Introduction to Radon & Health Risk
The Fundamentals of Radon & Radioactivity
Radon Occurrence & Behavior
Radon Measurement & Devices
1 of 3

Measurement Units

pCi/L (picocuries)- Measurement Units for Radon gas

In the United States, all radioactive materials are measured in Curies, a measurement named after Madame Curie, a French physicist famous for her work on radioactivity.  This measurement is the radioactivity associated with one gram of radium. One picoCurie is one trillionth of a Curie.  A picoCurie refers to an amount of radioactivity that emits 2.22 atomic disintegrations per minute. Radon gas is measured in picoCuries per liter of air (pCi/L), which is a measure of the number of radioactive disintegrations per minute per liter of air. (The international community measures in becquerels per cubic meter of air (Bq/³). One pCi/L is equivalent to 37 Bq/m³.)

1 pCi/L = 2.22 disintegrations/min/L

Since a picoCurie is 2.22 disintegrations per minute for each liter, a gallon container of air with 4 pCi/L radon would contain approximately 35.52 disintegrations per minute of radon atoms.  This equates to approximately 4 (quarts per gallon) times 4 (pCi/L) times 2.22 disintegrations per minute.

4 (gal) X 4 (pCi/L) X 2.22 (dpm) = 35.52

WL (Working Level) – Measurement Units for Radon Decay Products   

Radon decay products (RDPs) are measured in working levels (WL).  A WL unit is the concentration of short-lived radon decay products produced from one liter of air containing 100 pCi/L of radon. In other words, this is approximately the total alpha energy released from the short-lived decay products in equilibrium with 100 pCi/L of Rn-222.  Plated-out RDPs are immeasurable.

1 WL = 100 pCi/L  
Typically, 100 pCi/L will produce 0.3 to 0.7 WL of RDPs. Therefore, it is commonly assumed that the ER in typical house is 0.5 or 50%. Based on this assumption, a house with 50 pCi/L can be expected to have about 0.25 WL and a house with 4 pCi/L is likely to have about 0.02 WL.