National Safety Council Radiation


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Sources of Ionizing Radiation

Measuring Radiation
Exposure

Natural Sources

Manmade Sources Figures

Sources of Radiation Exposure

Radon Routes Into a Home

X-Rays Used in Medicine

Smoke Detector

Map of U.S. Nuclear Power Facilities

When energy particles and rays are expelled from the forces that bind them together in atoms, ionizing radiation is emitted. This process has been going on since the birth of the universe. Radiation has always been commonplace in our world. Natural radioactive materials were discovered in the 1890s. It was not until 1942 that physicist Enrico Fermi and his team created the first manmade radioactive materials in the world's first nuclear reactor at the University of Chicago. Figure 1: Sources of Radiation Exposure

Source: National Council on Radiation Protection and Measurements Measuring Radiation Exposure In the United States, we commonly measure human exposure to potentially harmful radiation in units called millirem (one one-thousandth of a rem). On average, each of us receives about 360 millirem of radiation each year. About 300 millirem, or 82 percent of the total, is natural background radiation (from radon and other natural sources). The remaining 18 percent of our radiation exposure is from manmade sources. (See Figure 1.)

X-rays and other medical and dental procedures
Consumer products (such as cigarettes, smoke detectors, color televisions)
Operation of nuclear power plants
Manufacture of nuclear weapons
Fallout from past atmospheric nuclear weapons testing

Natural Sources

Everything on Earth is exposed to a constant barrage of naturally occurring ionizing radiation from the sun, cosmic rays, and radioactive elements in the Earth's crust. The primary radioactive elements in the Earth's crust are uranium, thorium, potassium, radium, and their radioactive decay products or derivatives.

Radon
Radon is a naturally occurring gas formed from the radioactive decay of uranium-238 in rock and soil. Radon is colorless, odorless, tasteless, chemically inert, and radioactive. Radon also decays, emitting ionizing radiation in the form of alpha particles, and transforms into decay products, or "progeny" radioisotopes. The half-life of radon is about four days. Unlike radon, the progeny are not gases, and can easily attach to and be transported by dust and other particles in air. The decay of progeny continues until stable, non-radioactive progeny are formed. At each step in the decay process, radiation is released. Radon accounts for more than half (an average of 55 percent) of the radiation dose we receive each year and is the second leading cause of lung cancer, after cigarette smoking, in the United States.

Radon moves through air or water-filled pores in the soil to the soil surface and enters the air, while some remains below the surface and dissolves in ground water (water that collects and flows under the ground's surface). Radon has been found in drinking water from public ground water supplies in many states across the country. In the outside open air, most radon dilutes into relatively low concentrations (about 0.4 picocuries per liter of air, abbreviated pCi/L).

Radon becomes a serious public health problem when high levels are found in indoor air where people can breathe it — in homes, schools, and other buildings. Radon in the soil can seep through the basement or ground floor through cracks in a foundation or construction joints and build up indoors to levels substantially higher than outdoor air levels. (See Figure 2.) Indoor radon has become more of a problem in recent years because new homes are built more air-tight and Americans now spend an average of about 90 percent of their time indoors.

Figure 2: Radon Routes Into a Home

Source: U.S. Environmental Protection Agency

EPA and the Office of the U.S. Surgeon General recommend that citizens take steps to reduce indoor radon levels to below 4 pCi/L. EPA's National Residential Radon Survey completed in 1991 indicates that more than six percent of all homes nationwide have elevated radon levels, approximately one in every 15 homes (or six million homes) nationwide.

Radon can also be a problem in schools and other buildings. EPA's National School Radon Survey found that 20 percent of the schools nation-wide (about 15,000 institutions) have at least one schoolroom with a radon level greater than 4 pCi/L.

Although most of radon exposure indoors comes from soil, radon dissolved in tap water can be released into indoor air when it is used for showering, washing or other domestic uses, or when heated before being ingested. This adds to the airborne radon indoors. It is estimated that this source accounts for less than five percent of the total indoor air concentration in houses served by ground water sources. Because it takes about 10,000 pCi/L of radon dissolved in water to produce about one pCi/L of radon in household air, the levels of radon in drinking water need to be significantly elevated to substantially contribute to the level of radon in the indoor air.

Other Terrestrial Sources
Other naturally occurring radioactive materials in the Earth's crust, such as thorium, potassium, and radium, contribute about eight percent of our annual exposure to radiation. Radiation levels from these sources also vary in different parts of the country.

Cosmic Radiation
Cosmic radiation from outside the Earth's atmosphere includes high-energy protons, electrons, gamma rays, and X- rays that hit the Earth as it moves through space. Fortunately, the Earth's atmosphere absorbs much of the energy from cosmic radiation.

