EPA Radon Zones Explained
If you have looked up your county on RadonLookup, you have seen a number: Zone 1, 2, or 3. This guide explains what those zones actually mean, how the EPA determined them, and, more importantly, what they do and do not tell you about your specific home.
The short version: the zone tells you about area-wide risk. Your test result tells you about your home. Both matter, but only one of them is definitive.
What the EPA radon zones are
In 1993, the EPA published its Map of Radon Zones, classifying every US county into one of three categories based on predicted indoor radon levels. The zones have not changed since. They reflect the underlying geology, which is stable over human timescales.
The classification is based on three types of data: indoor radon measurements (actual test results from homes), aerial radioactivity surveys (measuring uranium and thorium concentrations in surface soil from aircraft), and soil parameters including geology, soil type, and permeability. The EPA combined these into a county-level prediction of what average indoor radon should be.
A prediction, not a measurement
Zone 1, 2, and 3: what each means
The EPA predicts average indoor radon levels above the agency's action level. Testing is especially important here, and if you've never tested, it should move to the top of your list.
Covers much of the Midwest, Northern Plains, and Mid-Atlantic, areas with uranium-bearing geology and older housing stock.
The EPA predicts averages between 2 and 4 pCi/L. Most homes here will test below the action level, but within Zone 2, individual results range widely. "Moderate" does not mean safe.
Covers a large portion of the US, including much of the South, West, and parts of the Northeast.
The EPA predicts averages well below the action level. Lower risk is good news, but it's not zero risk. Individual homes in Zone 3 can still test above 4 pCi/L.
Covers much of the Pacific Coast, parts of the South, and lower-elevation areas with less uranium-rich geology.
Search your county on RadonLookup to see your EPA zone alongside the CDC-measured average for your county, which gives you a second, independent data point.
What your zone does not tell you
This is the part that matters most, and it is what a lot of radon resources gloss over.
Zones are county-level averages. Radon is a hyper-local phenomenon. The specific factors that determine your home's radon level (foundation type, soil under your house, construction quality, ventilation patterns, and how many entry points exist) vary house by house, often dramatically.
- •A Zone 1 home on a poured concrete slab with good sealing might test at 1.5 pCi/L.
- •A Zone 3 home with a cracked basement floor and a sump pit might test at 9 pCi/L.
- •Your neighbor's test result tells you almost nothing about yours.
- •New construction is not safer. A tightly built home can trap radon more efficiently than an older drafty one.
Radon zones are the starting point for understanding your risk, not the ending point. A Zone 1 designation means you should test, not that your home is definitely dangerous. A Zone 3 designation does not mean testing is pointless.
The EPA is explicit about this: even in Zone 3, the agency recommends testing homes, particularly those with basements or other below-grade living spaces. No county is truly no-risk.
CDC data: a second look at your county
The EPA zone map is based largely on geological prediction. The CDC's Environmental Public Health Tracking Network offers a second, complementary dataset: actual measured indoor radon levels from real home tests, aggregated by county.
Where available, RadonLookup shows both: the EPA zone and the CDC county average in pCi/L. These sometimes diverge. A county in Zone 2 might have a CDC-measured average of 5 pCi/L (higher than the zone suggests), or a Zone 1 county might show a CDC average of 2.8 pCi/L (lower than the zone suggests). Neither is wrong. They're measuring different things with different methodologies.
Small sample size caveat
When zone and measured data disagree, both are worth noting. But neither replaces testing your specific home. They are useful context, worth knowing, but they are not the answer.
A short-term test kit costs around $15 and answers the question that zone data cannot: what is the radon level in this specific house? See Tipper's picks.