Radon-Reporter-2026-Issue-1FINAL

20 | APRIL 2026 National policies regarding radon remediation differ in relation to the location of fan exhaust, with Canada allowing emission at ground level and the US predominantly requiring expulsion above the roof line. Soon after the allowance of side-wall venting in Canada (2010), there was consideration of its applicability in the US. The following is a brief chronological summary of available studies regarding side-wall venting of radon. Using model houses and wind tunnel studies, Neff et al. (1994) reported a maximum concentration of 8 pCi/L along the side of a house exhausting radon of 1000 pCi/L at ground level and substantially less (max. 4 pCi/L) when the exhaust was emitted above the roof line. While the fraction of re-entrained radon was minuscule, Henschel (1995) reported mitigation systems “exhausting at grade level can contribute indoor radon concentrations 3 to 9 times greater than systems exhausting above the eave.” For radon exhaust of 1000 pCi/L, the study concluded “Wind tunnel tests of exhaust dispersion outdoors suggest that grade-level exhaust can contribute mean concentrations beside houses averaging 7 times greater than exhaust at the eave, and 25 to 50 times greater than exhaust midway up the roof slope.” Although they did not study ground-level exhaust, Maeda and Hobbs (1996) measured a ground-level maximum of 3 pCi/L in stable air outside a house with roof-level exhaust containing only 200 pCi/L of radon. This study showed that stable atmospheric conditions will reduce dispersion and allow local radon concentrations to elevate. In a comparison of lateral and vertical exhaust, Bernier and Brossard (2014) noted that, while radon re-entry had been shown to be higher for lateral ground-level discharge, a system could be effective where the basement-level fan was sealed and had proper fittings and sealed piping. Measurements also showed radon levels decrease rapidly to background (natural) concentrations following discharge at ground level. Lebel et al (2021) used plume modeling of vent exhaust, including a variety of scenarios, to clearly demonstrate the continuous exposure from ground-level radon exhaust to adjacent buildings. The study assessed the dilution of radon- laden exhaust gas and estimated the outdoor concentrations that can be expected under a variety of conditions, reaching as high as 7% of the radon level discharged in the exhaust (which can reach be as 1000 pCi/L, as noted by Neff). Later, using multiple parameters to model above-ground-level exhaust dispersion, Lebel et al. (2022) showed that re-entrainment is possible for the emitting house, and less likely for neighboring houses. The study evaluated plume dispersion for the exhaust-facing and non-exhaust-facing surfaces of the buildings under various meteorological, venting, and building geometry parameters. The direction of the prevailing wind affects locations where radon could accumulate. Outdoor radon concentrations can also be affected by wind speed, atmospheric stability, season (plume buoyancy), vent system velocity and building size. A video (KSU, 2022) of the dispersion of smoke exhaust, simulating radon emissions, dramatically demonstrated exposure scenarios. Sidewall venting enveloped the host, while venting above the roof line did not. This visual confirms that relatively little dispersion occurs during atmospherically calm periods. While other types of atmospheric conditions can increase dispersion, there is no evidence to support non-roof venting if it must rely on wind to reduce the exposure levels. Recent measurements conducted in PA (Brodhead (2020) noted a slight increase above background levels with sidewall venting. In a study of ten homes in PA over the course of a year, comparing radon levels every two weeks, alternating between above-roof and grade-level systems, Lewis et al. (2025) found ground-level radon concentrations were higher on the side of the home with fan exhaust, but with little re-entrainment. Interestingly, indoor radon levels above 3 pCi/L were rarer for the ground exhaust location than the roof exhaust location. Radon concentrations were near (or at) non-detectable levels within a few meters from the vent. However, radon levels in the exhaust were low (~230 pCi/L) relative to those that often occur. As shown here, most studies warn against the venting of radon emissions into the breathing zone. The potential for exposure from the vented radon is evident in the studies identified here. Exhaust from a fan rated at 120 ft3/min (3400 L/min) through a three-inch vent has a face velocity over 40 ft/sec (27 mph). It seems intuitive that this exhaust will impact a neighbor within 40 feet (in one second), especially during calm conditions. Exhaust can contain 500 pCi/L, in which case 28,000 pCi/L are vented each second. Even a fraction of possible re-entrainment accentuates the need to install exhaust on the downwind side of a house, if possible, and can be minimized by controlling the air infiltration Sidewall Venting – A Review of the Evidence RESEARCH