The study of the action of ionizing and nonionizing radiation on biological systems. Ionizing radiation includes highly energetic electromagnetic radiation (x-rays, gamma rays, or cosmic rays) and particulate radiation (alpha particles, beta particles, neutrons, or heavy charged ions). Nonionizing radiation includes ultraviolet radiation, microwaves, and extralow-frequency (ELF) electromagnetic radiation. These two types of radiation have different modes of action on biological material: ionizing radiation is sufficiently energetic to cause ionizations, whereas nonionizing radiation causes molecular excitations. In both cases, the result is that chemical bonds of molecules may be altered, causing mutations, cell death, or other biological changes. See also Electromagnetic radiation; Radiation.
Ionizing radiation originates from external sources (medical x-ray equipment, cathode-ray tubes in television sets or computer video displays) or from internal sources (ingested or inhaled radioisotopes, such as radon-222, strontium-90, and iodine-131), and is either anthropogenic or natural.
Nonionizing radiation originates from natural sources (sunlight, Earth's magnetic field, lightning, static electricity, endogenous body currents) and technological sources (computer video displays and television sets, microwave ovens, communications equipment, electric equipment and appliances, and high-voltage transmission lines).
Ionizing radiation
The action of ionizing radiation is best described by the three stages (physical, chemical, and biological) that occur as a result of energy release in the biological target material.
Physical stage
All ionizing radiation causes ionizations of atoms in the biological target material. The Compton effect, which predominates at the energies of electromagnetic radiation that are commonly encountered (for example, x-rays or gamma rays), strips orbital electrons from the atoms. These electrons (Compton electrons) travel through the target material, colliding with atoms and thereby releasing packets of energy. For low-energy x-rays, the photoelectric effect predominates, producing photoelectrons that transfer their energy in the same manner as Compton electrons. See also Compton effect; Electron; Gamma rays; X-rays.
The absorbed dose of ionizing radiation is measured as the gray (Gy, 1 joule of energy absorbed by 1 kilogram of material). Because of the very localized absorption of ionizing radiation, an amount of ionizing radiation energy equivalent to 1/100 the heat energy in a cup of coffee will result in a 50% chance that the person absorbing the radiation will die in 30 days.
Neutrons with energies between 10 keV and 10 MeV transfer energy mainly by elastic scattering, that is, billiard-ball-type collisions, of atomic nuclei in the target material. In this process the nucleus is torn free of some or all of the orbital electrons because its velocity is greater than that of the orbital electrons. The recoiling atomic nucleus behaves as a positively charged particle. Because the mass of the neutron is nearly the same as that of the hydrogen atom, hydrogenous materials are most effective for energy transfer. See also Neutron.
Chemical stage
Chemical changes in biological molecules are caused by the direct transfer of radiation energy (direct radiation action) or by the production of chemically reactive products from radiolysis of water that diffuses to the biological molecule (indirect radiation action). More than half the biological action of low linear-energy-transfer (LET) ionizing radiation (for example, x-rays and gamma rays) results from indirect radiation action, about 90% of which is due to the action of the hydroxyl radical (OH·). For high linear-energy-transfer radiation, direct radiation action predominates. Chemicals that react with hydroxyl radicals, rendering them unreactive, provide protection against indirect radiation damage. See also Radiation chemistry.
The most important biological targets for damage from ionizing radiation are probably the plasma membrane and DNA, because there is only one copy, or a few copies, in the cell; because they serve critical roles for the survival and propagation of cells; and because they are large. The last factor is important because ionizing radiation releases its energy in a random manner; thus the larger the target, the more likely that it will be damaged by radiation. Consequences of radiation damage in membranes are changes in ion permeability, with leakage of potassium ions; changes in active transport; and cell lysis. Lesions in DNA include single-strand breaks, double-strand breaks, base damage, interstrand cross-links, and DNA-protein cross-links. See also Deoxyribonucleic acid (DNA).
Biological stage
Various biological effects can result from the actions of ionizing radiation. Reproductive death is most pronounced in mammalian cells that are actively dividing and in nondifferentiated tissue. Thus, dividing tissues (bone marrow and the germinal cells of the ovary and testis) are radiosensitive, and nondividing tissues (liver, kidney, brain, muscle, cartilage, and connective tissue) are radioresistant. Developing embryos are quite radiosensitive. The radiosensitivity of organisms varies greatly, being related to their intrinsic sensitivity to radiobiological damage and to their ability to repair the damage. Radiation doses resulting in 10% survival range from 3 Gy (mouse and human cells), to greater than 1000 Gy (the bacterium Deinococcus radiodurans).
