EPA Radon Zone 1: What the Highest Radon Rating Actually Means

EPA Radon Zone 1 is the agency’s highest radon classification: counties where the predicted average indoor radon level is 4 picocuries per liter (pCi/L) or higher — at or above the level where the EPA says fix it. Roughly one in three US counties carries the Zone 1 designation, cutting across wealthy suburbs and farm towns alike.

Here’s what the rating does and doesn’t tell you about a specific house, what it means during a purchase, and why a $150 test settles what the map can only estimate.

What Zone 1 means — and what it doesn't

The EPA assigned every US county to one of three zones based on geology, aerial radioactivity surveys, soil permeability, and indoor measurements:

Two crucial caveats cut both ways:

• Zone 1 doesn't mean every house is high. The rating is a county-level prediction. Two houses on the same street can test 2 pCi/L and 12 pCi/L — construction, foundation type, and soil contact differ house to house.
• Zone 3 doesn't mean safe. Elevated homes are found in every zone, which is why the EPA recommends testing every home regardless of the map. The zone shifts your prior, not your answer.

What Zone 1 does mean: the odds are materially against you. In many Zone 1 counties, half or more of homes test above the action level. Walking into a Zone 1 purchase without a radon test is a bet you don't need to make.

Why this matters: radon is the #2 cause of lung cancer

Radon is an invisible, odorless radioactive gas produced by uranium decay in soil. It seeps in through foundation cracks, sump pits, and slab joints, and accumulates in lower living levels. The EPA attributes about 21,000 US lung cancer deaths per year to radon exposure — second only to smoking. The risk is cumulative and dose-dependent, which is why the number in your house is what matters, not the county average. See acceptable radon levels for how to read a test result.

Buying a home in Zone 1: the playbook

1. Make the offer contingent on a radon test. In Zone 1 counties this is normal practice, not paranoia. A professional test during inspection runs $150–$300; a 48-hour charcoal kit is under $30.
2. If the result is ≥ 4 pCi/L, negotiate mitigation — not panic. A standard active sub-slab depressurization system costs $800–$2,500 installed and reliably drops most homes below 2 pCi/L. Sellers in Zone 1 markets expect this ask.
3. Already-mitigated house? Verify, don't assume. Ask for the post-mitigation test result and check the system's manometer (the U-tube gauge on the pipe) shows suction. Fans last 10–15 years; budget ~$300–$500 for eventual replacement.
4. Re-test every 2 years and after any foundation or HVAC work — radon levels drift as houses settle and soil gas paths change.

Disclosure varies wildly by state

Some states require sellers to disclose known radon test results; many don't, and almost none require testing itself. In a Zone 1 county, “no known radon issues” on a disclosure form most often means “never tested.” Ask the direct question: has this home ever been tested for radon, and what was the number? Check where your state falls in our radon levels by state table.