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Radon (usually in the form of the radon-222 isotope) is a colorless and odorless radioactive gas formed from radioactive decay. Denoted by the atomic symbol, Rn. radon has an atomic number of 86 and the atomic weight of its most stable isotope is 222. It is a colorless, odorless gas that emits radioactivity. It is classified as a noble gas based on its location on the periodic table. Radon is the heaviest element in the family of inert, or noble, gases.

The discovery of radon is credited to Friedrich Dorn (1848-1916), a German physics professor. Marie Curie's experiments stimulated Dorn to begin studying the phenomenon of radioactivity. In 1900, he showed that radium emitted a radioactive gas that was called radium emanation for several years.

The most common geologic source of radon derives from the decay of uranium. Radon is commonly found at low levels in widely dispersed crustal formations, soil, and water samples. To some extent, radon can be detected throughout the United States. Specific geologic formations, however, frequently present elevated concentration of radon that may pose a significant health risk. The Surgeon General of the United States and the Environmental Protection Agency identify radon exposure as the second leading cause of lung cancer in the United States. Cancer risk rates are based upon magnitude and duration of exposure.

Produced underground, radon moves toward the surface and eventually diffuses into the atmosphere or in groundwater. Because radon has a half-life of approximately four days, half of any size sample deteriorates during that time. Regardless, because radon can be continually supplied, dangerous levels can accumulate in poorly ventilated spaces (e.g., underneath homes, buildings, etc.). Moreover, the deterioration of radon produces alpha particle radiation and radioactive decay products that can exhibit high surface adherence to dust particles. Radon detection tests are designed to detect radon gas in picocuries per liter of air (pCi/L). The picocurie is used to measure the magnitude of radiation in terms of disintegrations per minute. One pCi, one trillionth of a Curie, translates to 2.2 disintegrations per minute. EPA guidelines recommend remedial action (e.g., improved ventilation) if long term radon concentrations exceed 4 pCi/L.

Working level units (WL) are used to measure radon decay product levels. The working level unit is used to measure combined alpha radiation from all radon decay products. Commercial test kits designed for use by the general public are widely available. The most common forms include the use of charcoal canisters, alpha track detectors, liquid scintillation detectors, and ion chamber detectors. In most cases, these devices are allowed to measure cumulative radon and byproduct concentrations over a specific period of time (e.g., 60–90 days) that depends on the type of test and geographic radon risk levels. The tests are usually designed to be returned to a qualified laboratory for analysis.

The EPA estimates that nearly one out 15 homes in the United States has elevated radon levels.

Radon can be kept at low concentration levels by ventilation and the use of impermeable sheeting to prevent radon seepage into enclosed spaces. Radon in water does not pose nearly the health risk as does breathable radon gas. Regardless, radon removal protocols are increasingly a part of water treatment programs. Radon is removed from water by aeration or carbon filtration systems.

Exposure to radon is cumulative. Researchers are presently conducting extensive research into better profiling the mutagenic risks of long term, low-level radiation exposure.

Uranium miners must take special precautions to avoid radioactive poisoning by radon. The gas can also migrate upward into the soil and leak into buildings. Radon can seep into groundwater and so may be found in public drinking supplies.

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