The EPA radon zone map, explained

Our lookup is built on EPA’s Map of Radon Zones. Knowing how that map was made helps you use it well, and understand why it is only a starting point.

Why it exists

Congress directed EPA, under the Indoor Radon Abatement Act, to identify areas of the United States with the potential for elevated indoor radon. EPA released the Map of Radon Zones in 1993. EPA states its purpose plainly: to help national, state, and local organizations target their resources and to help building code officials decide where radon-resistant building practices are most needed.

How zones were assigned

EPA’s fact sheet explains that each county was assigned a zone based on five factors:

• indoor radon measurements,
• geology,
• aerial radioactivity,
• soil permeability, and
• foundation types.

These were combined into a radon potential assessment for geologic provinces, then adapted to county boundaries.

The three zones

• Zone 1: predicted average indoor screening level greater than 4 pCi/L (highest potential).
• Zone 2: predicted average between 2 and 4 pCi/L (moderate potential).
• Zone 3: predicted average less than 2 pCi/L (low potential).

In EPA’s current spreadsheet, the counties are split roughly evenly among the three zones.

The limits EPA stresses

EPA’s own text is clear that the map “should not be used to determine if individual homes need to be tested.” Three reasons:

1. It is an average. A county-wide prediction hides large variation from house to house.
2. It is old. The map dates from 1993 and has not been redrawn. States have collected many more test results since.
3. Elevated homes exist everywhere. EPA notes there are many thousands of homes with elevated radon in Zone 2 and Zone 3 counties.

Better local data

EPA encourages supplementing the map with state or local data. Many state radon programs publish test results by county or ZIP code, and CDC’s Environmental Public Health Tracking Network hosts radon testing data from participating states and labs. EPA also publishes state-by-state supporting documents describing the data and confidence behind each state’s map.

How to use the zone well

• Use it to gauge how common radon is in your area and how urgent testing is.
• Use it for community planning and building codes.
• Do not use it to skip testing. Test every home, and fix at 4 pCi/L or higher.

Check your county in the ZIP lookup or browse by state.

Why zone numbers don't predict your house

The zone map was built from county-level averages using indoor measurements, geology, soil permeability, aerial radioactivity data, and foundation types. A county average can hide huge variation from house to house. EPA's own guidance is that all homes should be tested, regardless of zone. In practice:

• Homes in Zone 3 counties (predicted average below 2 pCi/L) can test well above 4 pCi/L.
• Homes in Zone 1 counties (predicted average above 4 pCi/L) can test very low.
• Neighboring houses can differ dramatically because of soil, foundation cracks, construction details, and how each house "breathes."

How the zones compare to national averages

For context, EPA reports the average indoor radon level in U.S. homes is about 1.3 pCi/L, and the average outdoor level about 0.4 pCi/L. A Zone 1 county's predicted average of more than 4 pCi/L is roughly three times the national indoor average. A Zone 3 county's predicted average below 2 pCi/L is still above outdoor air.

Putting risk in perspective

EPA's risk tables estimate that among 1,000 people who never smoked and were exposed over a lifetime:

• at 4 pCi/L, about 7 could get lung cancer;
• at 2 pCi/L, about 4;
• at the average indoor level of 1.3 pCi/L, about 2.

For smokers, the numbers are much higher: about 62 per 1,000 at 4 pCi/L. Those estimates don't depend on what zone you live in. They depend on the radon level you actually breathe, which is why testing matters more than the map.

Better sources of local data

Many state radon programs publish testing results by county or ZIP code, sometimes with the percentage of homes above 4 pCi/L. These datasets are often more current and more detailed than the zone map, which dates from the 1990s. Use them as a guide to how common elevated levels are where you live, but still test your own home.

How builders and code officials use the map

Some state and local building codes require radon-resistant construction features in new homes in Zone 1 counties. The model residential code includes an appendix for radon control methods that jurisdictions can adopt. If you're building, ask your local building department whether RRNC is required where you live. See radon-resistant new construction.

A simple way to use the map

1. Look up your county's zone to understand regional potential.
2. Check your state radon program's local data.
3. Test your home, whatever the zone.
4. If your result is at or above 4 pCi/L, confirm it and fix it. If it's between 2 and 4 pCi/L, consider fixing.

Frequently asked questions

Is the zone map still current? It was developed in the early 1990s and hasn't been redrawn county by county since. EPA continues to point people to it as a general guide, together with supplemental information. State data are often more current.

My county is Zone 1. Does that mean my house has a radon problem? No. It means elevated levels are more likely in your area. Many homes in Zone 1 counties test below 4 pCi/L. Only a test can tell.

My county is Zone 3. Can I skip testing? EPA recommends testing all homes, whatever the zone. Low-potential areas still have homes with high levels.

Should a home buyer care about the zone? It's useful background, but the purchase decision should rest on a properly done test of that specific house. See radon when buying or selling a home.

Use the zone as context, then let your own test decide.