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Radiation, radioactivity, and radioisotopes
Radiation is a flow of energy through space or matter. It takes the form of particles (e.g., alpha and beta particles) or electromagnetic waves (e.g., X rays, gamma rays, and visible and ultraviolet [UV] light). Radiation can be classified as either ionizing or nonionizing depending on its ability to produce ions in the matter it interacts with. Ionizing radiation is more toxic than nonionizing radiation.
Radioactivity is the emission of radiation caused by the disintegration of unstable nuclei of radioisotopes. After disintegration, a radioisotope may become a radioisotope of another element, which will further disintegrate. The disintegration series continues until a stable isotope is formed.
Ionizing radiation
Ionizing radiation is radiation that produces ions in matter during interaction with atoms in the matter. The toxic effect of ionizing radiation is related to the ionization. It is believed that ionization of tissues, composed mainly of water, generates H2O+ and H2O− ions, which in turn form H and OH radicals. Because radicals are very reactive chemically, biological damage, such as attacks on DNA and proteins, results.
There are two classes of ionizing radiation: particulate and eletromagnetic. Alpha particles, beta particles, neutrons, and positrons are examples of particulate ionizing radiation. Gamma rays and X rays are electromagnetic ionizing radiation.
Among particulate ionizing radiation, alpha and beta particles are the forms most commonly encountered in the environment and are biologically the most significant. Composed of two neutrons and two protons and thus containing a 2+ charge, alpha particles are the heaviest ionizing particles. Although they do not penetrate tissue very well, alpha particles turn many atoms in their short paths into ions, producing intense tissue ionization.
In contrast to alpha particles, beta particles are electrons of little mass carrying only one negative charge. They penetrate up to several millimetres in soft tissues. Their low mass and low charge mean that only moderate ionization is produced in tissues when beta particles collide with atoms in its path.
Gamma rays and X rays are electromagnetic radiation of similar properties, with gamma rays having higher energy than X rays. Gamma rays usually accompany the formation of alpha or beta particles. Neither gamma rays nor X rays carry a charge, and neither have mass; consequently, they can penetrate tissues easily, creating moderate ionization along their paths.
Biological damage is related to the degree of tissue ionization produced by radiation. Thus, a physical dose of alpha particles does not produce the same amount of damage as that produced by the same dose of beta particles, gamma rays, or X rays.
Radiation sources
Radiation is either natural or man-made. Natural radiation includes cosmic radiation, terrestrial radiation, radioisotopes inside human bodies, and radon gas. Cosmic radiation consists of charged particles from outer space, and terrestrial radiation of gamma rays from radionuclides in the Earth. Radioisotopes in human bodies come from the food, water, and air consumed. Cosmic and terrestrial radiation, together with radioisotopes inside human bodies, contribute only one-third of the total natural radiation dose. The remaining two-thirds can be attributed to radon, a radioactive gas released from soil that may reach a high level inside buildings with poor ventilation. Man-made radiation consists of radiation from medical and dental diagnostic procedures, atmospheric tests of atomic bombs, emissions from nuclear plants, certain occupational activities, and some consumer products. The largest nonoccupational radiation sources are tobacco smoke for smokers and indoor radon gas for the nonsmoking population.
Emissions from nuclear power plants contribute only a very small portion of the total yearly radiation received. The low dose reflects the negligible amount of radionuclides released during normal operation, although the amount released can be much higher after a nuclear reactor accident. Not every reactor accident is a disaster, however. The 1979 accident at the Three Mile Island nuclear power station, near Harrisburg, Pa., released only a small amount of radiation (0.8 and 0.015 mSv within a 16- and 80-km radius, respectively), less than the background annual radiation dose. The nuclear reactor accident at Chernobyl in the Soviet Union, in 1986, however, was much more devastating, leading to more than 30 deaths and the evacuation of thousands of nearby residents.
Adverse effects of ionizing radiation
Ionizing radiation quickly kills rapidly dividing cells. In general, immature blood cells in bone marrow, cells lining the mucosa of the gastrointestinal tract, and cells in the lower layers of the epidermis and in hair follicles are the most rapidly dividing cells in the body. As a result, radiation leads to the decreased production of blood cells, nausea, vomiting, diarrhea, malabsorption by the intestine, skin burns, and hair loss. Because of its relatively selective lethal effect on rapidly dividing cells, however, ionizing radiation is used in the treatment of certain cancers. Some cells in the embryo and fetus also divide rapidly, and thus ionizing radiation can cause malformations and even fetal death. Ionizing radiation can also produce mutations by altering the DNA, and it can result in cancer.
