Radon Testing in Your New Home: Zones vs. Reality

The EPA's radon map predicts county averages, not houses. A home in low-risk Zone 3 can test high and a home in Zone 1 can test clean, which is why the EPA's own guidance is to test every home regardless of zone. A short-term test kit costs roughly $15 to $30, takes two to seven days, and is the only way to know your actual number.

Radon is a colorless, odorless radioactive gas produced as uranium breaks down in soil and rock. It seeps into homes through slab cracks, sump pits, crawl spaces, and utility penetrations, and it accumulates in the lowest lived-in level. The EPA calls it the second-leading cause of lung cancer in the United States after smoking and attributes roughly 21,000 lung cancer deaths a year to it. There is no smell, no symptom, and no visual cue — the entire problem is invisible without a test.

The zone map everyone cites divides US counties into three categories: Zone 1, where the predicted average indoor level exceeds 4 picocuries per liter (pCi/L, the standard unit of radon concentration); Zone 2, between 2 and 4; and Zone 3, below 2. Those are county-wide predictions built for builders and code officials, not parcel-level measurements. Geology shifts lot to lot, and two houses on the same street routinely test very differently depending on foundation type, soil contact, and how the house breathes.

The classic failure mode: a buyer sees Zone 3 on a map and waives the radon test to keep an offer competitive. The second classic: trusting the seller's test from five years ago. Radon levels change with seasons — winter readings often run higher because the house is sealed — and with renovations and HVAC changes. A finished basement that was unconditioned storage when last tested is a different exposure scenario entirely.

The numbers to remember: the EPA's action level is 4 pCi/L — fix the home at or above it. Between 2 and 4, the EPA says consider fixing. No level is considered fully safe; risk scales with concentration and years of exposure, which is exactly why the reading in the house you will sleep in matters more than any county average.

Testing is the easy part. Short-term tests run two to seven days under closed-house conditions: windows shut, exterior doors used normally but not propped, starting at least 12 hours before the test. Long-term tests run 90 days or more and give a truer annual average. In real estate transactions, a 48-hour continuous monitor placed by a professional is standard because it timestamps readings and resists tampering. Many state radon programs sell discounted kits or mail them free — search your new state's health department before paying retail.

If the result comes back high, the standard fix is sub-slab depressurization: a pipe through the slab and a continuously running fan that vents soil gas above the roofline. Quotes commonly land in the $800 to $2,500 range depending on region and foundation type — it varies, so get two bids — and installation usually takes a day. Retest after mitigation to confirm it worked, and glance at the system's manometer occasionally; fans fail after years of continuous duty.

For movers, timing is everything. If you're buying, the inspection contingency window is when a radon test has negotiating power — after closing, the cost is yours. If you're renting, you can still run a $20 kit on your own; some states have radon disclosure requirements for landlords and sellers, but coverage varies widely by state. New construction is not exempt either: radon-resistant construction features help, but the EPA still says test, because the features are passive until proven.

LocateFlow's New Home Dossier shows the EPA radon zone for a new address alongside the FEMA, EPA, and NWS layers — flagged explicitly as county-level context, paired with the to-do it should trigger: order a test. The zone is a reason to test promptly, never a reason to skip it.

First, check the EPA zone for context, not a verdict. Second, buy a short-term kit or schedule a 48-hour continuous monitor if you're inside an inspection window. Third, run the test on the lowest lived-in level under closed-house conditions. Fourth, mitigate at 4 pCi/L or above, and seriously consider it above 2. Fifth, retest every two years and after any renovation that touches the foundation or HVAC.