 12 14 (1956). The 12 millirem per year may be an underestimate because patients treated for malignances are not included. Martin (1958), who assumed that these patients might procreate after treatment, obtained a value of 28 for Australia. ❝Report of UNSCEAR, p. 11. |