Radon Maps: Useful or Misleading?
Radon Maps: Useful or Misleading?
Radon maps are useful for understanding regional radon potential, but they become misleading when used to judge a single home. The EPA’s Map of Radon Zones and similar state and county maps show where elevated radon is more or less likely across broad areas. They can’t tell you your own home’s level, because radon varies widely from house to house, even on the same street. The EPA states plainly that its map should not be used to decide whether a home needs testing. This article explains what radon maps show, where they help, where they mislead and how to use them alongside a test.
What a radon map actually shows
The best-known radon map in the United States is the EPA Map of Radon Zones, released in 1993. It assigns every county to one of three zones based on its predicted average indoor radon level:
- Zone 1: average indoor radon may be greater than 4 pCi/L
- Zone 2: average indoor radon may be between 2 and 4 pCi/L
- Zone 3: average indoor radon may be less than 2 pCi/L
The EPA built the map from indoor radon measurements, geology, aerial measurements of natural radioactivity, soil characteristics and common foundation types. The result is a prediction of average conditions across a whole county, not a measurement of any particular building.
Other maps work differently. Some state health departments publish maps of test results by county or ZIP code. Commercial sites and testing companies publish their own versions, often based on results from kits they’ve processed. Our own user-submitted radon readings show results homeowners have chosen to share. Each type has its own strengths and blind spots, so it helps to know where a map’s data comes from before reading too much into it.
Where radon maps are genuinely useful
The EPA designed its zone map for organizations, not individual homeowners. Its stated purpose is to help national, state and local groups target their resources and decide where to adopt radon-resistant building codes. For those decisions, a county-level view of radon potential is the right tool.
Maps also help homeowners understand context. If you live in a Zone 1 county, a high test result isn’t unusual for your area, and local contractors are likely familiar with radon mitigation. Colorado, for example, places all of its counties in Zone 1, and the state health department estimates that about half of Colorado homes are above the EPA action level of 4 pCi/L. Knowing that can help someone take a result seriously instead of assuming the test was wrong.
A map can also prompt people who have never thought about radon to test. That’s arguably its most valuable role for the public.
Where radon maps mislead
The problem starts when a map is treated as an answer about one house. Several things make that unreliable.
Averages hide individual homes
A county zone describes a predicted average. In any county, some homes will be well above it and others well below. A Zone 3 label means typical homes in the county may read under 2 pCi/L, not that every home does. The EPA notes that homes with elevated radon have been found in all three zones, and Colorado’s health department points out that many thousands of homes in Zones 2 and 3 have elevated levels.
Radon depends on the house as much as the location
Two neighboring homes can sit on different soil pockets, have different foundations and draw in soil gas differently. Cracks, sump pits, crawl spaces, heating patterns and how tightly a house is built all affect the reading. A map can’t see any of that.
The readings submitted to RadonResources.com show how much homes in the same area can differ. In Buffalo Grove, Illinois, 10 submitted readings ranged from 3 to 11 pCi/L. In Charles Town, West Virginia, 10 readings ranged from 2 to 16. These are self-reported results and not a representative sample, but they illustrate a pattern state radon programs describe as well: living in the same ZIP code doesn’t mean having the same radon level.
The data may be old or uneven
The EPA zone map dates to 1993, and the agency recommends supplementing it with any available local data. Maps built from test results can also be skewed by who tests. Areas with active testing programs or frequent home sales may have many results, while rural counties may have very few. Maps based on voluntarily submitted results tend to overrepresent high readings, because people with elevated results are more likely to look for information and share them.
A “low” color can discourage testing
The biggest risk is a reader seeing a yellow or green county and deciding not to test. That’s the conclusion the EPA warns against. Its zone map materials state that the map shouldn’t be used to decide whether a home should be tested, and that all homes should be tested regardless of zone. Colorado adds that the map is not to be used in place of testing during real estate transactions.
How to use a radon map well
A map works best as a starting point that leads to a test, not as a substitute for one. A practical approach looks like this:
- Use the map for context. Check your county’s zone and your state program’s data to understand how common elevated radon is near you.
- Test your home regardless of the map. A short-term test gives a quick answer, and a long-term test lasting more than 90 days better reflects year-round exposure.
- Read your result on its own terms. The EPA recommends fixing homes at or above 4 pCi/L and considering action between 2 and 4 pCi/L. No level is considered completely safe.
- Retest over time. Radon changes with seasons, renovations and foundation settling, so a past result isn’t permanent.
Our radon testing guide covers how to run a test, and our radon test kit comparison explains the differences between short-term kits, long-term kits and continuous monitors. If your result lands near the action level, our article on what 4.0 pCi/L actually means explains how to read it.
Reading local radon data on RadonResources.com
Our state radon pages combine official zone information with readings homeowners have submitted. Local readings can show that elevated results are common in your area and give you a feel for the range people are seeing. They can’t predict your own home’s number, and they lean toward higher results because of who chooses to share. Treat them as one more reason to test, not as a replacement for it.
Frequently asked questions
If I live in Zone 3, do I still need to test for radon?
Yes. The EPA recommends testing every home regardless of zone, and elevated radon has been found in homes in all three zones. A Zone 3 designation means the county’s predicted average is lower, not that individual homes are free of radon. A test is the only way to know your home’s level.
Is the EPA radon zone map still accurate?
The map was created in 1993 and still reflects broad regional patterns, since the geology behind radon doesn’t change quickly. It doesn’t include decades of newer test data, and it was never meant to describe individual homes. The EPA suggests pairing it with local data, such as results published by your state radon program.
Can I use a radon map instead of testing when buying a home?
No. A map can’t tell you the radon level in a specific house, and state programs such as Colorado’s say maps shouldn’t replace testing in real estate transactions. If you’re buying, consider a radon test during the inspection period. Our guide for real estate agents on radon covers how testing usually fits into a sale.
Why do radon maps from different websites disagree?
They’re built from different data. The EPA map combines geology and older measurements into county predictions. State maps often summarize test results reported to the state. Commercial maps may use results from one company’s kits. Different sources, sample sizes and time periods produce different pictures, which is another reason to rely on a test of your own home.
A map is a starting point
Radon maps are good at showing where radon is a regional concern and at encouraging people to test. They’re misleading when a county color is used to decide whether one home is safe. Use the map to understand your area, then test your house to learn your actual number. If you’d like help with testing or a high result, you can connect with a certified radon professional near you.
Sources and further reading
- U.S. Environmental Protection Agency, EPA Map of Radon Zones (1993 development, data sources, intended use, local data).
- U.S. Environmental Protection Agency, Radon Zones Map (zone definitions, elevated homes in all zones, test all homes).
- Colorado Department of Public Health and Environment, Understanding Radon (Zone 1 statewide, about half of homes above 4 pCi/L, maps not a substitute for testing).
- U.S. Environmental Protection Agency, A Citizen’s Guide to Radon (action levels, testing guidance, no known safe level).
- RadonResources.com user-submitted radon readings (self-reported and not a representative sample).
This article is for general educational purposes and is not medical advice. Long-term radon exposure causes lung cancer, but individual risk depends on the level, the length of exposure and smoking history. For guidance on your home, consult a certified radon professional.
