Natural Sources of Radiation

Key Points:

We are all exposed to small amounts of radiation from natural sources daily. Three common categories of natural radiation sources are internal, cosmic, and terrestrial.
The average amount of radiation a person receives each year from natural sources in the United States is about 310 millirem (mrem). A mrem is a measurement of the biological risks of exposure to radiation.
We receive about 10 mrem from a chest x-ray.

The average annual dose for each natural source is listed below (Table 1).

Table 1. Average Annual Dose for Natural Sources of Radiation

Average Annual Dose for Natural Sources of Radiation
Source	Average Annual Dose	Percent of Average Annual Dose
Internal (by Inhalation)	226 mrem	73%
External (from Cosmic Exposure)	34 mrem	11%
Internal (by Ingestion)	28 mrem	9%
External (from Terrestrial Exposure)	22 mrem	7%

Internal Radiation

We get internal radiation from the air we breathe and the food and water we consume. 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 we breathe and, on average, gives each of us 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 our upper atmosphere), find their way into our food and drinking water. Because we eat food and drink water, these radioactive elements give us a small dose of about 28 mrem per year.

Radiation can be naturally found in soil (click to enlarge)

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. We get exposure to cosmic radiation through our skin. The average annual dose of cosmic radiation that we receive is 34 mrem per year.

Terrestrial External Radiation

Terrestrial external radiation comes from rocks, soil, water, and vegetation. Exposure is through our skin. Some naturally occurring radioactive elements such as uranium, thorium, and radium give us a small external dose of 22 mrem per year.