Radon, a radioactive, colorless, odorless, tasteless noble gas, has been studied by a number of scientific and medical bodies for its effects on health. A naturally-occurring gas formed as a decay product of radium, radon is one of the densest substances that remains a gas under normal conditions, and is considered to be a health hazard due to its radioactivity. Its most stable isotope, radon-222 (), has a half-life of 3.8 days. Due to its high radioactivity, it has been less well studied by chemists, but a few compounds are known.
Radon is formed as part of the normal radioactive decay chain of uranium into . 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 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.
Occurrence
Concentration units
Radon concentration in the atmosphere is usually measured 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 progeny (, , , and ) 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 () released into the air decays to and other radioisotopes. The levels of can be measured. The rate of deposition of this radioisotope is dependent on the weather.
Natural
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 picogram per cubic meter. The average concentration of radon in the atmosphere is about 6 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 any one time is due to some tens of grams of radon, constantly being replaced by decay of larger amounts of radium and uranium. Its concentration 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 poorly ventilated dwellings, its concentration can climb to 20–2,000 Bq/m3.
In mining contexts, radon concentrations can be much higher. Ventilation regulations try to maintain concentrations in uranium mines under the "working level", and under 3 WL (546 pCi per liter of air; 20.2 kBq/m3 measured from 1976 to 1985) 95 percent of the time. The concentration in the air at the (unventilated) Gastein Healing Gallery averages 43 kBq/m3 (about 1.2 nCi/L) with maximal value of 160 kBq/m3 (about 4.3 nCi/L).
