# What Is Radon Gas

- **URL:** https://airsenseenvironmental.com/what-is-radon-gas/
- **Published:** 2026-09-07

Skip to content Radon Mitigation Services ▾ Radon Testing Commercial Radon Mitigation Commercial Radon Testing Indoor Air Quality Crawl Space Encapsulation Service Area Resources FAQ Blog Podcast Reviews & Projects About Us Contact 314-664-9807 Free Estimate Radon Mitigation Services Radon Testing Commercial Radon Mitigation Commercial Radon Testing Indoor Air Quality Crawl Space Encapsulation Company Service Area Resources FAQ Blog Podcast Reviews & Projects About Us Contact Call 314-664-9807 St. Louis Metro · Missouri & Illinois What Is Radon Gas? Radon Science Radon Testing NRPP-Certified Radon Pros MO & IL Radon gas is a colorless, odorless, radioactive noble gas produced by the decay of uranium in soil and rock, with a half-life of 3.8 days and a density roughly nine times that of air. Radon is element 86 on the periodic table. It is chemically inert, which is why it passes through filters and building materials without reacting, and it is radioactive, which is why it matters at all. Because radon is continuously produced underground, it cannot be eliminated at the source. The EPA estimates about 1 in 15 US homes test at or above the radon action level of 4.0 pCi/L. Call 314-664-9807 Get a Free In-Home Estimate IEMA Radon License #RNM20232346 Free Written Estimates Post-Mitigation Radon Testing When Radon Becomes a Problem EPA action level4.0 pCi/L Our design target< 2.0 pCi/L Systems are engineered to bring your home below the EPA action level of 4.0 pCi/L. Element 86 Properties What are the properties of radon gas? Radon is a noble gas, meaning it does not readily form chemical compounds, and it is the only radioactive element in that group that occurs in ordinary soil everywhere. Symbol and number: Rn, element 86 State at room temperature: gas, colorless and odorless Density: roughly nine times that of air, which is why it settles into basements Chemically inert: it does not react with filters, sealants, or building materials Radioactive: it decays by alpha emission into a series of solid particles Being inert is precisely what makes radon hard to remove. There is no chemical that neutralises it, which is why mitigation works by moving the gas rather than treating it. Half-Life What is the half-life of radon? The half-life of radon-222, the isotope found in homes, is 3.8 days. That means half of any quantity of radon decays into other elements within about ninety-two hours. A short half-life sounds reassuring and is not, for one reason: radon is replenished continuously. Uranium-238, the ultimate parent, has a half-life of about 4.5 billion years, so the supply in soil is effectively permanent on any human timescale. What decays away is immediately replaced by more radon migrating up from below. The short half-life does explain why outdoor radon is harmless. Once released into open air, it disperses long before concentration can build. Decay Can radon be a solid? Radon is a gas at any temperature found in a home, but it becomes a solid below roughly minus 71 degrees Celsius. The more relevant answer is that radon decays into solids, and those solids are what actually cause harm. When radon decays it produces a series of short-lived radioactive particles known as radon progeny or decay products. Unlike radon itself, these are solid metals, chiefly isotopes of polonium, lead, and bismuth. They attach to dust and airborne particles, and when inhaled they lodge in lung tissue rather than being exhaled. This distinction explains a common question. Air purifiers cannot capture radon gas, because it is a gas and chemically inert, but they can capture some decay products attached to dust. That is a partial benefit and does nothing about the gas still entering the building. Origin Where does radon gas come from? Radon gas comes from the decay of uranium in soil and rock. Uranium decays to radium, radium decays to radon, and radon migrates upward through pore spaces in the ground until it reaches the surface or a building foundation. Formations that generate more radon include granite, shale, and phosphate-bearing rock. Permeable soil and fractured bedrock let radon travel farther before decaying, which is why geology and soil type matter as much as the rock's uranium content. More detail is in our page on where radon is found. Measurement How is radon gas measured? Radon is measured in picocuries per liter of air, written pCi/L, which counts radioactive decays rather than mass. One picocurie equals 0.037 decays per second in a liter of air. Reference points that make the unit concrete: 0.4 pCi/L: typical outdoor air 1.3 pCi/L: the average indoor level in US homes 4.0 pCi/L: the EPA action level, above which the EPA recommends fixing the home Elsewhere in the world radon is reported in becquerels per cubic metre, where 1 pCi/L is about 37 Bq/m3. Local Conditions Radon gas in St. Louis area soil The geology under the St. Louis metro produces meaningful radon on both sides of the river. Much of St. Louis County and St. Charles County sits on limestone and shale with enough uranium content to generate elevated soil gas, and karst features common to the region give that gas pathways to travel. On the Illinois side, Madison and St. Clair counties carry their own elevated zones, and Illinois documents elevated readings in every part of the state. None of that predicts a specific address, which is why radon testing remains the only way to know. See radon mitigation for what happens when a reading comes back high. Common Questions Frequently Asked Questions About Radon Gas Is radon gas heavier than air? Yes. Radon gas is roughly nine times denser than air, which is why it settles into basements, crawl spaces, and the lowest levels of a building. Density is not the whole story though, because the stack effect and HVAC circulation carry radon upward through a house regardless, so levels on upper floors are lower but not zero. What does radon gas smell like? Radon gas has no smell. It is colorless, odorless, and tasteless, and it produces no immediate physical symptoms, so there is no sensory way to detect it at any concentration. Testing is the only method of detection, which is what makes it different from most household hazards. Is radon gas flammable? No. Radon is a noble gas and chemically inert, so it does not burn and does not react with other substances under normal conditions. Its hazard is entirely radiological rather than chemical, coming from alpha particles released as it and its decay products break down. How long does radon gas stay in a house? Individual radon atoms have a half-life of 3.8 days, but radon concentration in a house does not decline on its own, because new gas enters continuously from the soil below. Levels respond to ventilation and pressure rather than to time, which is why opening windows lowers readings temporarily and closing them lets levels return within hours. Is radon the same as radiation in basements? Radon is the main source of natural radiation found in basements, but they are not identical terms. Radiation in a basement can also come from building materials and from cosmic sources, though in almost all homes radon and its decay products account for the great majority of the dose. This is why basement radiation concerns are usually addressed by testing for radon specifically. References: EPA radon action-level guidance and A Citizen's Guide to Radon. From Science to a Number Find out what your home reads Understanding radon does not tell you your exposure. A short-term test in the lowest lived-in level does. Get a Free In-Home Estimate 5237 Old Alton Edwardsville Rd Edwardsville, IL 62025 314-664-9807 · Open 24 hours IEMA License #RNM20232346 Services Radon Mitigation Radon Testing Commercial Radon Radon Fan Replacement Sub-Slab Depressurization Crawl Space Encapsulation Company About Us Service Area Reviews & Projects FAQ Resources Contact © 2026 Air Sense Environmental. All rights reserved. Radon Mitigation & Testing · St. Louis, MO & Metro East, IL