What Is Radon — and Why Is It a Problem Indoors?
Radon is an invisible, odorless gas produced by the natural decay of uranium in soil and rock. It exists everywhere — more in some regions, less in others. Outdoors it dilutes instantly and is harmless. It only becomes a problem when it accumulates inside buildings.
How does radon get into a house?
Radon rises from the ground and enters buildings through every gap it can find: cracks in the slab, joints, pipe and cable penetrations, sump pits, crawl spaces, unfinished dirt-floor basements. Because warm indoor air rises, a slight vacuum forms in the house that literally pulls the gas out of the soil — strongest in winter, when the heat is on and windows stay closed. Levels are usually highest in basements and on ground floors.
Why is radon dangerous?
Radon itself is mostly exhaled again. Its short-lived decay products, however, attach to dust particles, lodge in the lungs, and irradiate the tissue there. The U.S. EPA attributes about 21,000 lung cancer deaths per year in the United States to radon — making it the second-leading cause of lung cancer after smoking, a ranking the WHO confirms worldwide. Roughly 1 in 15 American homes has an elevated radon level.
Important context: this is a long-term risk. Living for decades in rooms with high radon levels raises your lung cancer risk — especially in combination with smoking. A single afternoon in a basement is no reason to worry.
Which levels are too high?
| Reference | Level |
|---|---|
| EPA action level (USA) | 4 pCi/L (≈ 148 Bq/m³) — fix your home |
| EPA “consider fixing” range | 2–4 pCi/L |
| WHO reference level | 100 Bq/m³ (≈ 2.7 pCi/L) |
| Switzerland / Germany / Italy (existing homes) | 300 Bq/m³ (≈ 8 pCi/L) |
The EPA Map of Radon Zones shows which U.S. counties tend to run high — but radon is hyper-local: your neighbor’s reading tells you little about your own house.
First things first: test
You cannot see, smell, or taste radon — and levels vary drastically even between adjacent houses. The only way to know is to measure. How to do it, and what it costs, is covered in Testing for radon.