St. Louis Metro · Missouri & Illinois
Where Is Radon Found?
Radon is found in soil and rock almost everywhere on earth, and it is found indoors wherever that soil gas can seep into a building, which is why the highest concentrations in a house are in the basement or the lowest level touching the ground.
Radon forms from the natural decay of uranium in rock and soil, so it is present in all 50 states and in every kind of building. What changes from place to place is concentration, not presence.
The EPA estimates about 1 in 15 US homes, roughly 6.6 percent, test at or above the radon action level of 4.0 pCi/L. Elevated readings have been documented in every state, including throughout Illinois and across the St. Louis metro in Missouri.
When a Reading Matters
Systems are engineered to bring your home below the EPA action level of 4.0 pCi/L.
Found everywhere, measured locallyOrigin
Where does radon come from?
Radon comes from the radioactive decay of uranium in soil and rock. Uranium is present in nearly all soil and bedrock, it decays into radium, and radium decays into radon gas, which then migrates upward through pore spaces in the ground.
That chain is why radon cannot be eliminated at the source. It is produced continuously underground and it is replenished as fast as it escapes.
- Granite, shale, and phosphate-bearing rock generate more radon than most other formations
- Permeable soil and fractured bedrock let radon travel farther before it decays
- Limestone karst, common across the Missouri side of the St. Louis metro, provides pathways for soil gas to move
Indoors
Where is radon found in a house?
Radon is found in the lowest level of a house first, because that is the part in direct contact with soil, and concentrations then fall by roughly 50 percent for each floor above it. A basement almost always reads higher than the main floor of the same house on the same day.
The specific entry points are consistent:
- Cracks in concrete slabs and foundation walls
- The floor-to-wall joint where the slab meets the foundation
- Sump pits and unsealed sump covers
- Gaps around plumbing, electrical, and utility penetrations
- Exposed soil in a crawl space
- Construction joints and unsealed expansion gaps
Houses without basements are not exempt. Slab-on-grade and crawl-space homes regularly test above the EPA action level of 4.0 pCi/L, because the requirement is soil contact and a pressure difference, and every house has both. There is more detail in our guide to radon in basements.
Geography
Is radon found in every state?
Yes. Elevated radon has been found in all 50 states, and the EPA’s Map of Radon Zones identifies areas with the greatest potential for high indoor levels rather than areas where radon is absent.
State averages vary widely. Alaska, South Dakota, and Pennsylvania report the highest average indoor readings in the country, all well above the EPA action level of 4.0 pCi/L, while Hawaii, Louisiana, and Mississippi report the lowest.
The important caveat is that a state or county average tells you very little about one address. Two houses on the same street can differ by a factor of five, because foundation type, soil under that specific lot, and how tightly the house is sealed all matter more than the regional figure.
Local
Where is radon found in Missouri and Illinois?
Radon is found throughout both Missouri and Illinois, and the Illinois Emergency Management Agency reports that homes with elevated radon have been documented in every part of the state.
Within the St. Louis metro the picture is uneven:
- St. Louis County and St. Charles County sit largely on limestone and shale with enough uranium content to generate meaningful soil gas
- Jefferson and Franklin counties include areas of mineralized bedrock associated with historic lead mining
- Madison and St. Clair counties on the Illinois side carry their own elevated zones, and Illinois publishes measurements by county and zip code
Illinois requires radon measurement and mitigation professionals to be licensed through IEMA. Air Sense Environmental holds IEMA license #RNM20232346 and works both sides of the river. See our service area for the communities covered.
Water
Is radon found in water?
Radon is found in water, but for most homes it is a minor contributor compared with soil gas. Radon dissolves into groundwater as it passes through rock, and it is released into the air when that water is agitated by showering, dishwashing, or laundry.
The distinction that matters is the water source:
- Private wells drawing from bedrock can carry meaningful radon, and are worth testing separately if a home is on well water
- Municipal supplies drawn from surface water, which is what most of the St. Louis metro uses, lose nearly all radon to the air before delivery
For the overwhelming majority of homes, the soil beneath the foundation is the source that determines the indoor reading.
Outdoors
Where is radon found outdoors?
Radon is found in outdoor air everywhere, at very low concentrations that pose little risk. Typical outdoor levels sit around 0.4 pCi/L, roughly a tenth of the EPA action level of 4.0 pCi/L.
The reason outdoor radon is harmless and indoor radon is not comes down to dilution. Outside, radon disperses into an effectively unlimited volume of air the moment it leaves the ground. Inside, the same gas enters an enclosed volume, and a heated house actively draws it in through the foundation.
That is also why venting a mitigation system above the roofline works. The gas is not destroyed, it is returned to open air where it dilutes immediately.
Risk Factors
Which homes are most likely to have radon?
Homes most likely to have elevated radon are those with the most soil contact and the least air exchange. No single factor is diagnostic, but they stack.
- Basements and crawl spaces in direct contact with soil
- Uranium-bearing or highly permeable soil beneath the foundation
- Tightly sealed, energy-efficient construction that limits natural ventilation
- Single-story homes with a large footprint, which have more slab in contact with the ground per unit of interior volume
- Older homes with settled foundations where joints and cracks have opened over decades
Homes with every risk factor sometimes test low, and homes with none sometimes test high. Which is why radon testing is the only way to answer the question for a specific address.
Common Questions
Frequently Asked Questions About Where Radon Is Found
What are the signs of radon in a home?
There are no signs of radon in a home. Radon is colorless, odorless, and tasteless, and it produces no short-term symptoms, so there is nothing to see or smell. Musty odors and foundation cracks indicate soil-gas pathways rather than radon levels themselves. Testing is the only way to detect it.
Can radon be found in a house without a basement?
Yes. Radon is regularly found above the EPA action level of 4.0 pCi/L in slab-on-grade and crawl-space homes. A basement raises the odds of a high reading but is not required for one. What radon needs is soil contact and a pressure difference drawing gas inward, and every house has both.
Is radon found on upper floors?
Yes, radon is found on upper floors, at lower concentrations. Levels typically drop by roughly 50 percent per floor above the lowest lived-in level, so a basement at 8 pCi/L often corresponds to about 4 pCi/L on the main floor. Radon moves upward through stairwells and HVAC ductwork, so an unused basement does not isolate the rest of the house.
Where is radon found in the highest concentrations?
Radon is found in the highest indoor concentrations in the lowest level of a building that contacts soil, which in most homes is the basement or crawl space. Within that space, readings are usually highest near the entry points, meaning slab cracks, the floor-wall joint, and open sump pits. Radon is roughly nine times denser than air, so it settles into the lowest available space.
Does every home have radon?
Every home has some measurable radon, because radon is produced by ordinary uranium decay in ordinary soil. What varies is concentration. The EPA estimates about 1 in 15 US homes, roughly 6.6 percent, exceed the action level of 4.0 pCi/L, and only those homes need a mitigation system.
References: EPA radon action-level guidance and A Citizen’s Guide to Radon.
Answer It For Your Address
Find out where radon is in your home
State maps and county averages cannot tell you what one house reads. A short-term test in the lowest lived-in level can.