Radon levels by state: how to read EPA zone maps

Last updated: 2026-09-05 · 7 min read · How-to guide

The EPA Map of Radon Zones colors every US county by predicted average indoor radon potential. Use it to understand regional risk and building-code priorities — not to decide whether your house is safe. Every home should still be tested.

What you’ll learn

What Zone 1, 2, and 3 numbers mean in pCi/L
How the map was built (and what it cannot tell you)
A practical lookup workflow for your county or ZIP
When to mitigate once you have a real reading

The three zones, in plain numbers

EPA assigns each US county to one of three zones based on predicted average indoor screening levels in the lowest liveable area:

Zone 1 (highest potential) — predicted average greater than 4 pCi/L
Zone 2 (moderate) — predicted average between 2 and 4 pCi/L
Zone 3 (lowest potential) — predicted average less than 2 pCi/L

Those thresholds match the agency’s action language elsewhere: fix at or above 4 pCi/L; consider fixing between 2 and 4 pCi/L. Units: 4 pCi/L ≈ 150 Bq/m³.

Source: EPA Map of Radon Zones (agency page, last reviewed Aug 2026 on fetch). Zone colors on the printed map are typically red / orange / yellow for Zones 1 / 2 / 3.

What the map is for (and what it is not)

EPA built the map so governments and code officials can target radon-resistant construction and outreach. It is a county-level planning tool, not a home inspection.

Key limit Homes with elevated radon exist in all three zones. Neighboring houses on the same street can measure very differently because of soil, foundation type, airtightness, and HVAC paths.

Five factors went into zone assignment: indoor radon measurements, geology, aerial radioactivity, soil parameters, and foundation types. County lines were adapted from geologic provinces, so within-county variation is common — especially in large or geologically mixed counties.

How to look up your area

Open the EPA Map of Radon Zones page and download the PDF map or the text/XLS county list.
Find your county (or the county for the property you are buying). Note Zone 1, 2, or 3.
Check your state radon office or health department for local supplements — many states publish finer maps, average results by ZIP, or free kit programs.
Treat the zone as background odds only. Schedule a short-term or long-term test in the lowest lived-in level of the home.

Buying? Put a radon contingency in the inspection period in every zone. In Zone 1 counties it is close to non-negotiable for most buyers; in Zones 2 and 3 it is still the only way to know.

“By state” — what that phrase really means

Search results often say “radon levels by state,” but EPA zones are assigned at county level. A single state usually contains more than one zone. Iowa, Pennsylvania, and parts of the Mountain West, for example, have large Zone 1 footprints, while coastal and southern counties are often Zone 2 or 3 — with plenty of exceptions.

So: use the state to find the right map or health-department page, then read the county row. Do not assume a whole state is “safe” or “high.”

After you have a number

Once a kit or professional test returns a result, the zone map stops mattering for that decision. Compare the reading to EPA guidance:

≥ 4 pCi/L — EPA recommends fixing (mitigation, usually active soil depressurization).
2–4 pCi/L — consider fixing; still a meaningful exposure reduction for many households.
Below 2 pCi/L — lower potential, but retest after major renovations, HVAC changes, or if you move into a different lowest level.

Retest after any mitigation install. A system that was never commissioned, or a fan that failed later, can leave levels high even when the county is “only” Zone 2.

FAQ

Does Zone 3 mean I can skip testing?

No. EPA states elevated homes exist in Zone 3. The map predicts county averages, not your basement.

Is Zone 1 a guarantee my house is above 4 pCi/L?

No. It means the predicted county average screening level is above 4 pCi/L. Many Zone 1 homes still measure low — and many do not. Only a test answers it.

Are RadonFind listings paid?

Featured or affiliate placements (for example test-kit partners or mitigator referrals) will be labelled when used. Draft sample listings on this site are not endorsements.

Next steps

Browse RadonFind listings for testers and mitigators, or start with our guides index. If you already know you need a kit vs a pro test, that compare page is next on the week-1 plan.

Disclosure: We may earn a commission from test-kit partners or mitigator referrals. It does not change what you pay. Zone definitions and action levels above follow EPA public guidance; always confirm against the current EPA page before making a purchase or mitigation decision.