Ionizing Biological effects

Microwaves may be used to ionize gases when sufficient power is apphed, but only through the intermediate process of classical acceleration of plasma electrons. The electrons must have energy values exceeding the ioniza tion potential of molecules in the gas (see Plasma technology). Ionizing radiation exhibits more biological-effect potential whatever the power flux levels (2). 

U.S. radiation protection guidelines are estabHshed by the National CouncH on Radiation Protection and Measurement (NCRP) and are based on the recommendations of the International Commission on Radiological Protection (ICRP). The National Research CouncH also sponsors a report from its advisory committee on the biological effects of ionizing radiations (20). 

Radiation Dosimetry. Radioactive materials cause damage to tissue by the deposition of energy via their radioactive emissions. Thus, when they are internally deposited, all emissions are important. When external, only those emissions that are capable of penetrating the outer layer of skin pose an exposure threat. The biological effects of radiation exposure and dose are generally credited to the formation of free radicals in tissue as a result of the ionization produced (17). 

N.uional Research Council, 1972, The Biological Effects of Ionizing Radiations, National Academy of Sciences. 

The chemistry, and hence hazards, of hot, or radioactive, elements parallels that of their cold isotopes. However, the radiation poses additional toxicity hazards. A qualitative classification of selected isotopes in terms of their toxicity is given in Table 10.2. The biological effects of ionizing radiation stem mainly from damage to individual cells following ionization of the water content. Oxidizing species, e.g. hydrogen peroxide. 

Quality Factor (Q)—The linear-energy-transfer-dependent factor by which absorbed doses are multiplied to obtain (for radiation protection purposes) a quantity that expresses - on a common scale for all ionizing radiation - the approximate biological effectiveness of the absorbed dose. 

Relative Biological Effectiveness (RBE)—The RBE is a factor used to compare the biological effectiveness of absorbed radiation doses (i.e., rad) due to different types of ionizing radiation. More specifically, it is the experimentally determined ratio of an absorbed dose of a radiation in question to the absorbed dose of a reference radiation (typically 60Co gamma rays or 200 keV x rays) required to produce an identical biological effect in a particular experimental organism or tissue (see Quality Factor). 

Dose equivalent or rem is a special radiation protection quantity that is used, for administrative and radiation safety purposes only, to express the absorbed dose in a manner which considers the difference in biological effectiveness of various kinds of ionizing radiation. The ICRU has defined the dose equivalent, H, as the product of the absorbed dose, D, and the quality factor, Q, at the point of interest in biological tissue. This relationship is expressed as H = D x Q. The dose equivalent concept is applicable only to doses that are not great enough to produce biomedical effects. 

BEIRIII. 1980. The effects on populations of exposure to low levels of ionizing radiation. Committee on the Biological Effects of Ionizing Radiations, National Research Council. Washington, DC National Academy Press. 

UNSCEAR. 1982. United Nations Scientific Committee on the Effects of Atomic Radiation. Ionizing radiation Sources and biological effects. New York United Nations. 

UNSCEAR, Ionizing Radiation Sources and Biological Effects. United Nations, New York, NY (1982). 

US-National Academy of Sciences, National Research Council BEIR-II Biological Effects of Ionizing Radiation, Washington, D.C., USA (1972). 

One of the most sensitive biological effects of ionizing radiation is to increase the frequency of normally observed chromosome aberrations (but not to induce qualitatively special abnormalities). Peripheral blood lymphocytes are the most feasible cells for chromosome investigations, as blood samples are easy to obtain and the techniques to stimulate the lymphocytes to proliferate within a culture medium and to prepare suitable chromosome slides for microscopic analyzation have their routine protocoil (e. g. Yunis, 1965 Lloyd et al, 1982). 

Katz, R. and W. Hofmann, Biological Effects of Low Doses of Ionizing Radiations Particle Tracks in Radiobiology, Nuclear Instruments and Methods 203 433-442 (1982). 

Pohl-Ruling, J., P. Fischer, and E. Pohl, The Low-Level Shape of Dose Response for Chromosome Aberrations, in Proceedings of the International Symposium of Late Biological Effects of Ionizing Radiation (International Atomic Energy Agency) pp. 315-326, Vienna, Austria, (1978). 

Acute biological effects of the Chernobyl accident on local natural resources were documented by Sokolov et al. (1990). They concluded that the most sensitive ecosystems affected at Chernobyl were the soil fauna and pine forest communities and that the bulk of the terrestrial vertebrate community was not adversely affected by released ionizing radiation. Pine forests seemed to be the most sensitive ecosystem. One stand of 400 ha of Pirns silvestris died and probably received a dose of 80 to 100 Gy other stands experienced heavy mortality of 10- to 12-year-old trees and up to 95% necrotization of young shoots. These pines received an estimated dose of 8 to 10 Gy. Abnormal top shoots developed in some Pirns, and these probably received 3 to 4 Gy. In contrast, leafed trees such as birch, oak, and aspen in the Chernobyl Atomic Power Station zone survived undamaged, probably because they are about 10 times more radioresistant than pines. There was no increase in the mutation rate of the spiderwort, (Arabidopsis thaliana) a radiosensitive plant, suggesting that the dose rate was less than 0.05 Gy/h in the Chernobyl locale. 

Relative biological effectiveness (RBE) The biological effectiveness of any type of ionizing radiation in producing a specific damage (i.e., leukemia, anemia, carcinogenicity). See Radiation dose. 

Roentgen equivalent man (rem) The amount of ionizing radiation of any type that produces the same damage to humans as 1 roentgen of radiation. One rem = 1 roentgen equivalent physical (rep)/relative biological effectiveness (RBE). In the latest nomenclature, 100 rem = 1 Sievert (Sv). 

The effect of a biologically active compound is based on its ability to form a complex with a receptor. The intensity of the biological effect is proportional to the stability of this complex, which is dependent on the strength of the interaction of the effector molecule with the active centre of the receptor. The electron structure of the molecule can be decisive for this interaction and this may explain the correlation of ionization potentials and pharmacological properties of certain compounds. 

A ratio that assesses the biological effectiveness of absorbed radiation doses with respect to different types and energies of ionizing radiation. It is equal to the absorbed dose of a particular radiation divided by the absorbed dose of a standard radiation required to produce identical biological effects in a given organ, tissue, or organism. 

A unit of dose equal to the amount of ionizing radiation that produces in humans the same biological effect as one rad of X-rays or gamma rays. One rem is equal to 0.01 sievert. 

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