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A Brief Explanation of the Units Of Measurement

Radon is measured in picocuries per liter and written as (pCi/L). One picocurie is one-trillionth of 37 billion disintegrations per second. One curie, named for Marie Curie, the discoverer of metallic radium, is the amount of radiation given off by one gram of radium.
Radon decay products (RDPs) such as polonium(218), lead(214), bismuth(214), and polonium(214), lead(210), bismuth(210), polonium(210) are measured in working levels (WL). A working level is the amount of RDP which normally results when the decay products are in equilibrium (maximum concentration) with 100 picocuries of radon in the air.
RDPs are difficult to measure in a house though, because among other problems, RDPs have a static charge and tend to plate out (stick) to walls, furniture, clothing, dust, smoke, and other objects and substances.
One of the problems with understanding the amount of risk due to a specific radon level measurement is that the risk statistics are based on an average lifetime (70 years) spent in an exposed area, even though the average American moves every 7 years, and is thus exposed to many different radon levels.