Background radiation refers to ionizing radiation that is derived from cosmic radiation, terrestrial radiation, and radiation from sources internal to the body.
Some radiation has little biological effect. Visible light and infrared radiation, two types of electromagnetic radiation, do not cause ionization, are not mutagenic (mutation causing) and are not carcinogenic (cancer causing).
Ionizing radiation has the potential to kill cells or cause somatic (affecting the body) or germinal (relating to reproductive cells) mutations. It has this ability by virtue of its power to penetrate living cells and produce highly reactive charged ions. The ions (electrically charged atoms), within the ionizing radiation, directly cause cell damage. Radiation accidents and the potential for radiation from nuclear bombs (nuclear fission weapons, sometimes also called atomic bombs) and hydrogen bombs (nuclear fusion weapons) create a fear of radiation release around human activity.
However, people are subjected to diagnostic and therapeutic radiation each day. In addition, many older Americans were exposed to radioactive fallout from atmospheric testing of nuclear weapons in the middle part of the twentieth century. In the twenty-first century, there is a small amount of environmental contamination from nuclear fuel used in power plants. Accordingly, there is still considerable interest in radiation effects on biological systems and the sources of radiation in the environment.
Concern for radiation safety is certainly justified and most individuals seek to minimize their exposure to human-generated radiation. However, for most people, exposure levels to radiation from natural sources far exceed exposure to radiation produced by humans. In the United States, current estimates of human exposure levels of ionizing radiation suggest that less than 20 percent is of human origin. The remaining radiation (80 percent) is from natural sources and is referred to as “background radiation.” While radiation doses vary tremendously from person to person, the average human has an annual exposure to ionizing radiation of about 360 millirem. Millirem or mrem (one-thousandth of one rem, where rem stands for roentgen equivalent in man) is a measure of radiation absorbed by tissue multiplied by a factor that takes into account the biological effectiveness of a particular type of radiation and other factors such as the competence of radiation repair. One mrem is equal to 10 μSv; where mSv is an abbreviation for microsievert (or one millionth of one sievert), a unit that is used internationally.
Another source of background radiation is terrestrial radioactivity from naturally occurring minerals, such as uranium, thorium, and cesium, in soil and rocks. The abundance of these minerals differs greatly from one geographic area to another. Residents of the Colorado plateau receive approximately double the dose of terrestrial radiation as those who live in Iowa or Minnesota. The geographic variations are attributed to the local composition of Earth's crust and the kinds of rock, soil, and minerals present. Houses made of stone are more radioactive than houses made of wood. Limestones and sandstones are low in radioactivity when compared with granites and some shales. Naturally occurring radionuclides in soil may become incorporated into grains and vegetables and thus gain access to the human body. Radon is a radioactive gas produced by the disintegration of radium, which is produced from uranium. Radon escapes from Earth's crust and becomes incorporated into all living matter including humans. It is the largest source of inhaled radioactivity and comprises about 55 percent of total human radiation exposure (both background and human generated). Energy efficient homes, which do not leak appreciable amounts of air, may have a higher concentration of radon inside than is found in outside air. This is especially true of basement air. The radon in the home decays into radioactive “daughters” that become attached to aerosol particles which, when inhaled, lodge on lung and tracheal surfaces. Obviously, the level of radon in household air varies with construction material and with geographic location. Is radon in household air a hazard? Many people believe it is, since radon exposure (at a much higher level than occurs breathing household air) is responsible for lung cancer in nonsmoking uranium miners.
Naturally occurring radioactive carbon (carbon–14) similarly becomes incorporated into all living material. Thus, external radiation from terrestrial sources often becomes internalized via food, water, and air. Radioactive atoms (radionuclides) of carbon, uranium, thorium, and actinium and radon gas provide much of the terrestrial background radiation. The combined annual exposure to terrestrial sources, including internal radiation and radon, is about 266 mrem and far exceeds other, more feared sources of radiation.
Life on Earth evolved in the presence of ionizing radiation. It seems reasonable to assume that mutations can be attributed to this chronic, low level of radiation. Mutations are usually considered to be detrimental, but over the long course of human and other organic evolution, many useful mutations occurred, and it is these mutations that have contributed to the evolution of higher forms.
Nevertheless, it is to an organism's advantage to resist the deleterious effects associated with most mutations. The forms of life that inhabit Earth today are descendants of organisms that existed for millions of years on Earth. Inasmuch as background ionizing radiation has been on Earth longer than life, humans and all other organisms obviously cope with chronic low levels of radiation. Survival of a particular species is not due to a lack of genetic damage by background radiation. Rather, organisms survive because of a high degree of redundancy of cells in the body, which enables organ function even after the death of many cells (e.g., kidney and liver function, essential for life, does not fail with the loss of many cells; this statement is true for essentially all organs of the human body). Further, stem cells in many organs replace dead and discarded cells. Naturally occurring antioxidants are thought to protect against free radicals produced by ionizing radiation. Finally, repair mechanisms exist that can, in some cases, identify damage to the double helix and effect DNA (deoxyribonucleic acid) repair. Hence, while organisms are vulnerable to background radiation, mechanisms are present which assure survival.
See also Radiation ; Radiation exposure .
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Robert G. McKinnell