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Ionizing Radiation 81 IONIZING RADIATION 81 3. SUMMARY OF HEALTH EFFECTS OF IONIZING RADIATION 3.1 INTRODUCTION Ionizing radiation is a form of radiation with sufficient energy to remove electrons from their atomic or molecular orbital shells in the tissues they penetrate (Borek 1993). These ionizations, received in sufficient quantities over a period of time, can result in tissue damage and disruption of cellular function at the molecular level. Of particular interest is their effect on deoxyribonucleic acids (DNA). A special issue to consider when examining the health effects caused by ionizing radiation is the concept of dose and dose rate. The dose delivered to tissue from ionizing radiation can either be acute (the energy from the radiation is absorbed over a few hours or days) or chronic (the energy is absorbed over a longer period of months, years, or over a lifetime). The dose becomes particularly important when the individual is exposed is to radioactive materials inside the body. In distinguishing between acute and chronic exposure, both the intake rate and the physical, chemical, and biological aspects of the radionuclide kinetics must be considered. For radioactive materials with effective half-lives longer than a day, even if the intake is brief (minutes to a few days), the energy is deposited in tissue where it remains over a period longer than a few days, so that the exposure to the surrounding tissue is of a chronic duration. Depending on the size of the dose and the dose rate, the effects of ionizing radiation can either be acute (occurring within several hours to several months after exposure) or delayed (occurring several years after the exposure). The principles of dose are important to the interpretation of Tables 8-1 through 8-4, found in Chapter 8 (“Levels of Significant Exposure to Radiation and Radioactive Material”) in this profile. For example, Table 8-1 lists the observed health effects from radiation and radioactive material using inhalation as the route of exposure. Entry 109 shows a study in which Beagle dogs were exposed for 2 to 22 minutes to 90 SrCl2. Although these animals received the total amount of radionuclide within 2 to 22 minutes (an acute duration of exposure), the radionuclide was absorbed and redistributed to other tissues (in this case, bone), where it remained for a protracted period of time (chronic exposure). Delayed effects of osteosarcoma and other tumors were found in almost half of these animals (Gillett et al. 1987b). Without a clear understanding of both the dose and the toxicokinetics of the radionuclide, one might conclude 90 from this table that a 2- to 22-minute dose of radiation from SrCl2 will cause bone cancer in dogs. The 90 more appropriate conclusion to draw from this study is that after a 2- to 22-minute intake, SrCl2 IONIZING RADIATION 82 3. SUMMARY OF HEALTH EFFECTS OF IONIZING RADIATION appeared to have redistributed from the lungs to the bones and, given its long physical half-time (t1/2) of 28.6 years, would have irradiated the surrounding tissues for a lengthy period of time to produce a cancerous end point. Sources of ionizing radiation can be found at many waste sites in the United States and other countries. Exposure to these sources may have potential adverse health effects, depending on the isotope, the absorbed dose, and the dose rate. The predominant radionuclides found currently or in the past at Department of Energy (DOE) National Priorities List (NPL) waste sites are listed in Table 3-1. Table 3-1. ATSDR Priority Listing of Radionuclides Present at Department of Energy NPL Sites Ranking # Isotope Primary emission Physical half-life Target tissue(s) for soluble forms 1 Thorium-232 α 1.4 x 1010 years 2 Uranium-235 α 7.04 x 108 years Renal (proximal tubules)a 3 Radium-228 β 5.76 years Skeleton 4 Uranium-238 α 4.46 x 109 years Renal (proximal tubules) 5 Radium-226 α 1600 years Skeleton 6 Cobalt-60 β, γ 5.271 years Whole body 7 Krypton-85 β 10.72 years 8 Americium-241 α 432.2 years Lung 9 Uranium-234 α 2.45 x 105 years Renal (proximal tubules) 10 Potassium-40 β 1.26 x 109 years Skeleton 11 Europium-152 β 13.5 years 12 Neptunium-237 α 2.14 x 106 years 13 Cesium-137 β,γ 30 years Whole body 14 Protactinium-231 α 3.25 x 104 years 15 Strontium-90 β 28.6 years Skeleton 16 Krypton-88 β 2.84 hours 17 Thallium-208 β 3.053 minutes 18 Thorium-228 α 1.913 years 19 Protactinium-234 β 6.69 hours 20 Argon-41 β 1.82 hours 21 Plutonium-239 α 24,131 years Bone surface 22 Krypton-87 β/γ 76.3 minutes Whole body 23 Thorium-230 α 77,000 years Bone surface 24 Uranium-236 α 2.3415 x 107 years Bone surface 25 Plutonium-238 α 87.75 years Bone surface aRenal toxicity is more likely due its heavy metal properties rather than its radioactive properties. Source: Lide 1996; Schleien 1992 IONIZING RADIATION 83 3. SUMMARY OF HEALTH EFFECTS OF IONIZING RADIATION The scientific literature is filled with