Radon and Other Natural Sources of Radiation Exposure

Even though many people may not be aware of it, radiation is all around them. In fact, this natural “background” radiation has been here since the birth of the universe.

People are exposed to small amounts of radiation from natural sources every day. This radiation exposure can be measured. In the United States, the dose, which is the amount of radiation energy absorbed by the body, is most commonly reported in millirem (mrem). For example, a chest x-ray will cause a person to receive perhaps 10 mrem.

According to the U.S. Nuclear Regulatory Commission, people generally receive a total annual dose of about 620 mrem. Of this annual total, natural sources of radiation account for about 50 percent, while man-made sources account for the remaining 50 percent.

Three categories of natural radiation sources include internal, cosmic and terrestrial. The Centers for Disease Control and Prevention (CDC) shares the following information about these sources of natural radiation.

• Internal Radiation - Some naturally-occurring radioactive elements such as uranium, thorium and radium change into other radioactive elements known as radon. These radioactive elements are in rocks, soil and building materials all around us. Radon is in the air people breathe and, on average, gives each person a dose of 226 mrem per year. This is the largest source of natural radiation exposure. In addition, naturally-occurring radioactive elements such as carbon, potassium, uranium, thorium and radium, as well as radioactive elements such as Carbon-14 (produced by cosmic radiation in the upper atmosphere), find their way into people’s food and drinking water. These radioactive elements give people a small dose of about 28 mrem per year. 

• Cosmic or External Space Radiation - Cosmic radiation is high-energy radiation that comes from space. This radiation interacts with atoms in Earth’s upper atmosphere to produce other energetic particles and gamma rays. Doses of cosmic radiation vary with altitude. Cosmic radiation is more intense in the upper atmosphere and most intense in deep space beyond Earth’s magnetic field. People get exposure to cosmic radiation through their skin. The average annual dose of cosmic radiation that people receive is 34 mrem per year. 

• Terrestrial External Radiation - Terrestrial external radiation comes from rocks, soil, water and vegetation, and exposure occurs through the skin. Some naturally-occurring radioactive elements such as uranium, thorium and radium give people a small external dose of 22 mrem per year. 

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