Updated November 2026 · Sources
Radon comes from the ground, so it is usually highest in basements. Pennsylvania DEP’s database of more than 3 million home tests shows the difference clearly.
Radon by floor: what 3 million tests show
| Floor tested | Tests | Median (pCi/L) | At or above 4 pCi/L |
|---|---|---|---|
| Basement | 2,645,608 | 2.8 | 37.3% |
| First Floor | 379,493 | 1.5 | 20.4% |
| Slab On Grade | 103,274 | 2 | 24.8% |
| Second Floor | 52,367 | 1.2 | 15.7% |
| Crawl Space | 23,653 | 1.4 | 12.6% |
| Third Floor | 5,703 | 1.01 | 17.7% |
| Above Third Floor | 1,998 | 0.6 | 9.5% |
In Pennsylvania, a basement test was about 1.8 times as likely as a first-floor test to be at or above 4 pCi/L. New York State Department of Health data show the same pattern: across 58 counties, 36.1% of 84,677 basement tests were at or above 4 pCi/L, compared with 16.3% of 38,137 first-floor tests, and the basement average was higher in every county (see our New York section).
Why basements have more radon
- Radon moves up from the soil and enters through cracks in floors and walls, construction joints, gaps around pipes and sump pits (EPA).
- Basements have the most contact with the ground and often less ventilation.
- Lower air pressure inside the home, especially in winter, pulls soil gas in.
Finished basements and new bedrooms
If you start using the basement more, or plan to finish it, the EPA recommends testing that level. Buyers often ask for tests in areas such as a basement they plan to finish. If you find high radon, it is usually easier and cheaper to install a system before finishing the space.
How basement radon is fixed
The usual fix is sub-slab depressurization: a pipe through the basement floor and a fan pull radon from under the slab and vent it above the roof, and cracks are sealed. In PA DEP records for systems installed since 2016, the median test was 8 pCi/L before mitigation and 1.0 pCi/L after, and 95.3% of post-mitigation tests were below 4 pCi/L. See typical prices in our radon mitigation cost guide.
Frequently asked questions
Is radon only a problem in basements?
No. Basements usually have the highest levels, but homes on slabs and crawl spaces can have high radon too. In PA DEP data, 24.8% of slab-on-grade tests were at or above 4 pCi/L.
Is it safe to sleep in a basement with radon?
If the basement is a bedroom, test there. The EPA recommends fixing the home if the level is 4 pCi/L or higher.
Does a dehumidifier reduce radon?
No proven method other than a radon mitigation system reliably reduces radon. Sealing cracks helps, but the EPA does not recommend sealing alone.
Sources
- PA DEP: Radon Test Results (data.pa.gov) (retrieved November 2026)
- PA DEP: Radon Mitigation System database (data.pa.gov) (retrieved November 2026)
- U.S. EPA: A Citizen’s Guide to Radon (PDF, 2016)
- NYSDOH: Radon Test Results by County