Radon and Your Air Ducts: The Connection DMV Homeowners Need to Understand

Radon is a naturally occurring radioactive gas that seeps up from the ground into homes through foundation cracks, gaps around pipes, and other openings. The EPA estimates that radon causes approximately 21,000 lung cancer deaths annually in the United States, making it the second leading cause of lung cancer after smoking. The DMV area has varying radon risk levels. Much of Northern Virginia, particularly Fairfax County, Loudoun County, and western portions of Prince William County, falls in EPA Zone 1 (highest risk) or Zone 2 (moderate risk). Montgomery County and Frederick County in Maryland also have significant radon occurrence. Washington DC generally has lower radon levels due to its low-lying river valley geography, but individual buildings can still have elevated readings. The only way to know your home's radon level is to test. The EPA recommends that all homes be tested regardless of geographic risk level, since radon concentrations can vary dramatically between adjacent homes based on foundation type, soil composition, and HVAC system design.

Your HVAC ductwork can either help or hurt your radon situation depending on its condition and configuration. Return duct leaks in basements and crawl spaces are the biggest concern—when return ducts leak in below-grade spaces, they actively pull radon-laden air from the soil and distribute it throughout your home. A single significant leak in a basement return duct can dramatically increase radon exposure on every floor. Supply duct leaks in the basement have the opposite effect but still contribute to the problem. When conditioned air leaks out of supply ducts into the basement, it creates positive pressure that can actually reduce radon infiltration in that space—but you're losing conditioned air and energy in the process. The pressure dynamics of your HVAC system matter significantly. When the system runs, it creates slight negative pressure in areas with return ducts (pulling air) and positive pressure in areas with supply ducts (pushing air). If your home has more return capacity than supply capacity in the basement—common in older installations—the HVAC system actively draws radon-laden soil gas into the living space.

Active soil depressurization (ASD) is the most common radon mitigation method—a pipe and fan system that draws radon from beneath the foundation and vents it above the roofline. When properly installed, ASD systems reduce radon levels by 80-99%. However, the interaction between your ASD system and your HVAC system must be considered. An ASD system creates slight negative pressure beneath your foundation. If your HVAC system also creates negative pressure in the basement (through return duct operation), these pressures can either complement or conflict with each other. A radon mitigation professional should evaluate your HVAC system's pressure dynamics when designing the mitigation system. After radon mitigation installation, keep your HVAC system's ductwork in good condition. New duct leaks in below-grade spaces can compromise the effectiveness of your radon mitigation system by creating alternative pathways for soil gas to enter the living space.

Start with a radon test. Short-term test kits (2-7 days) provide a quick screening, while long-term tests (90+ days) give a more accurate annual average. Test in the lowest livable level of your home with windows closed. If results are above 4 pCi/L (the EPA action level), retest to confirm, then pursue mitigation. If your radon test comes back elevated, schedule a duct inspection focusing on below-grade ductwork. Our technicians can identify and document leaks in basement and crawl space ductwork that may be contributing to radon infiltration. Sealing these leaks is a relatively inexpensive step that can reduce radon levels and should be done whether or not you install a dedicated mitigation system. For comprehensive protection, combine radon mitigation with duct sealing and professional duct cleaning. This three-pronged approach addresses radon infiltration through both the foundation and the ductwork, while ensuring the air being distributed through your home is free of accumulated contaminants.