Ionizing radiation has the potential to kill cells or cause somatic or germinal mutations. It has this ability by virtue of its power to penetrate living cells and produce highly reactive charged ions. It is the charged ions which cause cell damage. Radiation accidents and the potential for radiation from nuclear (atomic) bombs create a fear of radiation release due to human activity. Many people are subjected to diagnostic and therapeutic radiation, and older Americans were exposed to radioactive fallout from atmospheric testing of nuclear weapons . There is some environmental contamination from nuclear fuel used in power plants . Accordingly, there is 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. Current estimates of human exposure levels of ionizing radiation suggest that only about 18% is of human origin. The remaining radiation (82%) 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 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; μSv is an abbreviation for microSievert, a unit that is used internationally.)
Some radiation has little biological effect. Visible light and infrared radiation do not cause ionization, are not mutagenic and are not carcinogenic. Consequently, background radiation refers to ionizing radiation which is derived from cosmic radiation, terrestrial radiation, and radiation from sources internal to the body. (Background radiation has the potential for producing inaccurate counts from devices such as a Geiger counter. For example, cosmic rays will be recorded when measuring the radioactive decay of a sample. This background "noise" must be subtracted from the indicated count level to give a true indication of activity of the sample.)
Cosmic rays are of galactic origin, entering the earth's atmosphere from outer space. Solar activity in the form of sunflares and sunspots affects the intensity of cosmic rays. The atmosphere of the earth serves as a protective layer for humans and anything that damages that protective layer will increase the radiation exposure of those who live under it. The dose of cosmic rays doubles at 4,920 ft (1,500 m) above sea level. Because of this, citizens of Denver, near the Rocky Mountains, receive more than twice the dose of radiation from cosmic rays as do citizens of coastal cities such as New Orleans. The aluminum shell of a jet airplane provides little protection from cosmic rays, and for this reason passengers and crews of high flying jet airplanes receive more radiation than their earth traveling compatriots. Even greater is the cosmic radiation encountered at 60,000 ft (18,300 m) where supersonic jets fly. The level of cosmic radiation there is 1,000 times that at sea level. While the cosmic ray dose for occasional flyers is minimal, flight and cabin crews of ordinary jet airliners receive an additional exposure of 160 mrem per year, an added radiation burden to professional flyers of more than 40%. Cosmic sources for non-flying citizens at sea level are responsible for about 8% (29–30 mrem) of background radiation exposure per annum.
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 the 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 the earth's crust and becomes incorporated into all living matter including humans. It is the largest source of inhaled radioactivity and comprises about 55% of total human radiation exposure (both background and human generated). Energy efficient homes, which do not leak 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 non-smoking 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 the 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 which can, in some cases, identify damage to the double helix and effect DNA repair. Hence, while organisms are vulnerable to background radiation, mechanisms are present which assure survival.
[Robert G. McKinnell ]
Benarde, M. A. Our Precarious Habitat: 15 Years Later. New York: Wiley, 1989.
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