The primary health risk associated with the presence of radon gas is due to the solid particles that radon decays into.  It is these radon decay products or  progeny that actually "stick" in the lungs that cause an increased potential for lung cancer.  Since the amount of radon decay products available for inhalation can vary with house and atmospheric conditions, a radon gas measurement alone does not fully characterize the actual health risk present in a building.  This is particularly true in buildings where high air circulation and the use of air filters can have a large impact of reducing exposure to radon decay products. Consequently, it is a good idea to measure both radon and radon decay products. Measuring radon decay products provides a measure of the actual risk and measuring radon provides the potential risk from radon gas. 

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