Radiation is a natural part of the earth's environment.
It comes from the sky above us, the earth beneath us and even from our own bodies. The air we breathe and the food we eat contain some naturally occurring radioactive materials. In fact, on average a person in the United States receives a radiation dose of about 300 millirem per year from natural sources compared to a dose of about 50 millirem per year from "artificially produced" sources such as medical x-rays.
Radioactive materials in rocks and soil
Natural radioactive material in rocks and soil account for about 28 millirem or 8% of the radiation dose a person typically receives in a year from all sources (including medical exposures). The earth's crust contains small amounts of uranium, thorium, and radium as well as radioactive isotopes of several elements including potassium. The radiation dose comes from the gamma rays which are emitted from the rocks, soil and some building materials (such as bricks and concrete).
Radon in indoor air
Small amounts of radon, a radioactive gas which comes from the radioactive decay of uranium, seep into the atmosphere from the soil. On average, inhalation of the radon in homes and other buildings accounts for 200 millirem per year. This is about 55% of the total radiation dose an individual receives in a year from all sources, including medical x-rays, cosmic rays, building materials, the earth's crust, and ingested radioactive materials.
Radioactive materials in the body
About 11% (40 millirem) of our radiation dose comes from naturally occurring radioactive materials in the body. Most of the dose comes from a radioactive isotope of potassium. Radioactive potassium-40, as well as other radioactive materials (such as carbon-14) which occur naturally in air, water, and soil are incorporated into the food we eat and then into our body tissues.
Cosmic radiation comes from outer space. The radiation dose from cosmic radiation increases with altitude, roughly doubling every 6,000 feet. Therefore, a resident of Florida (at sea level) on average receives about 26 millirem, one-half the dose from cosmic radiation as that recieved by a resident of Denver, Colorado, and about one-fifth of that by a resident of Leadville, Colorado (about two miles above sea level). A passenger in a jetliner traveling at 37,000 feet would receive about 60 times as much dose from cosmic radiation as would a person standing at sea level for the same length of time.
The millirem is the term used to describe the amount of radiation absorbed in the body, adjusting for radiation type.
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Glossary | Natural Radioactivity | NORM | Radon
Copyright HPS, 1997