Radon Levels by Zip Code: How to Read Your Result

Radon levels by zip code tell you how common high radon is in the homes around you. They do not tell you what is in your house. Radon rises from the soil and rock directly under each foundation and gets in through whatever cracks and openings that particular house has, so two houses on the same street can test far apart. Read a zip result as a reason to test sooner, never as a reason to skip the test. The only number that answers your real question comes from a test taken inside your own home.

Where radon levels by zip code come from

Almost every lookup you will find draws on one of two sources, and they mean different things.

The EPA map: counties, drawn in 1993

The EPA Map of Radon Zones was published in 1993, and it works by county, not zip code. A radon zone is EPA’s county-by-county estimate of how high indoor radon tends to run. Zone 1, shown in red, is where the predicted average indoor level is above 4 pCi/L. Zone 2, orange, falls between 2 and 4. Zone 3, yellow, is below 2. pCi/L (picocuries per liter) is the unit radon tests report: how much radon is in a liter of air. EPA built those predictions from indoor measurements, geology, soil permeability and common foundation types in each county. When a website turns your zip code into a color, it is usually placing your zip inside a county and reading that county’s zone.

Because the zones follow counties and the pattern follows the ground, high-radon areas show up as regional clusters rather than neat state borders. If you want to see how the zones fall across the country, read radon zones state by state. The rest of this page is about what to do with the result you already have.

EPA is direct about the map’s limits. On the same map page it says every home should be tested regardless of zone, that homes with elevated radon have been found in all three zones, and that the map is not meant to decide whether a particular home gets tested. A yellow county is a county where high readings are less common. It is not a county where they do not happen.

Test-company maps: averages of whoever tested

Most of the zip-level numbers online come from companies that sell radon tests or monitors. These test-company maps average the results from homes in a zip code that happened to be tested, often with the company’s own kits or devices. That makes them more local than a county zone, but the number carries a bias the map does not show. People test because they are buying a house, because a neighbor found high radon, or because they were already worried. A zip with only a handful of tests can swing on two or three houses.

An average also hides the spread. A zip average of 3 can mean most houses sit near 3, or it can mean a mix of houses at 1 and houses at 8. Some maps show the share of tests above 4 instead, which is the more useful figure because it answers how often houses near you run high. It still reflects only the houses that were tested.

What a zip code result can’t tell you about your house

The zip result describes the neighborhood. Your reading depends on things no map holds:

• What is directly under the house. Uranium-bearing rock, fractured bedrock and loose, permeable soil let more radon reach the foundation. That can change from one lot to the next.
• How the house meets the ground. Cracks in a slab, an open sump pit, gaps around pipes and wires, a dirt-floor crawlspace and hollow block walls all give soil gas a way in.
• How the house moves air. A heated house draws air up from the soil, and a tight, well-sealed house can hold radon in once it enters.

None of that follows a zip code line. New houses and old ones, basements and slabs, can all test high. So “my zip is yellow, I’m fine” and “my area averages 3, so my house is about 3” are the same mistake: treating a neighborhood figure as a measurement of one building.

What to do with your result

Start with what the numbers mean. The action level is the reading at which EPA recommends fixing the house: 4 pCi/L. EPA also says homeowners should consider fixing between 2 and 4, because there is no known safe level and lower is better (EPA on the radon action level). Here is how to act on each kind of lookup result:

• Red, Zone 1, or a zip average at or above 4. Test this season. Don’t wait for a sale, a renovation or next year. High readings are common enough near you that the odds of a problem are real.
• Orange, Zone 2, or an average between 2 and 4. Test within the next few months. Your neighborhood sits in the range where EPA says to consider fixing, and your own house could land on either side of 4.
• Yellow, Zone 3, or an average below 2. Still test, once, on your own schedule. A low area lowers the odds; it does not rule your house out.
• Buying a house, any color. Test as part of the inspection, before closing, when the seller can still be asked to fix it.

When you have your own number, read what radon levels are safe and which call for action for what it means for your household.

Why test at all if no one feels sick

Radon gives no warning. It has no smell, color or taste, and breathing it causes no symptoms you would notice. The harm is lung cancer after years of exposure, and EPA calls radon the second leading cause of lung cancer in the United States, and the leading cause among people who have never smoked (EPA on the health risk of radon). A zip map can’t measure that risk for your family. A test can.

How to test your own house

There are two kinds of test. A short-term test runs a few days. A long-term test runs three months or more and gives a better read of your average across the seasons. Either one is placed in the lowest level of the house you use as living space, with windows and outside doors kept closed while it runs.

You can do it yourself with a kit from a hardware store or a state radon program, or you can hire a professional who places a continuous monitor and reports the result. For a home sale, or when you want a result that holds up with a seller or buyer, a professional test is the usual choice. The tradeoffs are laid out in DIY test kits vs. a professional test. If you hire someone, ask whether they are certified or licensed where your state requires it.

Booking a test is not hard. Of the 503 radon companies in our directory, 286 offer testing, more than half of them, and 257 install mitigation systems if the number comes back high.

Timing matters a little. Readings tend to run higher when the house is closed up for the cold months, so a winter test leans toward the high side, which is the side you want to know about. More on that in why radon readings run higher in winter. For most houses, the right time to test is the week you read this.

If your test comes back high

A high reading is a fixable problem, not a reason to leave the house. The fix is mitigation: a system, usually a pipe and fan, that pulls radon from under the house and vents it outside above the roofline. Ask for a follow-up test after the system is installed so you know it worked, and retest every couple of years or after major work on the foundation.

Your next step is the same whatever color your zip came back: find a radon tester in your area using the city finder, and book a test for your own house.