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Fact Sheets Radiation in the Environment August 1998

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Radiation is a natural part of our environment. When our planet was formed, radiation was present - and radiation surrounds it still. Natural radiation showers down from the distant reaches of the cosmos and continuously radiates from the rocks, soil, and water on the Earth itself.

During the last century, mankind has discovered radiation, how to use it, and how to control it. As a result, some manmade radiation has been added to the natural amount is present in our environment.

graph1.gif Many materials - both natural and manmade -that we come into contact with in our everyday lives are radioactive. These materials are composed of atoms that release energetic particles or waves as they change into more stable forms. These particles and waves are referred to as radiation, and their emission as radioactivity.

As the chart on the left shows, most environmental radiation (82%) is from natural sources. By far the largest source is radon, an odorless, colorless gas given off by natural radium in the Earth’ s crust. While radon has always been present in the environment, its significance is better understood today. Manmade radiation - mostly from medical uses and consumer products - add about 18% to our total exposure.

Types of Ionizing Radiation
Radiation that has enough energy to disturb the electrical balance in the atoms of substances as it passes through is called ionizing radiation. There are three basic forms of ionizing radiation.

Alpha particles are the largest and slowest moving type of radiation. They are easily stoppedby a sheet of paper or the skin. Alpha particles can move through the air only a few inches before being stopped by air molecules. However, alpha radiation is dangerous to sensitive tissue inside the body.


Beta particles are much smaller and faster moving than alpha particles. Beta particles pass through paper and can travel in the air for about 10 feet. However, they can be stopped by thin shielding such as a sheet of aluminum foil.


Gamma radiation is a type of electromagnetic wave that travels at the speed of light. It takes a thick shield of steel, lead, or concrete to stop gamma rays. X-rays and cosmic rays are similar to gamma radiation. X-rays are produced by manmade devices; cosmic rays reach Earth from outer space.

Units of Measure

Radiation can be measured in a variety of ways. Units of measure show either (1) the total amount of radioactivity present in a substance or (2) the level of radiatron being given off.

The radioactivity of a substance is measured in terms of the number of transformations (changes into more stable forms) per unit of time. The curie is the standard unit for this measurement and is based on the amont of radioactivity contained in 1 gram of radium. The amounts of radioactivity that people normally work with are in the millicurie (one- thousandth of a curie) or microcurie (one-millionth of a curie) range.

Levels of radioactivity in the environment are in the picocurie or pCi (one trillionth) range.

Levels of radiation are measured in various units. The level of gamma radiation in the air is measured by the roentgen. This is a relatively large unit, so measurements are often calculated in milliroentgens. Radiation absorbed by humans is measured in either rad or rem. The rem is the most descriptive because it measures the ability of the specific type of radiation to do damage to biological tissue. Typical measurements will often be in the millirem (mrem), or one-thousandth of a rem, range.

Radiation in the Environment

Cosmic Radiation

Cosmic radiation is high-energy gamma radiation that originates in outer space and filters through our atmosphere.

Terrestrial Radiation

Terrestrial sources are naturally radioactive elements in the soil and water such as uranium, radium, and thorium. Average levels of these elements are 1 pCi/g of soil.


Many building materials, especially granite, contain naturally radioactive elements.


Radon levels in buildings vary, depending on geographic location, from 0.1 to 200 pCi/liter.

Because the radioactivity of individual samples varies, the numbers given here are approximte or represent an avaerage. They are shown to provide a perspective for concentrations and levels of radioactivity rather than dose.



Food contribues an average of 20 mrem/year, mostly from potassium-40, carbon-14, hydrogen-3, radium-226, and thorium-232.

Medical Treatment

The exposures from medical diagnoses vary widely according to the required procedure, the equipment, and film used for X-rays, and the skill of the operator.

Consumer Goods

Natural Radioactivity in Florida Phosphate Fertilizers
(in pCi/gram)
  xxxxx xxxxx xxxxx
Ra-226 21.3 21.0 33.0
U-238 20.1 58.0 6.0
Th-230 18.9 48.0 13.0
Th-232 0.6 1.3 0.3

International Nuclear Weapons Test Fallout

From pre-1980 atmospheric tests average for a U.S. citizen= 1 mrem/year

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