Radon, radioactive

CAS RN: 10043-92-2

Exposure Summary

Radon occurs naturally from the uranium-238 decay series present in rocks and soil of the earth. It is a radioactive, inert, monoatomic gas. Radon is not produced commercially. There are no stable nuclides. Radon-222 is the longest-lived known isotope of radon, with a half-life of 3.825 days. Radon-222 is an alpha-emitter and member of the decay series of uranium-238. The known artificial radioactive isotopes are 195-218, 221 and 223-228. Radon-222 exists everywhere in the atmosphere and varies widely at the global level with high concentrations noted particularly in areas where terrestrial gamma radiation is high. The average radon-222 concentration in air is 6X10-14 ppm by volume; the average concentration in igneous rock of the earth's crust is 1.7X10-10 ppm by weight. Radon-222 and radon-220 are the gaseous radioactive products of the decay of radium-226 and radium-228, respectively, which are present in all terrestrial materials. The largest share of normal background radiation is contributed by radon (54% radon, 27% other natural radiation). It is estimated that every square mile of soil to a depth of 6 inches contains about 1 g of radium, which releases radon in minute amounts to the atmosphere. The concentrations of radon in water may range over several orders of magnitude with the highest usually in well water, intermediate in groundwater and less in surface water. Radon dissolved in water may enter indoor air via de-emanation when the water is used. Occupational exposure to radon may occur through inhalation of this compound at workplaces where radon is produced or used. Radon inhalation and ingestion doses are small compared to doses from radioactive progeny. Of concern are the doses primarily from short-lived radon progeny (polonium-218, lead-214, bismuth-214, polonium 214) attached to inhaled dust particles. The most likely pathway by which the general public is exposed to radon is by inhalation due to the release of this substance from the earth's crust, especially in areas where terrestrial gamma radiation is high. Crystalline rock formation, especially granite, can be a source of elevated levels, whereas sedimentary rock usually yields lower levels. Radon-222 may pose a health problem for homeowners by contributing to indoor air radon levels. Monitoring data also indicate that the general population may be exposed to radon via ingestion of drinking water, although this pathway is considered minor when compared to inhalation. (SRC)
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