Radon exposure

Radon is formed as part of the normal radioactive decay chain of uranium into 206Pb. Uranium has been present since the earth was formed and its most common isotope has a very long half-life (4.5 billion years), which is the time required for one-half of uranium to break down. Thus, uranium and radon, will continue to occur for millions of years at about the same concentrations as they do now. Radon is responsible for the majority of the mean public exposure to ionizing radiation. It is often the single largest contributor to an individual's background radiation dose, and is the most variable from location to location. Radon gas from natural sources can accumulate in buildings, especially in confined areas such as attics, and basements. It can also be found in some spring waters and hot springs. According to a 2003 report EPA's Assessment of Risks from Radon in Homes from the United States Environmental Protection Agency, epidemiological evidence shows a clear link between lung cancer and high concentrations of radon, with 21,000 radon-induced U.S. lung cancer deaths per year—second only to cigarette smoking. Thus in geographic areas where radon is present in heightened concentrations, radon is considered a significant indoor air contaminant. Radon concentration is usually measured in the atmosphere in becquerels per cubic meter (Bq/m3), which is an SI derived unit. As a frame of reference, typical domestic exposures are about 100 Bq/m3 indoors and 10-20 Bq/m3 outdoors. In the US, radon concentrations are often measured in picocuries per liter (pCi/l), with 1 pCi/l = 37 Bq/m3. The mining industry traditionally measures exposure using the working level (WL) index, and the cumulative exposure in working level months (WLM): 1 WL equals any combination of short-lived 222Rn progeny (218Po, 214Pb, 214Bi, and 214Po) in 1 liter of air that releases 1.3 × 105 MeV of potential alpha energy; one WL is equivalent to 2.08 × 10−5 joules per cubic meter of air (J/m3). The SI unit of cumulative exposure is expressed in joule-hours per cubic meter (J·h/m3). One WLM is equivalent to 3.6 × 10−3 J·h/m3. An exposure to 1 WL for 1 working month (170 hours) equals 1 WLM cumulative exposure. A cumulative exposure of 1 WLM is roughly equivalent to living one year in an atmosphere with a radon concentration of 230 Bq/m3. The radon (222Rn) released into the air decays to 210Pb and other radioisotopes. The levels of 210Pb can be measured. The rate of deposition of this radioisotope is dependent on the weather. Radon concentrations found in natural environments are much too low to be detected by chemical means: for example, a 1000 Bq/m3 (relatively high) concentration corresponds to 0.17 pico-gram per cubic meter. The average concentration of radon in the atmosphere is about 6×10−20 atoms of radon for each molecule in the air, or about 150 atoms in each ml of air. The entire radon activity of the Earth's atmosphere at a time is due to some tens of grams of radon, consistently replaced by decay of larger amounts of radium and uranium. Concentrations can vary greatly from place to place. In the open air, it ranges from 1 to 100 Bq/m3, even less (0.1 Bq/m3) above the ocean. In caves, aerated mines, or in poorly ventilated dwellings, its concentration can climb to 20-2,000 Bq/m3.