All discussions of radon concentrations in the environment refer to 222Rn. While the average rate of production of 220Rn (from the thorium decay series) is about the same as 222Rn, the amount of 220Rn entering the environment is much less than that of 222Rn because of the short half-life of 220Rn (1 minute versus 4 days).
Radon concentrations found in natural environments are much too low to be detected by chemical means. A 1000 Bq/m3 (relatively high) concentration corresponds to 0.17 picogram 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 radon activity of the Earth atmosphere is due to some tens of grams of radon, consistently replaced by decay of larger amounts of radium and uranium.
Radon concentration is usually measured in the atmosphere, in becquerel per cubic meter (Bq/m3), the SI derived unit. Typical domestic exposures are about 100 Bq/m3 indoors, and 10-20 Bq/m3 outdoors.
It is often measured in picocuries per liter (pCi/L) in the USA, with 1 pCi/L=37 Bq/m3.
In the mining industry, the exposition is traditionally measured in working level (WL), and the cumulative exposition in working level month (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 exposition of 1 WLM is roughly equivalent to living one year in an atmosphere with a radon concentration of 230 Bq/m3.
Radon (222Rn), when 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 weather dependent.
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