About eight percent of our annual exposure comes from cosmic radiation. However, cosmic radiation increases at higher altitudes, roughly doubling every 6,000 feet. For example, the exposure to cosmic radiation is about twice as high in Denver as it is in Chicago.

Internal Radiation
About 11 percent of the average person's total annual exposure comes from radioactivity within our own bodies. Radioactive materials in the air, water, and soil are absorbed in food and then by the body's own tissues.

Potassium and carbon are two of the main sources of internal radiation exposures. They enter our bodies through the food we eat and the air we breathe.

Potassium, essential to life, is distributed throughout our bodies. A small portion (about one one-hundredth of a percent) of natural potassium consists of a naturally radioactive isotope called potassium-40. This isotope is the chief radioactive component in normal food and human tissue.
Carbon-14, a radioactive isotope of carbon created by cosmic radiation, makes up a small fraction of all carbon in our bodies.

(The section Major Uses of Isotopes has more information on this topic.)

Manmade Sources

As our use of radiation increases, so does our exposure to ionizing radiation from manmade sources. Lifestyle choices, including house construction, air travel, and smoking, also affect the level of our exposure. Airline crews experience greater exposures than people who live at sea level where they are protected by a thicker blanket of atmosphere.

Medical and dental X-rays account for most of the exposure from manmade sources, an average of about 11 percent of our total annual exposure.

Consumer products such as color television sets, video displays, and smoke detectors account for another three percent of annual exposure.

Other potential sources of small amounts of radiation are:

Mining and agricultural products, and ash from burned coal

Nuclear reactors and their supporting facilities (uranium mills and fuel preparation plants)

Federal facilities involved in nuclear weapons production

Fallout from past atmospheric weapons testing, which peaked in the mid-1960s

Medicine
About 15 percent of our total average exposure to ionizing radiation is from medical X-rays (11 percent) and nuclear medicine (4 percent).

Americans receive about 200 million medical X-rays every year. (See Figure 3.) X-rays are an important tool in medical diagnoses.

Figure 3: X-rays Used in Medicine

Nuclear medicine involves diagnostic procedures such as nuclear tracers, small amounts of radioactive materials that are injected into the blood stream to allow monitoring of their progress through the body with a radiation detector. Tracers can help locate blocked or restricted blood vessels and developing tumors.

Nuclear medicine also uses radiation to treat diseases. Precisely targeted cobalt radiation, for example, can destroy diseased cells without damaging healthy cells nearby. Injections into the bloodstream of radioactive iodine, which then concentrates in the thyroid, is an effective treatment for hyperthyroidism or Graves' disease, as well as thyroid cancer.

Average annual doses from medical applications are about one-sixth the average annual dose from background radiation. However, patients undergoing radiation therapy, where radiation is narrowly targeted to affected tissues, can be exposed to levels many times higher than background radiation. While medical uses of radiation offer important benefits, they can also pose risks.

Consumer Products
On average, we receive about three percent of our total radiation exposure from consumer products, about 11 millirem per year.

Figure 4: Smoke Detector

Smoke detectors that use americium-241 (See Figure 4.)

Lawn fertilizer containing potassium-40

Cigarettes

Gas lanterns

Exit signs

Natural gas appliances

Brick or stone houses

Color television sets

Radiation is also used in the manufacturing process for many consumer products. For example, cosmetics and medical supplies are sterilized by radiation. Radiation is also used to help determine the thickness of materials, how full cans are before they are sealed, and the quality of the welds in bridges and buildings. (See Industry and Consumer Products.)

Nuclear Power
Nuclear power reactors, which use uranium, supply about 20 percent of the electricity used in the United States. (See Figure 5.)

Figure 5: Map of U.S. Nuclear Power Facilities

Source: U.S. Department of Energy

Nuclear power plant operations account for less than one one-hundredth of a percent (less than one millirem per year) of the average American's total radiation exposure. However, workers at nuclear power plants can receive much higher doses and those who live near power plants may receive slightly higher doses.

Nuclear Weapons
For most people, nuclear weapons production and testing are responsible for only very small amounts of radiation exposure. However, past accidental and planned releases have exposed some employees and neighbors of weapons facilities to higher radiation doses.

Fallout from atmospheric testing of nuclear weapons reached its peak in the mid-1960s. While the effect on background radiation in the vicinity of these tests was significant in the days and weeks following an explosion, the effect on world-wide background radiation levels has been minor, although measurable. The longer half-life fission products from these tests, including cesium-137 and strontium-90, caused background levels of radiation around the world to increase slightly.

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