EPA Radon Zones by County Explained (Zones 1, 2, 3)

EPA radon zones by county are a screening map, not a verdict on any single house. The EPA Map of Radon Zones assigns every U.S. county to one of three zones based on predicted average indoor radon: Zone 1 counties have a predicted average above 4 pCi/L (the EPA action level), Zone 2 falls between 2 and 4 pCi/L, and Zone 3 is below 2 pCi/L. The map was built in 1993 to help states target resources and building codes — EPA itself says it "should not be used to determine if individual homes need to be tested." Every home should be tested, in every zone, because radon varies house to house on the same street.

What Radon Is and Why It Ends Up in Houses

Radon is a naturally occurring radioactive gas produced when uranium in soil and rock breaks down. You can't see it, smell it, or taste it. It seeps out of the ground everywhere, and outdoors it disperses harmlessly. Problems start when it enters an enclosed building — through cracks in slabs and foundation walls, sump pits, crawlspace floors, utility penetrations, and construction joints — and accumulates.

The health stakes are why EPA treats radon as a top-tier indoor hazard. According to EPA's health risk page, radon is the leading cause of lung cancer among nonsmokers and is estimated to cause about 21,000 lung cancer deaths in the U.S. each year. EPA also estimates that roughly 1 in 15 American homes has an elevated radon level (at or above the 4 pCi/L action level).

Two numbers frame every radon conversation:

• 4 pCi/L — EPA's action level. Fix the home at or above this level.
• 2–4 pCi/L — EPA says consider fixing, since no level of radon is considered risk-free. For reference, the World Health Organization recommends a lower reference level of 100 Bq/m³, roughly 2.7 pCi/L.

The EPA Map of Radon Zones, County by County

The Map of Radon Zones was developed in 1993 by EPA working with the U.S. Geological Survey. It combined indoor radon measurements, geology, aerial radioactivity surveys, soil permeability, and foundation types to estimate the radon potential of each county. The purpose was practical: help national, state, and local organizations target radon programs, and help code officials decide where radon-resistant new construction requirements make the most sense.

What Zone 1, Zone 2, and Zone 3 Mean

Zone	Map color	Predicted average indoor screening level	Radon potential
Zone 1	Red	Greater than 4 pCi/L	Highest
Zone 2	Orange	2 to 4 pCi/L	Moderate
Zone 3	Yellow	Less than 2 pCi/L	Lowest

Read that middle column carefully: the zone describes a predicted county-wide average, not a measurement of your house. A Zone 1 county will contain plenty of homes that test low. A Zone 3 county will contain homes that test high — sometimes very high. Geology shifts over short distances, and construction details matter enormously: a foundation crack, a leaky crawlspace, or a sump pit that connects directly to soil gas can push one house far above its neighbors.

How to Look Up Your County's Radon Zone (Free)

1. Open EPA's Map of Radon Zones and Supplemental Information page.
2. Use the printable national map for a quick look, or open the state-by-state maps and supporting documents to find your specific county.
3. Cross-check with state data. Many state radon programs publish county-level results from actual indoor tests, which are more current than the 1993 predictions. EPA's same page links each state's radon contact through the ten EPA regional offices.
4. For measured (not predicted) data, EPA points to the CDC Environmental Public Health Tracking Network, which aggregates test results from participating states and national radon labs.

What the Zone Map Is — and Is Not — For

The map is for policy and prioritization: where states should spend outreach money, and where builders should be required to install radon-resistant features in new construction. It is not a substitute for testing, and EPA says so directly on the map page: no matter where you live, test your home.

This is the single most misunderstood point about EPA radon zones by county. Buyers in Zone 3 counties routinely skip radon testing because "the map says we're low risk." That reasoning fails because the map predicts an average, and no one lives in an average house. EPA's A Citizen's Guide to Radon recommends testing all homes below the third floor — regardless of zone, age, or foundation type.

Radon Testing in a Real Estate Transaction

If you're buying a house, the inspection contingency window is when radon testing happens. Here's how it typically works:

1. Order the test with your inspection. Many home inspectors are certified to run radon tests, or you can hire a dedicated radon measurement professional. A professional test during a home purchase generally costs roughly $150–$400, and sometimes more depending on region and device — HomeAdvisor puts the full professional testing range at about $150–$800.
2. Use a short-term closed-house test. Real estate transactions almost always use a 48-hour (minimum) short-term test, usually with a continuous radon monitor (CRM) that logs hourly readings. EPA testing protocol requires closed-house conditions: windows shut and exterior doors closed except for normal entry and exit, starting at least 12 hours before the test begins and continuing throughout.
3. Place the device correctly. The test goes in the lowest level of the home that is or could be lived in — typically the basement if it could be finished, otherwise the first floor — away from drafts, exterior walls, high humidity, and sumps.
4. Get the report and negotiate. If the result is at or above 4 pCi/L, EPA's guidance for real estate is to fix the home. In practice, buyers request that the seller install a mitigation system or credit the cost at closing. This is a routine negotiation, not a deal-killer.