The three organ systems that generally contribute to the death of mammals following a single dose of whole-body irradiation are, in decreasing order of radiosensitivity, the hematopoietic system, the gastrointestinal system, and the cerebrovascular system. Late somatic effects may take years or decades to appear and include genetic mutations transmitted to subsequent generations, tumor development and carcinogenesis, and shortening of life span. See also Mutagens and carcinogens; Mutation; Tumor.
Nonionizing radiation
Of all the nonionizing radiations, only ultraviolet radiation, microwaves, and high-voltage electromagnetic radiation are considered in the study of radiation biology.
Ultraviolet radiation
Since it can penetrate only several layers of cells, the effects of ultraviolet (UV) radiation on humans are restricted to the skin and the eyes. Ultraviolet radiation is divided into UV-C (wavelength of 200–280 nanometers), UV-B (280– 320 nm), and UV-A (320–400 nm). The most biologically damaging is UV-C, and the least damaging is UV-A. The solar spectrum at the Earth's surface contains only the UV-B and UV-A radiations.
Biological effects can arise only when absorption of ultraviolet radiation occurs. Absorption is dependent on the chemical bonds of the material, and it is highly specific. Sunburn is a form of erythema produced by overexposure to the UV-B portion of the solar spectrum. A rare but deadly form of skin cancer in humans, malignant melanoma, is induced by exposure to sunlight, with occurrences localized on those regions of the body that are most frequently exposed. Ultraviolet light can also cause photochemical damage. Cyclobutane pyrimidine dimers are the main photoproduct following exposure to UV-C and UV-B, and they can lead to cell death and precarcinogenic lesions. Other types of dimers are considered to be especially mutagenic. DNA-protein cross-links that are observed after ultraviolet radiation can be lethal. See also Photochemistry.
Survival from ultraviolet irradiation is reduced as the dose of radiation is increased. The shapes of survival curves are similar to those for lethality from ionizing radiation, they are dependent on the presence or absence of repair systems. The four repair systems that enhance biological survival include photoreactivation (splitting of cyclobutane dimers in the DNA of cells that have been irradiated by ultraviolet light); DNA excision repair; DNA recombination repair; and an inducible repair system of bacteria known as SOS repair. See also Ultraviolet radiation; Ultraviolet radiation (biology).
Microwaves
Microwaves are electromagnetic radiation in the region from 30 MHz to 300 GHz. They originate from devices such as telecommunications equipment and microwave ovens. Thermal effects of microwaves occur at exposure rates greater than 10 mW/cm2 (70 mW/in.2), while nonthermal effects are associated with exposure rates less than 10 mW/cm2. Material with a high water content will have a higher absorption coefficient for microwaves, and thus a greater thermal response to microwave action. Microwave absorption is high in skin, muscle, and internal organs, and lower in bone and fat tissue. See also Microwave.
Cultured mammalian cells exposed to microwaves at a high power density show chromosome abnormalities after 15 min of exposure. Progression through the cell cycle is also temporarily interrupted, which interrupts DNA synthesis. Chromosome aberrations in peripheral blood lymphocytes are significantly greater for persons who are occupationally exposed to microwaves. Microwaves can be lethal when the power intensity and exposure time are sufficient to cause a rise in temperature that exceeds an organism's homeostatic capabilities.
There are also some nonthermal effects associated with microwaves. A list of clinical symptoms includes increased fatigue, periodic or constant headaches, extreme irritability, decreased hearing acuity, and drowsiness during work. Laboratory studies involving exposure of animals to microwaves have produced changes in the electroencephalogram, blood-brain barrier, central nervous system, hematology, and behavior. Cell membrane permeability is also altered.
Extremely low frequency electromagnetic fields
This type of radiation is generated by the electric and magnetic fields associated with high-voltage current in power transmission lines, and also some household and industrial electrical equipment. Biological effects from ELF radiation are the least understood, and the potential consequences are the most controversial. The issue of potential biological damage from this type of radiation has arisen only since the introduction of very high voltage electric power transmission lines (440 kV and above) and the occurrence of widespread use of various electrical and electronic equipment. See also Electromagnetic pulse (EMP).
Biological studies on ELF electromagnetic fields have been performed on cells and whole animals; and epidemiological studies have been carried out on populations exposed occupationally. The results share some common features: (1) There is not always a clear dose response; that is, increasing the exposure does not necessarily give rise to an increased biological effect. (2) Some biological effects are seen only at certain frequencies and dose rates. Some of the reported effects are subjective, and may be related to normal physiological adaptation to environmental changes. In humans, qualitative biological effects of low-frequency radiation (0 to 300 Hz) include headaches, lethargy, and decreased sex drive. Humans have been noted to perceive the presence of a 60-Hz electric field when the intensity is in the range of 2 to 12 kV/m (0.6 to 3.6 kV/ft), and animals were observed to avoid entering an area where the electric field was greater than 4 kV/m (1.2 kV/ft).
