Introduction to Radon & Health Risk
The Fundamentals of Radon & Radioactivity
Radon Occurrence & Behavior
Radon Measurement & Devices
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Factors that Affect the Ability to Measure Radon

Equilibrium Ratio

The concentration of airborne RDPs versus those that subsequently plate-out is called Equilibrium Ratio (ER). In other words, ER is the measurable fraction of RDPs remaining in the air after some fraction has plated-out on solid objects such as walls, floors, etc.  The equilibrium ratio will decrease when the concentration of airborne RDPs is reduced. Stagnant air containing an abundance of suspended particles will provide more airborne particles to which the RDPs can attach, creating a higher ER.  All of these environmental factors can influence the concentration of airborne progeny without affecting the radon-222 gas concentration, making perfect equilibrium unattainable in home atmospheres.

Figure 2-9
Plate-out vs Measurable/Breathable RDPs
© 2012-2021 Capstone Creative

Studies of equilibrium ratios (ER) in homes indicate that typically 30% to 70% (0.3 to 0.7) of the RDPs plate out and therefore do not remain airborne. An ER of 0.50 (or 50%) is commonly assumed to be average. Based on this 0.50 ER assumption, the house with a radon gas concentration of 100 pCi/L would only produce one-half of a working level, meaning it would take approximately 200 pCi/L to generate one full working level. (However, considering the range of 30% to 70% 1 WL in any given house could, in fact, correspond to anywhere between 150 and 300 pCi/L). Based on an average 50% equilibrium ratio assumption, the EPA “Action Level” of 4 pCi/L is equal to 0.02 WL.  The formula for converting radon gas (pCi/L) to Working Levels (WL) is Equilibrium Ratio x picoCuries per Liter  ÷  100 as depicted below.

Factors Affecting ER

Many factors can impact the equilibrium ratio such as: Air circulation or lack thereof, excessive ventilation, and air filtration impact the amount of radon decay products remaining in the air. Another factor to consider is an excessive amount of airborne particles that can keep RDPs suspended in the breathing zone and subsequently elevate health risk. Since measuring ER is not within the scope of activity for measurement specialists, a standard 50% ER is factored in to most measurement devices whether calculated automatically by electronic equipment in the field or at a laboratory.

Equilibrium Ratio and Calculating Measurement Results

Because ER is not measured during a radon test, an ER of 50% is used to convert from WL to pCi/L to determine an approximate radon level. The following formula would be used to obtain the pCi/L from a known WL of 0.02:                                 

pCi/L = (WL x 100) ÷ ER

pCi/L = (0.02 x 100) ÷ ER

4.0 pCi/L = (0.02 x 100) ÷ 0.50

This conversion is automatically calculated in mechanical devices such as continuous radon monitors and during laboratory analysis of passive devices. A conversion factor from WL to pCi/L is also necessary in other radon calculations, such as when determining worker exposure levels. An in depth study on WL conversions can be found in chapters 4 and 5 of this course.

Secular Equilibrium

Secular Equilibrium must be considered in the accurate measurement of indoor radon levels. It is the state in which the formation of atoms by decay of a parent radioactive isotope is equal to its rate of disintegration by radioactive decay.  In other words, when radon gas and RDP levels are equal they are said to be in a state of perfect Secular Equilibrium.  Consequently, one working level is equal to 100 pCi/L of radon-222 gas and 100 pCi/L of each of the progeny.  It takes approximately 3-4 hours for radon gas and its RDPs to achieve Secular Equilibrium.

Secular Equilibrium: Stable state when radon gas and RDP levels are equal. Takes 4 Hours to Achieve.

Dynamic Equilibrium

When the maximum possible RDP concentration produced by a given radon concentration is present, the equilibrium ratio (ER) of radon gas to the RDPs would equal one (1). However, RDPs can’t reach perfect equilibrium with the radon in homes or other buildings due to the natural infiltration of outdoor air.  This ventilation does not allow radon to remain in the house long enough (3 to 4 hours) to reach equilibrium with their progeny.  All of these factors can prevent the decay products from reaching Dynamic Equilibrium, which is the balance of the amount of RDPs produced and lost.  Typically, it takes 12 hours for Dynamic Equilibrium to be achieved after doors and windows have been closed. 

Dynamic Equilibrium: The stable state at which radon gas and RDP levels stabilize once the house has been closed. Takes 12 Hours to Achieve. Justifies the need for Closed-Building Conditions 12-hours prior to measurement.
Figure 2-10
Dynamic Equilibrium in the Home
Source: J. Burkhart