in-depth discussions and reviews on the effects of ionizing radiation in humans and animals, and it would be difficult, if not impractical, to summarize all of the known information about the effects of each radionuclide in every animal. Although the database of biological, radiological, toxicological, and toxicokinetic information is substantial and much is known, much remains to be learned about the specific mechanisms by which ionizing radiation produces its effects, how these effects can be minimized in living tissues, and what the long-term effects of very low doses of ionizing radiation are over the normal human lifespan. In this profile, some of the information about the effects of ionizing radiation has been obtained from human epidemiological and medical studies, but a sizable portion has come from studies conducted in laboratory animals and then extrapolated to humans. In addition to data from epidemiological studies, there is a substantial human database derived from therapeutic applications of radiation. Because of this large database of information, and in an effort to provide a useable overview of the health effects caused by exposure to radionuclides, this toxicological profile will summarize the adverse effects of ionizing radiation from alpha (α), beta (β), and gamma (γ) radiation, using representative radionuclides to illustrate the effects on specific organs and tissues. Other radionuclides with similar emissions and kinetics may produce similar end points. This profile will not provide an in-depth discussion of the more subtle points of radiation biology and toxicology. It will, however, provide the reader with a comprehensive and informative overview of a cross-section of the scientific literature that pertains to the potential adverse carcinogenic and non-carcinogenic effects of α, β, and γ radiation, focusing on key human and animal studies and using representative radionuclides for illustration purposes. Readers are encouraged to consult both the glossary and Chapter 2 of this profile to become familiar with the terminology used in discussing exposure to ionizing radiation and the characteristics of these three radiations. Several excellent texts and review papers are also available in the open literature that provide the salient background material for many of the sections of this profile (BEIR IV 1988; BEIR V 1990; Faw and Shultis 1993; Harley 1991; Roesch 1987; UNSCEAR 1993; Raabe 1994). 3.2 HEALTH EFFECTS FROM EXPOSURE TO IONIZING RADIATION High doses of ionizing radiation can lead to various effects, such as skin burns, hair loss, birth defects, illness, cancer, and death. The basic principle of toxicology, “the dose determines poison,” applies to the toxicology of ionizing radiation as well as to all other branches of toxicology. In the case of threshold effects (“deterministic effects” in the language of radiation toxicology), such as skin burns, hair loss, sterility, nausea, and cataracts, a certain minimum dose (the threshold dose), usually on the order of hundreds or thousands of rad, must be exceeded in order for the effect to be expressed. An increase in the size of the dose above the threshold dose will increase the severity of the effect. IONIZING RADIATION 84 3. SUMMARY OF HEALTH EFFECTS OF IONIZING RADIATION For cancer induction, increasing the radiation dose does not increase the severity of the cancer; instead it increases the chance of cancer induction. In the case of carcinogens generally, whether chemical or radiological, safety standards are based on a postulated zero threshold (i.e., any increment of carcinogen, no matter how small, is assumed to carry with it a corresponding increase in the chance of causing cancer). Increasing the size of the dose increases the probability of inducing a cancer with that carcinogen. Cancers that are, in fact, caused by radiation are completely indistinguishable from those that seem to occur spontaneously or are caused by other known or suspected carcinogens. In a given population, such as the Japanese survivors of the atomic bombings of 1945, investigators identified the carcinogenicity of ionizing radiation only by measuring the frequency of occurrence of cancer. In the case of the survivors of the atomic bombings in Japan, there was no observed statistically significant increase in cancer frequency among people whose radiation dose did not exceed 0.4 Gy (40 rad) and no increase in leukemia among those whose radiation dose did not exceed 0.1 Gy (10 rad). Because investigators could not uniquely identify any cancer as having been caused by the radiation, and because there was no observed increase in cancer frequency following low-level irradiation, the calculated cancer risk coefficient (i.e., the probability of getting cancer per unit of radiation dose) is usually estimated by extrapolation of data from observations on populations that received high doses of radiation.
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