Basements usually have the highest radon levels in a home because they are closest to the ground, where radon comes from (National Cancer Institute). Levels are typically lower the further up you go (Washington State Department of Health). Whether your basement needs testing depends mainly on how you use it, or how a buyer might.
Homes without basements can have radon problems too (EPA, Basic Radon Facts). Having no basement isn’t a reason to skip the test.
Why radon collects in basements
Radon comes mainly from the soil and gets in through cracks and other holes in the foundation (EPA, Citizen’s Guide). A basement has more of its walls and floor against the soil, with entry points such as floor-wall joints, gaps around pipes and cables, hollow-block walls, sumps and drains (WHO).
What is a safe radon level in a basement?
There’s no separate safe level for basements: EPA’s guidance uses the same numbers throughout the home.
| Level | What EPA says |
|---|---|
| 4 pCi/L or more | Fix your home |
| 2 to 4 pCi/L | Think about fixing your home |
| About 1.3 pCi/L | The average US home, not a safe limit |
From EPA, Basic Radon Facts. EPA believes any radon exposure carries some risk and that no level of radon is safe (EPA, Citizen’s Guide). See radon levels explained for the full chart.
Should you test the basement?
EPA’s advice depends on your situation:
| Your situation | Where to test |
|---|---|
| You use the basement often | In the basement. EPA says to test the lowest lived-in level (EPA, Citizen’s Guide) |
| You rarely use it | The first floor, which is then your lowest lived-in level (EPA, Citizen’s Guide) |
| You’re buying or selling | The lowest level that could be used regularly, even if nobody uses it now (EPA, Home Buyer’s and Seller’s Guide) |
| You plan to finish it | In the basement, before work starts, and again after (EPA, Home Buyer’s and Seller’s Guide) |
| You’ve started using it | In the basement, even if you tested upstairs before (EPA) |
Virginia doesn’t count crawl spaces or cellars that lack utilities or enough headroom, or that have exposed dirt or rock, as livable space to test (Virginia Department of Health).
How to test in a basement
- Pick a room that’s used regularly, such as a family room, playroom or bedroom. Avoid the kitchen, bathroom and laundry room (Montana DEQ).
- Place the kit at least 20 inches above the floor, away from drafts, high heat, high humidity and exterior walls (EPA, Citizen’s Guide).
- Keep it away from openings in the floor, such as sumps and plumbing cut-outs (Virginia Department of Health).
See how to test for radon for the full steps, including keeping windows and doors closed, and what a test costs.
Basement result high? What it means upstairs
A high result in the basement doesn’t tell you the level on the floors above, which is often lower, though considerable levels can be found above the ground floor too (WHO). If the lowest level isn’t used much, Montana suggests also testing a higher-use area to better estimate your long-term exposure (Montana DEQ).
A single short-term result is also not the final word. Take a second test before deciding, as how to test for radon explains.
How to fix radon in a basement
In most cases, the fix is a system with a vent pipe and fan that vents radon outside (EPA, Home Buyer’s and Seller’s Guide). A typical system has one pipe through the basement floor, though the pipe can also run outside the house (EPA, Citizen’s Guide). See how radon mitigation systems work and what they cost.
Sealing cracks on its own rarely fixes a radon problem. A sump pump cover does the most good as part of a mitigation system, not on its own.
Finishing a basement
Test before you start. If the result is high, radon-resistant techniques can be included inexpensively as part of the renovation. Major renovations can change the radon level in any home, so test again when the work is done (EPA, Home Buyer’s and Seller’s Guide; CDC).