Radon Science • Iowa
Why Does Iowa Have the Highest Radon Levels in the Country?
Iowa's average indoor radon level is 8.5 pCi/L, more than six times the U.S. average. Every one of the state's 99 counties is in the EPA's highest-risk zone. The answer is under your feet: ice-age geology, clay-rich soil, and the way Iowa homes are built and heated.
Test Your Home's Radon LevelIowa Radon by the Numbers
The Iowa Department of Health and Human Services puts the state's total average indoor radon level at 8.5 picocuries per liter (pCi/L). The U.S. average is 1.3 pCi/L. The EPA recommends fixing any home at 4 pCi/L or above, so the typical Iowa home sits more than twice the action level before anyone has tested it.
Based on home test data, Iowa HHS estimates that as many as 5 in 7 Iowa homes have elevated radon. The older State/EPA Residential Radon Survey, cited by the U.S. Geological Survey, found about 71% of Iowa homes elevated, the highest share of any state and ahead of North Dakota at 63% and Minnesota at 46%.
The EPA has placed all 99 Iowa counties in Zone 1, its highest-potential category, meaning the predicted average indoor screening level is above 4 pCi/L. Most states have a mix of zones. Iowa has none.
Health-wise, the EPA estimates radon causes about 21,000 lung cancer deaths a year in the United States and roughly 400 a year in Iowa. It is the leading cause of lung cancer in nonsmokers.
Where Iowa Stands
- 8.5 pCi/LIowa average indoor radon level (Iowa HHS).
- 1.3 pCi/LU.S. average indoor radon level (Iowa HHS).
- 4.0 pCi/LEPA action level. Remediation is recommended at or above this reading.
- 99 of 99Iowa counties in EPA Zone 1, the highest radon-potential classification.
- 94 of 99Iowa counties whose average test result is at or above 4.0 pCi/L (CDC Tracking Network, 2008–2017).
- 3.2 – 13.6Range of county averages, from Decatur County to Sioux County. Polk County averages 7.3 pCi/L.
The Geology: Glaciers Did This
Radon is a decay product of uranium. Iowa's soil has more of it, in a form that moves more easily.
1. Uranium in the ground
Radon gas occurs naturally in soil. It is produced by the radioactive decay of uranium and radium, which are present in small amounts in most rock and soil. Iowa HHS notes that long ago, glacial activity left behind ground-up deposits of many minerals, including uranium, in Iowa's soil and upper crust.
2. Ice sheets ground it fine
Over repeated ice ages, glaciers scraped bedrock from the north, crushed it, and dropped the debris across Iowa as a thick blanket called till. The U.S. Geological Survey reports that this crushing and grinding increases the mobility of uranium and radium in the resulting tills, letting them move down through the soil and keep generating radon close to the surface.
3. Clay holds the gas
Not all till is equal. The USGS found that clay-rich tills in Iowa and the Dakotas generate significant numbers of elevated indoor readings, while the sandy tills of Michigan typically produce low to moderate levels. Dense clay soil holds radon in place under a foundation instead of letting it vent to open air, so the gas is waiting when a house creates a path indoors.
How Radon Gets Into a House
Radon is a gas, so it travels wherever air travels. The primary source of high indoor radon is the soil directly below and around the home. As Iowa HHS describes it, warm air inside a house rises, which creates a small vacuum in the lower levels. Soil gas moves from the higher pressure in the ground toward the lower pressure inside the house, and radon rides along.
It enters the same way any soil gas does: through cracks in the foundation, floor, or walls; through hollow-block walls; around floor drains, pipes, and sump pump openings; and up through crawl spaces. No foundation is airtight, and the pressure difference is constant, so the flow never fully stops on its own.
That is also why the fix works. Radon mitigation in Des Moines and across central Iowa uses a sealed pipe and a continuously running fan to reverse that pressure difference under the slab, pulling soil gas out before it can enter and venting it above the roofline.
Common Entry Points
- Cracks in slab floors and foundation walls
- The joint where the floor meets the wall
- Hollow concrete-block walls
- Openings around floor drains and plumbing
- Sump pits and sump pump penetrations
- Exposed soil or gravel in crawl spaces
Source: Iowa Department of Health and Human Services, Iowa Radon Home Buyers and Sellers Fact Sheet.
