Radon Testing: Basement vs. First Floor — Where and How to Test for Accurate Results

Radon enters homes from soil and rock through foundation cracks, gaps around pipes, and porous building materials. Because it rises from below, radon concentrations are highest in the lowest areas of a home and decrease with each floor up. This means where you place your radon test matters enormously — potentially the difference between getting a result that triggers action and missing a problem entirely.

The EPA's Official Testing Guidance

The EPA and EPA-affiliated testing programs are clear on test placement: test in the lowest level of your home that is currently used — or that could be used — as a living space. Their specific guidance:

• If you have a finished basement you use regularly, test there
• If you have an unfinished basement you use even occasionally (as storage, a laundry room, a workshop), test there
• If you have a basement you never enter and have no plans to use, test on the first floor
• If you have no basement, test on the first floor

The underlying principle: you want to know the radon concentration in the air you actually breathe. Radon in a basement you never enter doesn't accumulate exposure. But many people underestimate how much time they spend in partial-use basements — a laundry room visit twice a week is still meaningful exposure over years.

Why Not Test Upstairs to "Get a Better Result"?

Testing on upper floors produces lower readings — sometimes dramatically lower. Radon concentrations on a second floor can be 50–80% lower than basement levels in the same home. This isn't a "safe" result; it's a misleading one. The basement still has the elevated radon, and that radon is present in air that circulates throughout the home via the HVAC system, stairwells, and natural air movement.

Testing upstairs is essentially like checking your car's tire pressure by measuring the one that looks fullest. The number might be reassuring, but it doesn't tell you whether you have a problem.

Test Placement Within the Lowest Level

Once you've identified which floor to test, placement within that floor also matters:

• Height: Place the test 20 inches to 6 feet above the floor. Don't place it directly on the floor — radon settles near the ground, and a floor-level reading may be higher than average breathing zone concentrations.
• Away from exterior walls: Keep tests at least 20 inches from exterior walls. Near-wall placement can introduce outdoor air effects.
• Away from drafts: Avoid areas near windows, vents, and HVAC registers. Airflow disrupts radon accumulation and can artificially lower readings.
• Not in the sump pit area: Sump pits can be significant radon entry points, but testing directly above them may overstate levels in the occupied space. Test in a representative location instead.

Short-Term vs. Long-Term Tests: Which to Use

Short-term tests (48–96 hours for charcoal canisters) are useful for initial screening or real estate transactions when time is limited. They're accurate to within about ±10–15% in controlled conditions, but natural radon fluctuation — driven by weather, barometric pressure, and heating/cooling cycles — means any single short-term result captures only a slice of the real picture.

Long-term tests (90+ days using alpha-track detectors) average out these fluctuations and give you a more accurate representation of your actual annual exposure. For homes where you'll be making a mitigation decision worth hundreds or thousands of dollars, a long-term test is worth the extra time.

The EPA guidance: if a short-term test shows 4 pCi/L or higher, conduct a second test (either another short-term or a long-term) before deciding on mitigation. If the first result is at 8 pCi/L or higher, fix the home using the initial short-term test results without waiting for confirmation.

Testing Multiple Levels

For homes where multiple levels are used regularly, testing each level provides a complete picture. First-floor levels are typically lower than basement levels but not always negligible, particularly in:

• Homes with crawlspaces (which can channel radon into first-floor areas)
• Slab-on-grade homes (no basement, but radon can enter through the slab)
• First floors in homes with passive HVAC systems that draw basement air upward

Multi-level testing adds cost but is worthwhile when the first-floor result matters for occupancy decisions — such as when a bedroom is on the ground floor in a home where the basement has elevated radon.

After Mitigation: Where to Retest

After a radon mitigation system is installed, test again in the same location you originally tested, using the same test type. The comparison needs to be apples-to-apples. If the original test was a short-term charcoal kit in the basement, retest with a short-term kit in the same basement location. Professional mitigators should provide a post-mitigation test as part of their service — ideally a third-party test rather than one they conduct themselves.