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What houses are most at risk for radon?

By Radon411 Editorial TeamUpdated How we research

Short answer

Any house can have high radon: EPA says new and old homes, well-sealed and drafty homes, and homes with or without basements can all have a problem. Risk is greater in areas EPA rates Zone 1, in homes built on uranium-rich soil, and on the lowest floors, but only a test shows a given home's level.

Next step: a test is the only way to know your home's level. How to test your home

EPA's Citizen's Guide is direct: any home may have a radon problem. Answering the myth that radon only affects certain kinds of homes, it says house construction can affect radon levels, but radon can be a problem in homes of all types, and that local geology, construction materials, and how the home was built are among the factors that can affect radon levels in homes.

What homes usually have radon? All of them have some: CDC says all outdoor and indoor air has some radon in it. What varies is the level. The Citizen's Guide says nearly 1 out of every 15 homes in the U.S. is estimated to have elevated radon levels, and the National Cancer Institute says basement and first floors typically have the highest radon levels because of their closeness to the ground.

What to do next: test your own home rather than judging it by its type or its neighbors, starting with the lowest lived-in level, and look up your county's zone on the radon map.

Foundation type

Minnesota's Department of Health says any home can have a radon problem, no matter the type of foundation, and describes how each type lets radon in.

How each foundation type lets radon in, as Minnesota's Department of Health describes it
FoundationWhat Minnesota says
BasementProvides a large surface area in contact with the soil, and radon can enter through different pathways; taller homes add potential for a greater stack effect
Slab-on-gradeHas many openings that allow radon to enter, similar to a basement
Crawl spaceDirectly connected to the soil, creating a pathway for radon to enter the home
Manufactured home with solid skirtingActs like a crawl space and provides a direct connection to the soil

No basement does not mean no radon. See can a house have radon without a basement.

Location: where radon potential is highest

EPA's Map of Radon Zones identifies areas of the U.S. with the greatest potential for elevated indoor radon levels. In , EPA's zone map document says, average indoor radon levels may be greater than 4 ; in Zone 2, between 2 and 4 pCi/L; in Zone 3, less than 2 pCi/L. The Citizen's Guide says EPA recommends building new homes with radon-resistant features in high radon potential (Zone 1) areas.

Bar chart of how many U.S. counties are in EPA radon Zone 1, Zone 2 and Zone 3, and how many have no zone on file, from the EPA Map of Radon Zones.
Read this figure as text

Predicted potential

Counties per EPA radon zone
ZoneBarCountiesShare of all counties
Zone 1Highest predicted potential1,07133.1%
Zone 2Moderate predicted potential1,03432.0%
Zone 3Lower predicted potential1,03332.0%
No zoneNo EPA zone on file932.9%

3,231 counties in all.

An EPA radon zone describes predicted potential across a whole county. It is not intended to determine whether any individual home should be tested, and homes with elevated levels occur in all three zones. EPA recommends testing every home regardless of zone, and testing is the only way to know the radon level in a specific building.

How many U.S. counties fall in each EPA zone, from the zone data Radon411 holds. A zone is a prediction for an area, not a reading of any home.

Construction, sealing and soil

What can raise a home's radon level
FactorDetail
Soil rich in uranium, thorium and radiumRadon levels can be higher in homes built on it (National Cancer Institute)
A well-insulated, tightly sealed houseRadon levels can be higher (National Cancer Institute); radon can also be a problem in drafty homes
Lower floorsBasement and first floors typically have the highest radon levels (National Cancer Institute)
A taller houseAdds potential for a greater stack effect (Minnesota Department of Health)
A home closed up or heated most of the yearCan result in higher levels of radon (Minnesota Department of Health)
A private well or ground water supplyHigh radon levels in the water supply are more likely when its source is ground water

Which floor, and which room

WHO says radon levels are usually higher in basements, cellars and living spaces in contact with the ground, and CDC notes that radon levels are often higher in basements and lower levels. That is why EPA's Citizen's Guide says to place a test kit in the lowest lived-in level of the home, in a room that is used regularly but not your kitchen or bathroom.

Questions about which homes have radon

What homes usually have radon?

Every home has some, because CDC says all outdoor and indoor air has some radon in it. The Citizen's Guide estimates that nearly 1 out of every 15 homes in the U.S. has an elevated level.

Are older homes more at risk for radon than new ones?

Neither is safe by age. The Citizen's Guide says any home may have a radon problem, new and old homes alike, and that every new home should be tested after occupancy, even if it was built radon-resistant.

Can a house without a basement have radon?

Yes. Minnesota's Department of Health says slab-on-grade homes have many openings that allow radon to enter, similar to a basement, and crawl spaces are directly connected to the soil.

Does an energy-efficient, tightly sealed house have more radon?

It can. The National Cancer Institute says radon levels can be higher in homes that are well insulated and tightly sealed. The Citizen's Guide adds that drafty homes can have a problem too.

Can I go by my neighbor's radon result?

No. The Citizen's Guide says radon levels can vary greatly from home to home, and the only way to know if your home has a radon problem is to test it.

Do I need to test if I live in Zone 3?

Yes. EPA says all homes should be tested regardless of geographic location, and homes with elevated levels have been found in all three zones.

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