Audeen W. Fentiman
Brian K. Hajek
Joyce E. Meredith
Naturally occurring radioactive materials are common in the environment and in the human body. These materials are continuously emitting ionizing radiation. Ionizing radiation from outer space (cosmic radiation) bombards the earth constantly. Collectively, the ionizing radiation from these and similar sources is called background radiation. Human activities, such as making medical x-rays, generating nuclear power, testing nuclear weapons, and producing smoke detectors which contain radioactive materials, cause additional exposure to ionizing radiation.
The percentage of the average annual radiation exposure contributed by each major source is illustrated in Figure 1. About 82 percent is from nature, and 18 percent is from industrial, medical, and consumer sources. The values given in Figure 1 are averages for the United States. Actual values vary depending on where people live and how they spend their time.
Figure 1. Average Annual Exposure to Ionizing Radiation
This fact sheet describes sources of ionizing radiation and gives some reasons for the variations in radiation exposure received.
The largest natural source of radiation exposure to humans is radon gas. While radon gas has always been in the environment, its contribution to human radiation exposure has increased in recent years. Radon's primary pathway is from the earth, through the basements of houses and other buildings, and into inside air that people breathe. To conserve energy, buildings are more tightly constructed so there is less exchange of inside air with fresh air from outside. This tends to trap radon inside. The average American receives about 200 millirem (mrem) per year from radon.
Radon exposures can vary depending on the soil and rock structure beneath buildings. A line of Ohio Black Shale runs through the center of the state from south to north, along part of the Lake Erie shore, and in northwestern parts of the state (see Figure 2). Many people who live over this geological structure experience higher doses from radon than those who live elsewhere in Ohio.
Figure 2. Distribution of Ohio Black Shale
Other terrestrial sources include naturally occurring radioactive materials that exist in rocks and soil. The main contributors are the radioactive isotope of potassium and the isotopes that are products of the decay of uranium and thorium. The average American receives about 28 mrem per year from terrestrial sources.
As the earth moves through space it is bombarded by high energy particles and gamma rays. These particles and rays add to the background radiation. The earth's atmosphere acts as a shield, absorbing much of the energy from cosmic radiation. People who live close to sea level are protected by a thicker blanket of atmosphere than those who live at high elevations, and thus have a lower exposure to cosmic radiation. For example, the exposure to cosmic radiation is about twice as high in Denver as it is in Ohio. Flying in an airplane also adds to exposure because of the high altitude. The average American receives 27 mrem per year from cosmic radiation.
Internal radiation comes from radioactive materials that occur naturally in the human body. Potassium and carbon are the primary sources of internal radiation exposures.
Potassium is an essential mineral for life. The Potassium-40 isotope (0.01 percent of all potassium) is naturally radioactive. It enters the human body through the food chain.
Carbon makes up about 23 percent, by weight, of the human body. Cosmic radiation creates Carbon-14, which is a small percentage of all carbon. Carbon enters the body both through the food chain and by breathing. The average American receives a dose of about 40 mrem per year from internal radiation.
The largest source of medical exposure, when averaged over all individuals, is from diagnostic x-rays, including both chest or limb x-rays and dental x-rays. Other sources are lumped into the category of nuclear medicine. These include diagnostic procedures such as the use of nuclear tracers. (Very small amounts of radioactive materials, called tracers, are put into the blood stream, and their progress through the body is monitored with a radiation detector. Blocked or restricted blood vessels can be identified, as can developing tumors.) Nuclear medicine also includes treatment of disease. Some examples are cobalt irradiation for the treatment of cancers, or the injection of radioactive iodine which concentrates in the thyroid for treatment of Graves' disease.
Radiation is used in the manufacturing of many consumer products. It is used to sterilize products such as cosmetics and medical supplies and for shrink-wrap packaging. It can be used to determine the thickness of materials, how full cans are before sealing them, and the quality of welds in structures such as bridges and buildings.
Radioactive materials also are used in many consumer products. The most common of these is the smoke detector which uses radioactive Americium-241 to detect smoke particles in the air.
Some other examples of activities from which people receive exposures from ionizing radiation include smoking cigarettes, burning gas lanterns, using natural gas for heating and cooking, living in brick or stone houses, using phosphate fertilizers, and watching color television.
The dose from medical, industrial, and consumer product sources varies from person to person, depending upon how many medical/dental x-rays received, type of employment, smoking habits, etc. The dose for the average American is about 10 mrem per year from consumer products and 53 mrem from medical sources.
This category includes the generation of electricity from both coal and nuclear power plants, the transportation of nuclear materials, and the storage of nuclear wastes. It also includes the exposures from fallout from the international nuclear weapons testing programs. These sources make up less than one percent of the annual radiation exposure to the average American.
If you want to read more about sources of ionizing radiation, some of the references and other fact sheets listed below may be helpful.
Eric J. Hall, Radiation and Life, Second Edition, Pergamon Press, New York, 1984.
Fred A. Mettler, Jr. and Robert D. Moseley, Jr., Medical Effects of Ionizing Radiation, Grune & Stratton, Inc., Orlando, Florida, 1985.
National Research Council, Committee on the Biological Effects of Ionizing Radiation, Health Effects of Exposure to Low Levels of Ionizing Radiation, BEIR V, National Academy Press, Washington, D.C., 1990.
RER-20 What is radioactive material and how does it decay?
RER-21 What is ionizing radiation?
RER-23 How is ionizing radiation measured?
Dr. Audeen W. Fentiman is an Assistant Professor in Nuclear Engineering at The Ohio State University. Brian K. Hajek is a member of the Nuclear Engineering faculty. Joyce E. Meredith is a Graduate Research Associate, Ohio State University Extension.
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Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.
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