What the EPA Radon Zone Map Actually Tells You

Sooner or later everyone looking into radon finds the EPA Map of Radon Zones - the one that shades every U.S. county red, orange, or tan for Zone 1, 2, or 3. And the very first thing people do with it is the one thing the EPA specifically says not to do: use it to decide whether their own house needs testing.

What the map is

The EPA built the Map of Radon Zones back in 1993 to identify areas of the country with the potential for elevated indoor radon. It was made using indoor radon measurements, geology, aerial radioactivity, soil parameters, and foundation types, then boiled down to three categories:

Those are predicted county averages. The whole purpose, in the EPA's own words, is to help national, state, and local organizations target their resources and implement radon-resistant building codes. It's a planning tool for agencies, not a diagnostic for your address.

What the map is not

The EPA is blunt about this: the Map of Radon Zones "should not be used to determine if individual homes need to be tested." Two houses on the same street can test completely differently depending on their foundation, how they're sealed, the soil right under them, and how the furnace and exhaust fans move air. A county average tells you nothing reliable about the box you actually live in.

This is the trap. Someone in a Zone 3 county reads "low potential" and decides they're off the hook, skips the test, and never finds the 8 pCi/L their basement is actually sitting at. Radon is the second-leading cause of lung cancer in the U.S. behind smoking, and the map's coarse county shading is exactly the wrong tool for a house-level decision.

The numbers that actually matter

Forget your zone color for a second. The action levels are the same everywhere:

• Fix your home if it tests at 4 pCi/L or higher. That's the EPA action level, full stop, in every zone.
• Consider fixing between 2 and 4 pCi/L. There's no level of radon that's officially "safe" - the risk just drops as the number drops.
• Test no matter where you live. The EPA's own guidance on the map says it plainly: no matter the zone, test your home. A test kit is cheap and the test is easy.

How to actually use the map

The map is genuinely useful for one thing at the homeowner level: setting your expectations. If you're in a Zone 1 county, don't be shocked by a high number - plan on testing and budget mentally for the possibility of a mitigation system. If you're in a Zone 3 county, don't get comfortable - you still test, because your specific house is the only data point that counts. The EPA even recommends supplementing the map with local data, because state and county radon programs often have finer-grained measurements than a 1993 national map ever could.

Want to look up your own county's zone? The EPA Map of Radon Zones is right there - just read it for what it is: a starting point, not an answer. The answer comes from a test in your house.