Toxicities of whole-body ionizing radiation
X rays and gamma rays irradiate the body uniformly and acutely affect all of the tissues discussed above. At sufficiently high doses, this type of radiation can lead to a condition known as acute radiation syndrome. The most sensitive tissue is the bone marrow, where blood cells are generated. The next tissue affected is the gastrointestinal tract. If the dose is high, the central nervous system is affected and the person becomes uncoordinated and disoriented and experiences tremors, convulsions, and coma. At even higher doses, the skin, eyes, and ovaries and testes are affected. Death may follow from 2 to 35 days after exposure. Exposure to radiation can also result in cancers of the bone marrow (leading to leukemia), lungs, kidneys, bladder, esophagus, stomach, colon, thyroid, or breasts.
Radioisotopes that are absorbed and distributed evenly throughout the body also can result in whole-body irradiation. Examples are tritium and cesium-137, both of which release beta particles that can lead to bone marrow toxicities and even, in the case of cesium-137, to death. The toxicity of tritium is less severe than that of cesium-137 because the beta particles generated by tritium are less energetic and because cesium-137 also releases gamma rays.
Local toxicities of common beta-particle emitters
Unlike tritium and cesium-137, the isotopes strontium-90, iodine-131, and cerium-144 emit beta particles that are not distributed evenly in the body. Strontium-90 releases only beta particles, while iodine-131 and cerium-144 release both beta particles and gamma rays, but their toxicities are primarily caused by the beta particles. These radioisotopes produce toxicities in the tissues where they are stored or concentrated. Strontium-90 and cerium-144 chemically resemble calcium and as a result are stored in bone. Therefore, these two radioisotopes produce bone cancer and leukemia, which is a result of the irradiation of bone marrow. Iodine-131 is concentrated in the thyroid and produces thyroid damage and tumours.
Local toxicities of common alpha-particle emitters
There are radioisotopes that emit primarily alpha particles, together with some gamma rays. Because the destructive effect on tissues of alpha particles is far greater than that of gamma rays, the toxicities of these radioisotopes are contributed mainly by the alpha particles. Because of the limited penetrability of alpha particles, only tissues in the near vicinity of the isotopic molecules are affected. These radioisotopes typically produce tumours at the storage site.
Most of the common alpha-particle emitters belong to the uranium series, which consists of radioisotopes that form one after another, via a nuclear decay reaction, and release mainly alpha particles. The series starts with uranium-238. The nuclear disintegration of uranium-238 forms radium-226 which disintegrates to form radon gas (radon-222). Radon decays to form a series of daughter nuclides, most of which are alpha-particle-releasing isotopes, such as polonium-210. The radioisotopes in the uranium series are important because uranium is the starting fuel for many nuclear reactors and because daughter nuclides in this series are commonly found in the environment.
The toxicity of uranium-238 depends on the water solubility of the uranium compound. Water-soluble forms mainly cause kidney injury, while the insoluble forms produce fibrosis and cancer of the lung. Because of its similarity to calcium, radium-226 is stored mainly in the bone, and it produces abnormal changes in the bone marrow, including anemia and leukemia, cancers of the bone, and paranasal sinuses. The next radioisotope in the uranium series is radon, radon-222. Although radon is radioactive, its toxicity is not due to retention of the gas by the lungs but rather to the series of radioactive daughter nuclides in particulate form. These particulate daughter nuclides are deposited on the respiratory tract when inhaled, the respiratory tract is irradiated by the alpha particles released, and lung cancer can result.
Other radioisotopes do not belong to the uranium series. For example, radium-224, which is deposited mainly on bone surfaces, has been used in Europe to treat ankylosing spondylitis. Because of its short half-life (3.6 days), it affects only the bone surface and not the bone marrow. Its major toxicity is the production of bone cancer. Like uranium-238, plutonium-239, which is used in some nuclear reactors and in nuclear bombs, primarily releases alpha particles. Although there are no human data, animal studies indicate that the toxicity of plutonium-239 is similar to that of insoluble uranium-238 in causing fibrosis and cancer of the lung.
Nonionizing radiation
Nonionizing radiation includes ultraviolet light, infrared radiation, microwaves, and radio frequencies, all of which are electromagnetic waves. The toxicity of radio frequencies is rather low. On the whole, nonionizing radiation is not as toxic as ionizing radiation, and the various forms of nonionization radiation share common target organs; particularly the skin and eyes.