What does the EPA radon zone tell me about a house?
The EPA radon zone is a county-level prediction of average indoor radon, made in 1993 from geology, aerial radioactivity, soil permeability and the indoor measurements that existed then. It says whether the county as a whole is expected to run high, moderate or low. It does not say what the level is in any particular house, and the EPA says so directly: every home should be tested, whatever its zone.
Reviewed by Spatia's editor on 2026-09-30; sources checked 2026-09-30.
The mistake almost everyone makes
Skipping the test because the county is Zone 3. Radon enters through the particular slab, crawlspace and foundation cracks of a particular house, and two houses on the same street routinely test very differently. The EPA's map exists to help governments target resources, and its own page states it should not be used to decide whether an individual home needs testing. The zone tells you how surprised to be by a high result; only a test tells you the result. A test costs little and takes days, so the map is never the reason to skip it.
How to check it
Read the zone as a county-wide prior
Zone 1 counties have a predicted average indoor screening level above 4 picocuries per litre, Zone 2 between 2 and 4, Zone 3 below 2. Those are county averages from a 1993 assessment, and the map has not been redrawn since; treat it as a prior about the area, not a reading for the house.
Test the house, inside the contingency period
A short-term test under closed-house conditions takes two to seven days and can be done by a certified tester as part of the inspection; most state radon programs list who is certified. Ask for the result in writing, with the test location and dates.
Act on the number, not the zone
The EPA's advice is to fix a home at 4 picocuries per litre or above, and to consider fixing between 2 and 4. Mitigation is routine work, usually a sub-slab depressurisation system, and its cost is a negotiable item in a purchase.
Ask for existing results and systems
Ask the seller for any prior test and for the service history of any mitigation system already installed. Some states require radon results to be disclosed on sale; where none does, the question still is.
What this still does not tell you
- It does not measure anything in the house; it predicts a county average from 1993 data.
- It is not a health assessment for the people who will live there.
- It does not say what a state's disclosure rules require of the seller.
- It is not a substitute for a radon test, and it is not legal or medical advice.
Who actually decides
- US EPA — Map of Radon Zones, and its statement on how it should be used
- US EPA — radon: testing and the action level
- Your state radon program — lists certified testers and mitigation contractors, and states any disclosure rule