Radiation Background

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Radiation occurs naturally in a wide variety of forms. Our bodies are constantly being bombarded by radiation, although we are unaware of it. The National Council on Radiation Protection cites three general types of radiation sources. These are:

1. Natural Sources

  • Cosmic radiation from outer space
  • Terrestrial radiation from the ground
  • Radionuclides naturally present in the body.

2. Enhanced Natural Sources

These are sources for which human exposures have been increased as a result of human activities. This includes the higher cosmic radiation levels experienced during air travel, especially at high altitudes; and the buildup of radon gas which is trapped in homes or other buildings.

3. "Man-made" Sources

  • X-rays in medicine
  • Radiopharmaceuticals in medicine
  • Atmospheric nuclear bomb testing
  • Nuclear power

The following table summarizes the levels of exposure to each type of radiation for the average member of the U.S. population (NCRP Report No. 93).


Natural Background Exposure

Radon ~ 200 mrem
Terrestrial ~ 28 mrem
Cosmic ~ 28 mrem
Internal (K-40) ~ 26 mrem
Medical ~ 10 mrem
Dental ~   5 mrem
Fallout from Weapons Testing < 0.3 mrem
TOTAL ~300 mrem

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