Radioactivity presents special hazards because it cannot be detected by the normal human senses. Strict safety standards have been established by international organizations to ensure that exposure to workers is minimized and that the public is not exposed to radiation from other than the natural background (ICRP 1979; NCRPM 1959, 1987a,b). This background radiation, from naturally occurring radioisotopes and cosmic rays, is the base exposure level, and cannot be practically reduced.
All types of radiation share the property of losing energy by absorption in passing through matter. The degree of absorption depends upon the type of radiation, but all types are absorbed to some extent (Figure 11.23.1). The process of absorption always results in ionization. This process of stripping electrons from atoms causes damage to human tissues. It also allows for the design of instruments for detection and measurement of radioactivity. The
properties of the various radiations determine the protective measures needed and the methods of measurement. Three types of radiation exist.
Alpha Radiation: Radiation from alpha particles loses energy very quickly when passing through matter. As a result, alpha radiation travels only a few inches in air and can easily be stopped by the outer layer of human skin. Alpha radiation sources are most harmful to humans if they are ingested. Alpha radiation can be very damaging to body organs, especially the lungs if the alpha source is inhaled as fine particles (BEIR 1988). Beta Radiation: Radiation produced by beta particles travels much farther in air than alpha radiation, and can penetrate several layers of human skin. The human body can be damaged by being near a source of beta radiation for a long period of time or by ingesting a source of beta radiation. Beta radiation can be stopped by absorbing materials. Gamma Radiation: Gamma radiation travels great distances and easily penetrates matter. It can pass completely through the human body, damaging cells en route, or be absorbed by tissue and bone. Three feet of concrete or two inches of lead are required to stop 90% of typical gamma radiation. Excessive external gamma radiation can cause serious internal damage to the human body, but cannot induce radioactivity in it.
The biological effect of radiation is measured in units called rems. A rem is the amount of beta/gamma radiation that transfers a specific quantity of energy to a kilogram of matter. A single exposure to 300 rems would result in death within thirty days for 50% of the persons exposed. The unit of dose is difficult to put into perspective, however, a comparison of the allowable doses helps. The permissi ble level for occupational radiation exposure is five rems per year to the whole body. It is believed that this level can be absorbed for a working lifetime without any sign of biological damage. Background radiation is measured in millirems (0.001 rem).
The average person is exposed to ionizing radiation from many sources. The environment, and even the human body, contains naturally occurring radioactive materials. Cosmic radiation contributes additional exposure. The use of x-rays and radioisotopes in medicine and den-
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