A few things can distort a transaction test, so treat them as part of step 2 rather than an afterthought: severe storms and high winds can swing readings, so professionals may note weather holds in the report; closed-house violations (someone airing out the basement mid-test) invalidate results; and CRM hourly logs make tampering visible as sudden dips. Interfering with a radon test during a real estate transaction is actually illegal in some states — Illinois, for example, imposes penalties for it. If conditions were compromised, the fix is simple: retest, usually for the cost of another measurement visit.

EPA's Radon Resources for the Real Estate Community page collects the transaction-specific guidance, including the current Home Buyer's and Seller's Guide to Radon (revised March 2024). Radon is one line on a longer list of checks during your contingency period — our home buyer due-diligence checklist walks through the rest, from flood zones to nearby Superfund sites.

Radon Mitigation Systems and What They Cost

The standard fix is an active soil depressurization (ASD) system: a PVC pipe is run from a sealed penetration in the slab (or from under a membrane in a crawlspace) to an inline fan that continuously pulls soil gas from beneath the foundation and vents it above the roofline. EPA notes that properly designed systems can reduce indoor radon by up to 99%. Installation typically takes a day.

Costs are moderate and fairly predictable. Per 2026 cost data, HomeAdvisor puts the national average at about $1,000, with a typical range of roughly $790–$1,275 and complex installs running up to about $2,500; HomeGuide's 2026 survey puts a typical active soil depressurization install at $1,200–$2,000, with $2,500+ possible for larger homes or difficult foundations. Crawlspace encapsulation, multiple suction points, or routing pipe through finished space push toward the top of those ranges.

Budget guidance for buyers: if a transaction test comes back elevated, a roughly $1,000–$2,000 line item covers mitigation for most single-family homes, and that's a reasonable number to bring to the negotiating table.

After installation:

• Retest within 30 days (but no sooner than 24 hours after the fan starts) to confirm the system works.
• Check the manometer — the U-shaped gauge on the pipe — periodically; it confirms the fan is pulling suction.
• Retest every two years, per EPA guidance, and after any major renovation or HVAC change.

New construction in many high-potential areas includes passive radon-resistant features (a vent pipe and sealed sub-slab layer without a fan). These help but do not guarantee low levels — passive systems should be tested too, and can be activated by adding a fan if needed.

State Radon Programs, Licensing, and Free Help

Radon oversight is largely state-level, funded in part by EPA's State and Tribal Indoor Radon Grants (SIRG) program. A number of states license or otherwise regulate radon testers and mitigators — requirements range from full state licensure to mandatory national certification — while others rely entirely on voluntary credentials. The two national certifying bodies are the National Radon Proficiency Program (NRPP) and the National Radon Safety Board (NRSB); Kansas State University's National Radon Program Services maintains state-by-state certification information. Before hiring anyone, check your state's rules through your state radon contact — many state programs also offer free or discounted test kits to residents.

Free national resources, listed on EPA's radon hotlines page:

• 1-800-SOS-RADON (1-800-767-7236) — order low-cost test kits by automated line.
• 1-800-55RADON (1-800-557-2366) — talk to a live information specialist.
• EPA's guide to finding a test kit or qualified professional.

Everything above is free public information — the zone maps, the state contacts, the testing protocols. The work is in checking all of it, for one specific address, before your contingency clock runs out.

Frequently Asked Questions

Does a Zone 1 county mean my house has high radon?

No. Zone 1 means the predicted county average screening level exceeds 4 pCi/L, so the odds of an elevated result are higher — but only a test tells you about your house. Plenty of Zone 1 homes test low. If yours tests high, mitigation typically runs about $1,000–$2,000 for most single-family homes and reliably brings levels down.

Can I skip the radon test because my county is Zone 3?

EPA says no. The zone map was never designed to make house-level decisions, and elevated homes are found in Zone 3 counties every year. A 48-hour test during your inspection period costs a small fraction of what you're spending on the inspection itself, and it's the only way to know.

How long does a radon test take when buying a house?

The minimum is 48 hours under closed-house conditions, with windows and doors shut starting at least 12 hours before the test. Most transaction tests use a continuous radon monitor and fit inside a standard 7–10 day inspection contingency with room to spare, even if a retest is needed.

Do radon mitigation systems affect resale value?

Generally they help. A documented, functioning system with a post-installation test result removes an unknown for future buyers, and in disclosure states you'd have to report a known elevated level anyway. Keep the installation paperwork and your most recent test result with your house records.

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Radon is one of ten categories worth checking before you commit to a house — and like flood zones, wildfire exposure, and nearby contamination sites, the official data is scattered across separate federal tools. Property Red Flags runs all of them in one pass for a single U.S. address: a $19 one-time report with a verdict, evidence for every finding (source, date, distance — including your county's EPA radon zone), and a what-to-verify action plan for before the offer and before closing. See a sample report or check your address.