The Equilibrium Factor (EF), often denoted as , is a critical parameter in radon dosimetry, representing the state of radioactive equilibrium between radon-222 gas and its short-lived progeny in the air. While radon gas itself can be measured in Becquerels per cubic metre (), the [potential alpha energy concentration](/potential-alpha-energy-concentration) (PAEC) of its decay products is more directly related to the dose received by the lungs. The EF bridges the gap between these two measurements.
In a closed system, over sufficient time, a parent radionuclide and its decay products will reach radioactive equilibrium. For radon-222, with a half-life of 3.8 days, and its short-lived progeny (Polonium-218, Lead-214, Bismuth-214, and Polonium-214, all with half-lives much shorter than radon), this means that their activity concentrations would become equal. In such a state of full equilibrium (), of radon would be accompanied by of each of its short-lived decay products.
In practical indoor environments, full radioactive equilibrium between radon and its progeny is rarely achieved. Several environmental factors contribute to disequilibrium:
Consequently, the activity concentrations of radon's short-lived progeny are typically lower than that of the parent radon, leading to an Equilibrium Factor less than 1 ().
The Equilibrium Factor is formally defined as the ratio of the actual potential alpha energy concentration (PAEC) of the short-lived radon decay products to the PAEC that would exist if these progeny were in full radioactive equilibrium with the measured radon concentration. The PAEC is typically expressed in Joules per cubic metre ().
If is the measured activity concentration of radon-222 () and is the measured potential alpha energy concentration of its decay products (), the EF is calculated as:
The constant represents the potential alpha energy concentration that of radon-222 would generate if it were in full radioactive equilibrium with all its short-lived decay products.
In indoor environments, the Equilibrium Factor typically ranges from 0.2 to 0.7, with an average often cited around 0.4. This variability underscores the importance of not assuming full equilibrium when assessing radon hazards. The EF is crucial for: