# Does a Passive Radon System Work? When to Add a Fan

- **URL:** https://airsenseenvironmental.com/does-a-passive-radon-system-work/
- **Published:** 2026-11-09

A passive radon system can lower radon somewhat, but it relies on natural airflow, so the only way to know whether it is enough is to test, and if the result is 4.0 pCi/L or higher, a licensed contractor can activate it with a fan. If you own a newer home in the Metro East, there is a good chance you have one. Illinois has required passive radon-resistant construction in new homes since June 1, 2013, so houses built since then in Madison County and St. Clair County, from Edwardsville to O'Fallon, IL, usually have a labeled PVC pipe running from the basement slab up through the roof. Buyers of new construction across St. Louis, MO, St. Louis County and St. Charles County find the same pipe when a builder has included radon-resistant features. The pipe raises fair questions. Is it doing anything? Does it need a fan? Who is allowed to add one? This guide covers what a passive system is, what the EPA says about how well it works, why Illinois homes have one, how to tell whether yours is enough, and what activation involves. What is a passive radon system? A passive radon system is a vent pipe that runs from beneath the foundation, through the warm interior of the house, and out above the roof, with no fan. It relies on the natural upward flow of air in the pipe to draw soil gas from under the slab and release it outdoors. Illinois law defines it precisely. Under 420 ILCS 52/15, a "passive new construction pipe" is "a pipe installed in new construction that relies solely on the convective flow of air upward for soil gas depressurization," routed "through conditioned space from below the foundation to above the roof." The EPA's radon-resistant construction basics page lists the five features builders should include: Gravel. A 4-inch layer of clean, coarse gravel under the slab so soil gas can move freely. Plastic sheeting. Heavy-duty 6-mil polyethylene or a vapor retarder over the gravel. A vent pipe. A 3-inch or 4-inch Schedule 40 PVC pipe from the gravel layer through the conditioned space and roof, labeled "Radon System." Sealing and caulking. All openings, cracks and crevices in the slab and walls sealed. A junction box. An electrical outlet in the attic "for use with a vent fan, should, after testing for radon, a more robust system be needed." That last item tells you how the EPA sees a passive system: a head start, built so that a fan can be added later if a test calls for one. Does a passive radon system work? A passive radon system does work, but less reliably than an active one. The EPA's A Consumer's Guide to Radon Reduction rates passive subslab suction at a typical 30 to 70 percent radon reduction, compared with 50 to 99 percent for active subslab suction with a fan (p. 18). The EPA explains the gap in the same guide: passive subslab suction "relies on natural pressure differentials and air currents instead of a fan," and "is generally not as effective in reducing high radon levels as active subslab suction" (p. 12). The guide's comparison table also notes that passive suction "may be more effective in cold climates," because the warm pipe draws harder when it is cold outside. What that means for a St. Louis-area home: A lower starting level is more likely to land below the action level. A 30 to 70 percent reduction takes a moderate reading down further than a high one. Performance varies with the weather. A passive pipe draws harder in a January cold snap than on a mild spring day. Nobody can tell from the pipe alone. The reduction depends on the soil, the slab and how well the house was sealed, so the result has to be measured, and radon mitigation for St. Louis homes starts from that number. Why do new homes in Illinois have a passive radon pipe? New homes in Illinois have a passive radon pipe because state law requires one. 420 ILCS 52/20 states: "All new residential construction in this State shall include passive radon resistant construction." The requirement came from Public Act 97-953 and took effect June 1, 2013. The act covers single-family homes and buildings with 2 or fewer units, which 52/15 defines as "new residential construction." For the Metro East, that means most houses built since mid-2013 in Edwardsville, Glen Carbon, O'Fallon, Shiloh, Collinsville and the rest of Madison and St. Clair counties should have the pipe. A builder or a radon contractor may install it during construction. Section 20 requires the pipe, not a fan. A passive pipe gives an Illinois home a lower starting point, but the house still has to be tested to know where it stands. What about the Missouri side? The Missouri Department of Health and Senior Services' radon guidance does not list a radon-resistant construction requirement. It focuses on testing, with free test kits for Missouri residents, and on choosing a certified contractor. New homes in St. Louis County, St. Charles County or Jefferson County may or may not have a pipe, depending on the builder. The EPA notes that a new-home buyer "may ask the builder about these features, and if not provided, may ask the builder to include them." How do you know if your passive radon system is enough? Testing is the only way to know if a passive radon system is enough. The EPA's Consumer's Guide states that "even new homes built with radon-resistant features should be tested after occupancy to ensure that radon levels are below 4 pCi/L" (p. 16). The EPA recommends fixing a home when radon is 4.0 pCi/L or higher and considering a fix between 2.0 and 4.0 pCi/L. For a home with a passive pipe, the test results map to three outcomes: Test resultWhat it meansNext stepBelow 2.0 pCi/LThe passive pipe is doing its jobRetest periodically and after any remodel2.0 to 3.9 pCi/LBelow the action level, but in EPA's "consider fixing" rangeConsider activation; a long-term test gives a year-round average4.0 pCi/L or higherAt or above the EPA action levelHave a licensed contractor activate the system with a fan Test the lowest lived-in level, with windows and outside doors closed. Heating season, when the house stays closed, is a convenient time to do it. How do you activate a passive radon system? A passive radon system is activated by adding an inline radon fan to the existing pipe, and in Illinois only a licensed radon contractor may do it. 420 ILCS 52/25 states: "Only a radon contractor may install a radon vent fan or upgrade a passive new construction pipe to an active mitigation system." At a high level, activation follows the same sequence the EPA describes for any active system: Confirm the result. A test at or above 4.0 pCi/L, or a confirmed reading in the 2.0 to 4.0 range, is the reason to act. Assess the existing pipe. The contractor checks the pipe, the sealing and the attic or exterior run. Install the fan outside the living space. The EPA lists "unconditioned home and garage spaces, including attics and the exterior of the home" as common fan locations (p. 12), which is why radon-resistant construction puts the junction box in the attic. Add a warning device. The EPA says a warning device "must be installed to alert you if an active system stops working properly," such as a liquid gauge, usually a manometer (p. 14). Retest. The EPA advises a post-mitigation test "within 30 days of system installation, but no sooner than 24 hours after your system is in operation," and recommends an independent follow-up measurement as well (p. 15). Illinois homeowners can check any contractor against the licensee lists on the IEMA radon program page. Air Sense Environmental works from its Edwardsville headquarters, and owner Jake Lusby holds IEMA Mitigation Professional license #RNM20232346, as noted on the Metro East radon mitigation page. On the Missouri side, the Illinois licensing rule does not apply. Missouri DHSS mitigation guidance tells homeowners to look for contractors certified by the National Radon Proficiency Program and to ask for proof of NRPP or NRSB certification. Passive vs. active radon system: what changes when you add a fan? Adding a fan changes a passive radon system from weather-dependent suction to constant suction under the slab. Everything else in the pipe stays the same: the suction point, the sealed slab and the roof discharge. Passive (no fan)Active (fan added)What moves the soil gasNatural upward airflow in a warm pipeAn inline fan creating constant suctionTypical radon reduction (EPA)30 to 70 percent50 to 99 percentElectricityNoneThe fan runs continuouslyWarning deviceNot requiredRequired by EPA guidance, usually a manometerWho can install it in IllinoisBuilder, subcontractor or radon contractor during constructionLicensed radon contractor onlyTestingTest after occupancyTest 24 hours to 30 days after activation, then periodically The full comparison of system designs, including crawl space and block-wall systems, is in our overview of radon mitigation system types. For a home that already has the pipe, activation can be a smaller job than a new installation, because the suction point and the roof penetration are already in place. The cost depends on the house, and a written estimate after an on-site assessment is the reliable number. Frequently asked questions Does a passive radon system need a fan? Not always. A passive radon system needs a fan only if a test shows radon at or above the EPA action level of 4.0 pCi/L, or in the 2.0 to 4.0 pCi/L range if you choose to act there. Can I add a radon fan myself? Not in Illinois. Under 420 ILCS 52/25, only a licensed radon contractor may install a radon vent fan or upgrade a passive pipe to an active system. In Missouri, DHSS guidance points homeowners to NRPP- or NRSB-certified contractors. How can I tell if my house has a passive radon pipe? Look for a 3-inch or 4-inch PVC pipe rising from the basement slab and running up through the house with no fan attached. The EPA says the pipe should be labeled "Radon System," and an attic outlet is often mounted nearby for a future fan. Does a passive radon system use electricity? No. A passive radon system has no fan, so it uses no electricity. An activated system adds a fan that runs around the clock. Should I retest after activating a passive system? Yes. The EPA recommends a post-mitigation test within 30 days of activation, no sooner than 24 hours after the fan starts, and suggests an independent follow-up measurement as well. Does Missouri require radon-resistant construction? Missouri's state health department guidance does not list a radon-resistant construction requirement, unlike the Illinois act in force since 2013. Ask your builder whether radon-resistant features were installed in a new Missouri home, then test after you move in. Find out whether your passive pipe is enough If your home has a passive radon pipe and no recent test, start with the number. Air Sense Environmental tests homes across the Metro East and the Missouri side, and designs radon mitigation across the St. Louis metro when a result calls for it. Call 314-664-9807 or request a free estimate.