A gaseous byproduct of decaying uranium and radium, radon itself decays when it enters the atmosphere, forming a series of "daughter" particles. Two of them, polonium 218 and polonium 214, are alpha emitters, meaning that, when inhaled in high concentrations, they emit alpha particles, known to initiate cancers in the bronchial epithelia.
Radon concentrations are traditionally expressed in picocuries (pCi) per liter, whereby 1 pCi represents the amount of material needed to produce 2.2 radioactive decays per minute. For comparison to epidemiological studies of uranium miners, picocuries are converted into a measure called a Working Level Month (WLM). A WLM is defined as a radon concentration of 100 picocuries per liter of air inhaled for 170 hours. A person breathing the air in a home with a radon level of 4 picocuries per liter (the action level set by EPA) would receive an annual dose equivalent to one WLM.
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