Why Iowa Homes Concentrate It
Soil explains where the radon comes from. The way Iowa houses are built and lived in explains why so much of it ends up indoors. Three things about the typical Iowa home work against it.
Basements are the norm. A basement puts hundreds of square feet of concrete floor and wall in direct contact with the soil, well below grade, where soil-gas pressure is highest. More contact area means more cracks, joints, and penetrations for gas to follow.
The heating season is long. From fall through spring, Iowa houses stay closed up with the furnace running. Warm air rising through the house pulls replacement air in from the lowest point it can find, and under a basement that point is the soil. This stack effect is the vacuum Iowa HHS describes, and it runs hardest in exactly the months when windows are shut.
Tight construction keeps it in. Energy-efficient homes exchange less air with the outdoors. That is good for heating bills, but it means radon that enters is diluted more slowly and builds to a higher steady level.
Soil Is Not the Whole Story
Iowa's state radon program has made the point that most soil in Iowa will test high for radon, but that by itself does not tell you how much will get into a given house. What matters is how easily gas can pass from the soil into the structure.
That is why Iowa HHS reports that radon levels vary considerably from house to house, even on the same street, and that a high and a low reading can be found in homes directly next to each other. It is found in new and old homes, and in homes with and without basements. A neighbor's result, a county average, or the age of the house tells you the odds, not your number.
The only way to know is a certified radon test in your own home.
Iowa Radon — Common Questions
Why is radon so high in Iowa compared to other states?
Iowa’s soil is largely glacial till — rock that was crushed and ground by ice sheets and left behind as fine, clay-rich deposits. The Iowa Department of Health and Human Services notes that glacial activity left ground-up deposits of minerals such as uranium across the state, and the U.S. Geological Survey has found that this grinding makes uranium and radium more mobile in the resulting soil. Uranium decays into radium and then into radon gas, which rises through the soil and into homes. The result is a statewide average indoor level of 8.5 pCi/L, more than six times the U.S. average of 1.3 pCi/L.
Is every county in Iowa a high-radon area?
Yes. The EPA has placed all 99 Iowa counties in Zone 1, its highest-potential category, meaning the predicted average indoor screening level is above 4 pCi/L. County averages from the CDC Environmental Public Health Tracking Network (2008–2017) range from 3.2 pCi/L in Decatur County to 13.6 pCi/L in Sioux County, and 94 of 99 counties average at or above the 4.0 pCi/L action level. Polk County averages 7.3 pCi/L.
Does a new home or a home without a basement have less radon?
Not reliably. Iowa HHS is clear that radon is found in new and old homes, and in homes with and without basements. The source is the soil under and around the house, and any foundation in contact with that soil can draw radon in. A new home built with a passive radon system has a head start on fixing the problem, but it still needs to be tested.
My neighbor tested low. Does that mean my house is fine?
No. Iowa HHS reports that radon levels vary considerably from house to house, even on the same street, and that a high and a low reading can be found in homes directly next to each other. Foundation type, cracks and openings, soil permeability under the slab, and how the house is heated and ventilated all change how much gas gets inside. The only way to know your number is to test your home.
How serious is the health risk?
Radon is the leading cause of lung cancer among nonsmokers and the second leading cause overall. The EPA estimates radon is responsible for about 21,000 lung cancer deaths per year in the United States, and roughly 400 per year in Iowa. The risk builds with long-term exposure, which is why testing and, if needed, mitigation are recommended for every Iowa home.
Sources
- Iowa Department of Health and Human Services, Iowa Radon Home Buyers and Sellers Fact Sheet (statewide and U.S. averages, Zone 1 status, 5-in-7 estimate, entry mechanism, health estimates, house-to-house variation).
- U.S. Geological Survey, Geologic Radon Potential of the Midwest (glacial till, uranium and radium mobility, clay-rich versus sandy tills, State/EPA Residential Radon Survey percentages).
- CDC Environmental Public Health Tracking Network, mean pre-mitigation radon test results by Iowa county, 2008–2017. Full table on our Iowa radon levels by county page.
The geology is fixed. Your home's radon level isn't.
A certified 48 to 72 hour test tells you where your house stands. If it's high, a mitigation system brings it down and keeps it there.