The three major sources of naturally occurring radiation are:

• cosmic radiation
• sources in the earth's crust, also referred to as terrestrial radiation
• sources in the human body, also referred to as internal sources.

Cosmic radiation comes from the sun and outer space and consists of positively charged particles, as well as gamma radiation. At sea level, the average cosmic radiation dose is about 26 mrem per year. At higher elevations the amount of atmosphere shielding cosmic rays decreases and thus the dose increases. The average dose in the United States is approximately 28 mrem/year.

There are natural sources of radiation in the ground, rocks, building materials and drinking water supplies. This is called terrestrial radiation. Some of the contributors to terrestrial sources are natural radium, uranium and thorium. Radon gas is a current health concern. This gas is from the decay of natural uranium in soil. Radon, which emits alpha radiation, rises from the soil under houses and can build up in homes, particularly well-insulated homes. In the USA, the average effective whole body dose from radon is about 200 mrem per year while the lungs receive approximately 2000 mrem per year.

The difference between man-made sources of radiation and naturally occurring sources is the place from which the radiation originates. The following information briefly describes some examples of human-made radiation sources.

Medical radiation sources

X rays are identical to gamma rays; however, they are produced by a different mechanism. X rays are an ionizing radiation hazard. A typical radiation dose from a chest x ray is about 10 mrem. The total average dose from medical x rays is 40 mrem in a year. In addition to x rays, radioactive isotopes are used in medicine for diagnosis and therapy. The total average dose is 14 mrem in a year.

Consumer products

Examples include TV's, older luminous dial watches, some smoke detectors, and lantern mantles. This dose is relatively small as compared to other naturally occurring sources of radiation and averages 10 mrem in a year.

Atmospheric testing of nuclear weapons

Another man-made source of radiation includes residual fallout from atmospheric nuclear weapons testing in the 1950's and early 1960's. Atmospheric testing is now banned by most nations. The average dose from residual fallout is about 2 mrem in a year.

Industrial uses

Industrial uses of radiation include x-ray machines and radioactive sources (radiography) used to test pipe welds, bore-holes, etc. Most people receive little if any dose from these sources.

As a whole, these sources of natural and human-made radiation are referred to as background.

The average annual radiation dose to a member of the general population from ALL background sources is about 360 millirem.