View from inside bottom of forced air unit
Radon laden soil can be drawn into a home through very small openings in the ductwork, especially when a forced air unit sits over a large opening in the slab.
Not all homes that have under-slab return ducts will have radon concerns. The only way to know is to test. But, if a long-term test confirms elevated radon levels in a home that also has under-slab air returns, the following options should be considered.
REMOVAL AND REPLACEMENT OF DUCTWORK
This is the last resort and can be very expensive. It may be prudent to explore a few other options before replacing ductwork.
MEASURE RADON DECAY PRODUCTS
Although people usually test for radon and take action based upon the results, it is important in this situation to recognize that the primary health risk comes from radon decay products -- the radioactive particles created when radon itself decays.
These decay products are the elements that lodge in the lungs and cause harm. However, these radon decay products also have electrostatic charges that can cause many of them to attach to walls, carpeting, furniture, etc. Once attached, the decay products remain there and are no longer in the breathable air; hence, they no longer present a health risk.
Air circulation increases the amount of attachment to walls, which can reduce the risk from radon, even though the level of indoor radon is still elevated.
While under-slab ducts can increase radon entry when the air conditioner fan is on, the reduction in airborne radon decay products may, in effect, offset some radon that enter through unsealed ductwork.
One approach is to measure the radon decay products to see if the actual risk warrants mitigation. The graph to the left depicts results of simultaneous radon and radon decay product measurements in a building in Tucson where under-slab ducts were primarily responsible for the radon entry. One can see from the graph that although the radon was elevated, the movement of air in the building was responsible for reducing the radon decay products to well below the action level. In this case, no mitigation was conducted.
ADDING FRESH AIR TO SYSTEM
It is also possible, if the air conditioning system has sufficient cooling capacity, to allow some outdoor air to enter the return duct. Adding outdoor air can reduce radon by:
- Slightly pressurizing the interior of the building thereby reducing soil gas entry
- reducing vacuum in the return duct that is causing radon entry, and
- diluting radon levels in the building.
Note: modifications to air conditioning systems should be done by a qualified contractor to insure the air conditioner is not overloaded.
ATTACH HIGH EFFICIENCY PARTICULATE AIR FILTER TO FORCED AIR UNIT
You could consider attaching a HEPA filter to your forced air unit to filter out radon decay products as well as other indoor air pollutants. These can either be stand-alone units or more efficient units that take a portion of the air from the return ducts, filter it and re-inject the filtered air back into the system. When considering this approach, a homeowner should seek the advice of a qualified mechanical contractor who is familiar with the installation of HEPA filters.
INSTALL ASD (ACTIVE SOIL DEPRESSURIZATION)
It is possible that the large vacuums exerted on the soil by the air conditioner will not allow an ASD system (vent pipe with radon fan) to fully reduce radon levels. However, if this is combined with a little fresh air make-up and sealing the plenum under the air handler, the combination of these may be sufficient, especially if radon decay products are measured.
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