Underslab Duct 1

Buildings and homes where the return duct of a forced air furnace or air conditioning system can be very difficult to mitigate.  With return duct being on the suction side of the blower very high negative pressures can exist within the duct.  When this duct is buried in the soil, very small holes in the ductwork can cause radon laden soil gas to be drawn into the ductwork and distributed into the building.  An active soil depressurization system that attempts to draw radon laden soil gas from beneath a foundation often does not have enough vacuum capacity or pressure field extension to be able to overcome the severe vacuums created within the vicinity of the buried return ducts.  This was the situation for the case below and also in Case 2  for a residential structure.

Case 1: Office Building with Under-slab Return Ducts

An office building in the southwest portion of the country had buried return ducts as a part of its air conditioning system.  Radon measurements by two NEHA certified measurement professionals indicated indoor radon levels between 6 and 10 pCi/L on a short-term basis.  Employees within the building were concerned enough about the health risk exposure that many were allowed to work at home that significantly reduced office efficiencies.  Local mitigation contractors were unwilling to attempt active soil depressurization techniques due to the severe negative pressures being created by the return ducts.  A technician from Colorado Vintage Companies, Inc. was asked to investigate alternative mitigation strategies.  As part of this effort the technician measured radon decay products in addition to radon gas to determine the actual exposures present within the building.  The results are shown in the thumbnail gallery below (click on each thumbnail to enlarge the picture).

Diagram The severe negative pressure within a buried return duct can draw radon in.	Forced Air Unit Cabinet was placed directly on concrete slab, under which the buried return duct entered the blower cabinet.	Under-Slab Duct Radon laden soil gas can be drawn through very small openings in the buried duct and distributed throughout the building.	Results Although the radon was in excess of the EPA guidance of 4.0 pCi/L, the radon decay products were well below the 0.02WL guidance.

Discussion: At the same time that radon was being drawn in by the air handling unit, the air movement caused by the air handling unit decreased the airborne levels of radon decay products through plate-out in ductwork and on fixed objects.  Once attached to objects the radon decay products do not detach and therefore no longer present a breathable exposure.

At times, the air movement caused by forced air systems can cause enough radon decay product reduction to offset the increased radon entry.

Outcome: Employees were presented with data and were very relived that health risk was much lower than perceived by radon gas measurement alone.  Subsequently all employees returned to work.  Facility added fresh air make-up to air conditioning system to further reduce risk, but under non-emergency conditions.

Caution: When doing follow-up measurements using radon decay product levels as a criteria it is important to also measure radon.  This insures that the lower radon decay product levels are a function of lower equilibrium factors and plate-out, rather than an a lower radon gas level during the time of the follow-up measurement.  That is why PGL provides devices through its manufacturing partners that measure BOTH radon and radon decay products!

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Last modified: 06/05/08