Radon is the exposure almost nobody checks, which is strange, because it is the leading cause of lung cancer among people who don't smoke. The EPA attributes about 21,000 US lung cancer deaths a year to radon. It has no smell, no taste, and no symptoms until the damage is done. The only reason it stays off your radar is that you cannot see it.
Here is the part most people don't know: the EPA already mapped the whole country for radon potential. Every county in the US sits in one of three zones, and you can check yours in ten seconds with your ZIP code - it appears right on your water report, alongside lead and air quality.
The EPA Map of Radon Zones predicts the average indoor radon level for homes in each county:
Zone 1 (highest potential). Homes are predicted to average at or above 4 picocuries per liter (pCi/L) - the level at which the EPA says fix your home. If you live in a Zone 1 county, testing is not optional. Whole regions of the Mountain West, the upper Midwest, and Appalachia sit here.
Zone 2 (moderate potential). Predicted averages between 2 and 4 pCi/L. That is an average, which means plenty of individual homes in Zone 2 counties test above the action level.
Zone 3 (low potential). Predicted averages under 2 pCi/L. Better odds, not a guarantee.
The zone map is a screening tool built on geology, aerial radioactivity surveys, and indoor measurements. It predicts county averages, not your house. Radon enters through the specific cracks, sumps, and soil contact of your foundation, so two houses on the same street can test wildly differently. The EPA is blunt about this: homes with elevated radon have been found in all three zones, which is why the official recommendation is that every home below the third floor gets tested regardless of zone.
Think of the zone as your prior. Zone 1 means "assume you have a problem until a test says otherwise." Zone 3 means "probably fine, but a one-time test is cheap insurance."
This is one of the cheapest home-safety wins there is. A short-term charcoal test kit typically costs about $15 to 30, and some state radon programs mail free kits. Place it in the lowest level of your home that people actually use - a finished basement counts, a crawlspace does not - keep windows closed as much as normal, and send it to the lab. Cooler months give the most realistic reading because the house is sealed.
If the result comes back at 4 pCi/L or higher, retest before doing anything. Radon swings day to day, and the EPA wants a confirmed number before you spend money. Two elevated results means it is time for mitigation: an active sub-slab depressurization system, essentially a pipe and fan that vents soil gas above the roofline, typically installed for about $800 to 2,500. Mitigation reliably drops levels dramatically, often below 2 pCi/L.
Radon is not a one-and-done test. Retest after any major renovation, foundation work, or new HVAC, after finishing a basement, and before selling - many states require radon disclosure in real-estate transactions. A digital radon monitor (about $100 to 200) is worth it in Zone 1 counties for continuous peace of mind.
What level of radon is dangerous?
There is no known safe level - risk scales with exposure. The EPA's action level is 4 pCi/L: at or above it, fix your home. Between 2 and 4, the EPA says consider fixing, especially if anyone smokes (smoking multiplies radon's lung-cancer risk).
Do I need to test if my neighbor's house was fine?
Yes. Radon entry depends on your foundation's specific cracks and soil contact. Neighboring homes routinely test an order of magnitude apart.
How long does a radon test take?
Short-term charcoal kits run 2 to 7 days and give a lab result about a week after mailing. Digital monitors show a first estimate within days and get more accurate over weeks. For real-estate deadlines, certified testers do 48-hour closed-house tests.
Does a radon zone affect my water too?
Mostly no - the zone map is about soil gas entering the air of your home. Private wells in high-radon areas can carry waterborne radon, which is one more reason well owners should test their water. Start with your free ZIP report to see your county's zone, then check our radon guide for the full testing walkthrough and your county's air quality while you're there.