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  Origins of radon
The Earth's crust contains small amounts of naturally radioactive materials such as uranium and thorium. Uranium and thorium decay to other radioactive atoms including radium which then decays to radon gas. Since it is an inert (i.e., chemically stable) gas, radon moves from the soil, where it is produced, and into the air. Radon is a natural part of the earth's atmosphere. The amount of uranium and radium in soil varies greatly with geographic location and soil type. Therefore, the amount of radon gas released to the atmosphere also varies across the United States.
    Movement of radon.
Radon gas can get into homes and other buildings through cracks or other openings in the foundation and by passing through the foundation materials. Radon also decays into radioactive atoms called "progeny" or "decay products". The radon gas and its decay products become part of the air that people breathe both inside and outside buildings. However, the amount of radon in air inside buildings is generally much higher than the amount outside because the buildings trap the radon which moves from the soil.
    Doses from radon.
The "decay products" of radon can be trapped in the lungs where they produce a radiation dose. Breathing air containing radon accounts on average for about 55% of the total radiation dose received by a person in the United States. The radiation dose from "indoor radon" can vary a great deal depending on the age of the exposed individual, the length of exposure, location, season of the year, weather, and the type of building. In general, the amount of radon in air in a "tight" building is greater than the amount in a well ventilated building. Certain types of construction can prevent much of the radon in soil from entering the air in buildings.
    Studies on radon.
The National Council on Radiation Protection and Measurements (NCRP), the International Commission on Radiological Protection (ICRP) and the Environmental Protection Agency (EPA) have studied the potential health risk from exposure to radon gas. In 1988, the National Academy of Sciences (NAS) published a study of the potential health risks of exposure to radon. Since 1988, a substantial number of studies have appeared in the scientific literature. As a result, the NAS is currently taking a second look at this issue.
    Limits on radon exposure.
The EPA has recommended that individuals test their homes for radon and take steps to reduce the amount of radon if it exceeds an "action level" of 4pCi/liter (0.15 Bq/L). This action level is approximately four times the average radon concentration in homes in the United States. About 1 in 12 homes in the United States - nearly 6 million in all - have radon concentrations above this level. Considerable controversy surrounds this recommendation because there is tremendous uncertainty in calculating the radiation dose from this concentration. It is a problem that is difficult to solve, and as a result the EPA's action levels are liable to be revised.
    Picocuries (pCi) or bequerels (Bq) are the units used to describe the number of atoms of a radioactive material which will decay (emit radiation) in a given period of time. One pCi of radioactive material means that, on average, 2.2 atoms of the material will decay per minute. One Bq of radioactive material means that, on average, 1 atom of the material will decay per second.
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     Glossary | Natural Radioactivity | NORM | Radon

Copyright HPS, 1997


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