Equilibrium Ratio

  • Radon gas is constantly decaying and reoccurring.
  • What happens to RDPs?
    • Some are ventilated
    • Some remain suspended in the air
    • Some plate out on other objects


Plate out is the process of airborne RDPs leaving the air and attaching themselves to solid objects such as dust, smoke, walls, floors, clothing, or any other object. When an RDP leaves the air and attaches itself to a solid object, it is no longer breathable or measurable. In order to accurately measure radon gas levels indoors, we need to account for the lack of equilibrium between the fraction of RDPs created, compared to the fraction that remains. 

Plate-Out Determines Equilibrium Ratio

It is important to understand the ratio of RDPs created vs the ratio that still remain airborne. We know that typically, 30 – 70 % of RDPs created indoors will plate out on solid objects. Because measuring the fraction of airborne RDPs is impractical in most situations, it is commonly assumed that 50% (the average between 30-70 %) of RDPs will remain airborne and 50% will plate out.

Although a 50% Equilibrium Ratio (ER) is typically assumed, we need to understand how to determine the actual ER, which allows for the conversion of pCi/l to WL. We also need to consider how higher or lower RDP levels in air will affect the radon measurement. Radon gas is constantly decaying and reoccurring. What happens to RDPs that don’t plate out? They are ventilated or remain suspended in the air.

What Can Cause ER to Be Above or Below Average?

  • Air circulation
    • Increases plate-out, decreases ER  
  • Electronic air filters
    • Decreases airborne RDPs, decreases ER
  • Recent ventilation
    • Inadequate time for RDPs to form, decreases ER  
  • Suspended dust, smoke, humidity
    • Decreases plate-out, increases ER

Equilibrium Ratio Impacts Measurement Protocols

  • ER will never be measured in the Field.
  • Therefore, special protocols must be observed to protect the test environment.
  • For Example:
    • Short-Term Tests Conducted Under Closed Building Conditions
    • Air Handling Mechanisms Used Minimally
    • Initiate Closed Building Conditions 12-hours Prior