A radon map shows where high radon is more likely. A well test shows what’s in your water. Neither tells you what’s in the air of your home, so both questions lead back to the same step: test the indoor air.
The two topics sit together here because both get used as a stand-in for that test, and both can mislead. If you’re deciding whether to test, or someone has quoted your county’s zone at you, start with the map. If your water comes from a well, or you’re holding a water result, read the water section too. Levels below are in picocuries per liter (pCi/L), of air or of water, and the two scales are not the same.
Radon Handbook sells no tests or systems and refers no one. The figures here come from the two guides linked below, where each has its full source and date.
What the zone map says
EPA’s Map of Radon Zones, which EPA still dates to 1993, sorts every US county into one of three zones. The levels are predictions for a whole county, not measurements of any home.
| Zone | EPA’s description | Average indoor level | Counties (our count) |
|---|---|---|---|
| Zone 1 | Highest potential | May be greater than 4 pCi/L | 1,072 |
| Zone 2 | Moderate potential | May be between 2 and 4 pCi/L | 1,035 |
| Zone 3 | Low potential | May be less than 2 pCi/L | 1,037 |
The map was made for planners, to help organizations target their resources and implement radon-resistant building codes. EPA says plainly that it “should not be used to determine if individual homes need to be tested.” So “zone 3, no need to test” is a claim EPA’s own page contradicts, and “we’re in Zone 1” doesn’t prove a problem in one house either.
Why a low zone isn’t an answer
High homes turn up outside the top zone. EPA’s 1993 report estimated that about 3.8 million of 5.4 million homes at or above 4 pCi/L were in Zone 1 counties. By our subtraction, that leaves about 1.6 million, or 3 in 10, elsewhere.
Newer work points the same way. A 2025 PNAS paper used more than 6 million short-term tests from 2001 to 2021 to estimate levels by ZIP-code area across the lower 48 states. In its lowest band, areas estimated under 1 pCi/L, 7.5% of tests were still above 4 pCi/L. The authors say their community estimates shouldn’t be applied to individual buildings.
Our guide to what radon maps can’t tell you has the full picture: a county count for every state, how the map was built, what the newer study found, and why Oregon shows the limits of a 1993 file.
State and ZIP code data
State data can be a closer map, because it comes from actual tests in the area. It still stops short of your house: Oregon rates risk by ZIP code and says its data “should not be used to substitute for radon testing of an individual home”. Three checks help with any local figure:
- How many tests? A handful can move an average a long way, and a blank area may simply be short of tests.
- Who tested? Most data comes from people who chose to test or were buying a house, not a random sample.
- What share is above 4? An average can look low while some homes test high.
That’s why this site has no ZIP lookup. A box that returned a low-sounding word for your address would imply something no map can know.
When water matters
Radon in water is a concern mainly for homes on a private well or another groundwater supply. The Consumer’s Guide (2025 cover) says it usually isn’t a problem when the source is surface water. The gas escapes when you shower, wash clothes or run the dishwasher, and EPA says most of the risk comes from breathing that released radon, not from drinking the water. In the Citizen’s Guide (2025 cover), EPA says your risk of lung cancer from breathing radon in air is “much larger than your risk of stomach cancer from swallowing water with radon in it.”
EPA also says radon from the soil is usually a much larger risk than radon from water. The zone map won’t help here, since it’s about indoor air, and Maine says air radon doesn’t predict water radon, or the other way round. Only a test of your own water will tell you. Our full guide to radon in water and well testing covers sampling, the state numbers, treatment and published prices.
A water number isn’t an air number
This is the mistake to avoid. EPA’s 2012 report to Congress says that with typical water use, on average, 10,000 pCi/L in tap water adds about 1 pCi/L to indoor air. So 4,000 in water is not the same kind of number as 4 in air: on that average, it would take about 40,000 pCi/L in water to add 4 pCi/L to the air. It’s a rough average, not a measurement of your home, and yours can land higher or lower.
Advice, not limits
There is no federal limit on radon in water. EPA proposed one in 1999, and its rule record, updated 9 June 2026, says: “A Final Rule has not published yet.” EPA says of private wells: “These wells are not regulated by EPA or required to follow EPA’s standards.” The six states we read give advice instead, with numbers from 2,000 to 10,000 pCi/L, and they don’t agree with each other. Use your own state’s number, and ask your state radon office if you can’t find one.
Testing and treating water
An air kit can’t test water: you need a lab’s water kit. Treatment is a separate job from an air system too. A soil-gas system doesn’t take radon out of the water, and water treatment doesn’t stop soil gas. EPA says a tap filter isn’t effective against radon released from all the water used in the home, so treatment goes where water enters the house.
EPA says the usual choices are aeration, typically removing 95 to 99 percent, and granular activated carbon, typically 85 to 95 percent. Sources differ most on carbon: Vermont “discourages the use of GAC systems to remove radon,” because radon on the filter could create a radiation hazard.
What to do next
Whatever the map shows and whatever your water source, EPA’s advice on its zone page is the same:
“No matter where you live, test your home for radon—it’s easy and inexpensive. Fix your home if your radon level is 4 picocuries per liter (pCi/L) (150 becquerels per meter cubed (Bq/m3)) or higher. Consider fixing if your level is between 2 and 4 pCi/L (75 – 150 Bq/m3).”
4 pCi/L is EPA’s action level, a decision point and not a safety line. For wells, EPA’s Citizen’s Guide adds: “If you’ve tested the air in your home and found a radon problem, and your water comes from a well, have your water tested.” Connecticut and Maine go further and recommend a water test for homes on wells, so if you’re on one, testing both at once is the simple route: two numbers show which source matters more.
Once you have an air number, the radon test result reader gives EPA’s next step. Then ask:
- Your state radon office: what do tests near me show, how many are there, what level do you use for water, and which labs do you accept?
- Your water supplier, on public water: do you use groundwater, and what were your latest radon results?
- A seller or agent: is there an actual test result for this house, on which floor, when, and was a system running?
This page is general information, not medical, engineering or legal advice. Sources checked on 4 November 2026.
