Parcels · Hazard layers · Radon · October 16, 2026

Radon Map: How to Check Radon Risk for a Property

How to use a radon map: EPA radon zones by county, state test data, how to check a parcel, and why only a radon test tells you a home's real level.

A radon map shows which parts of the country are more likely to have high indoor radon, a colorless, odorless radioactive gas that seeps up from soil and rock and can build up inside houses. Buyers look one up before making an offer, and owners check one when a neighbor mentions a radon system or a home inspector recommends a test. The map is a useful first look, but it works at the county level and cannot tell you the radon level in any single house. This guide explains where to find radon maps, how to check a property, what the zones mean and why a test is still the only real answer.

What radon is and why it matters

Radon forms when uranium that occurs naturally in soil and rock breaks down. Outdoors it thins out quickly. Indoors it can collect, entering through cracks in slabs and foundation walls, gaps around pipes, sump pits and crawl spaces. The U.S. Environmental Protection Agency calls radon the second leading cause of lung cancer in the United States and the leading cause among people who have never smoked. Radon is measured in picocuries per liter of air (pCi/L).

The EPA Map of Radon Zones

The best-known radon map is the EPA Map of Radon Zones, first released in 1993. It assigns every county to one of three zones based on indoor radon measurements, geology, aerial radioactivity readings, soil permeability and common foundation types.

• Zone 1 (red): highest potential. Average indoor radon may be greater than 4 pCi/L.
• Zone 2 (orange): moderate potential. Average indoor levels may fall between 2 and 4 pCi/L.
• Zone 3 (yellow): low potential. Average indoor levels may be below 2 pCi/L.

EPA offers the map as a PDF, as an interactive web map with links to each state’s radon program, and as a downloadable county table you can search. EPA is clear about what the map is for: helping governments target outreach and radon-resistant building codes. It says the map should not be used to decide whether a home needs testing, because homes with high radon have turned up in all three zones.

State radon maps and test data

Every state has a radon program, usually in the health or environmental department, and many publish more detail than the national map. Depending on the state, you may find:

• County maps of results from homes that were tested, often showing the share of tests at or above 4 pCi/L.
• Test results grouped by ZIP code, which give a finer picture than one color per county.
• Geologic radon potential maps from the state geological survey, drawn from rock and soil types.
• Lists of certified testers and mitigation contractors.

Treat these the same way as the EPA map. They describe patterns across many homes, and the homes that get tested are not a random sample.

How to check radon for a specific property

1. Confirm the county. Parcels near a county line are easy to mix up. Pull the lot up in the county parcel viewer; see how to find a parcel number by address, or start from our land map.
2. Look up the zone. Find the county in EPA’s zone table or on the interactive map and note its zone.
3. Check the state program. Look for county or ZIP code test results and any geologic radon map for the area around the parcel.
4. Look at the geology. Granite, some shales and phosphate-bearing rock are often tied to higher radon, but local soil and how a house is built can matter as much. The soil survey shows soil types and drainage for the parcel.
5. Ask about past tests. Ask the seller for any radon test results and whether a mitigation system was installed, and look for a vent pipe running up the outside of the house or through the roof with a small fan.
6. Test the house. This is the step that actually answers the question.

How to read a radon map

• One color covers a whole county. A county can hold thousands of homes on very different ground. Two houses on the same street can test far apart.
• Zone 3 is not a pass. EPA recommends testing every home, in every zone.
• Zone 1 is not a verdict. Plenty of homes in red counties test low.
• The map is old in places. The zones date from the early 1990s. State data gathered since then may give a better picture for your area.

Testing: the only way to know

• Short-term tests run from 2 to 90 days, and most kits take a few days. They are common in home sales because they are quick.
• Long-term tests run more than 90 days and give a better sense of the year-round average, since radon rises and falls with weather and season.
• Continuous monitors record levels hour by hour. Many home inspectors and radon testers use them during a sale.
• Where to test: the lowest level of the home that is lived in or could be, away from drafts, with windows and doors kept closed as the test instructions say.

EPA advises fixing a home at 4 pCi/L or higher and considering a fix between 2 and 4 pCi/L.

Radon when buying a property

• Put a test in the contract. Many buyers make the offer depend on a radon test along with the home inspection, so there is time to negotiate if the result is high.
• Read the disclosures. Some states, Illinois among them, require sellers to disclose radon test results or hazards they know about. Others say nothing about radon, so ask directly.
• Check an existing system. If a mitigation system is in place, ask for the most recent test after it was installed and make sure the fan runs and the gauge on the pipe shows suction.
• Well water. Radon can also dissolve into groundwater and be released inside the house when the water is used. If the property is on a private well, ask about a water test too; see how to find well records.
• Vacant land and new builds. You cannot test a house that does not exist yet. In higher-radon areas, ask the builder about radon-resistant construction, such as a gas-permeable layer, sealed slab and a vent pipe roughed in under the floor. Some local building codes require these features.

If the radon level is high

High radon is usually fixable. The most common method, called sub-slab or active soil depressurization, uses a pipe and fan to pull soil gas from under the foundation and vent it above the roof. Sealing cracks helps but is rarely enough on its own. Hire a contractor certified by your state or by a national radon certification program, and retest after the work is done.

What a radon map cannot tell you

• The radon level inside a particular house.
• Whether a past test was done correctly or a system still works.
• Where the property lines are. For that, see how to read a property survey.

Radon belongs on the same due-diligence list as the other hazard checks for a parcel: the flood zone by address, a wildfire risk map and a sinkhole map.

Questions people ask

Related guides

• Sinkhole map: how to check a property for sinkholes
• Wildfire risk map: check a property’s fire hazard zone
• How to find a flood zone by address
• What is a Phase 1 environmental site assessment?
• How to find well records for a property