Radon Levels by County: EPA Zones and County Records for the United States

What are the key radon statistics by county?

These findings are the page's fastest, independently quotable findings. Every line identifies the figure, source vintage, qualifier and named primary source so it remains accurate when read outside the surrounding article.

1. 1EPA's September 2025 county radon file contains 3,144 county and county-equivalent source records: 1,072 Zone 1 (34.10%), 1,035 Zone 2 (32.92%) and 1,037 Zone 3 (32.98%). Source: Cedar Rapids Radon Mitigation Research calculation from the U.S. EPA Map of Radon Zones county file, recomputed July 24, 2026.
2. 2EPA radon zones are predicted potential, not measurements: Zone 1 means average indoor radon levels may be greater than 4 pCi/L; Zone 2 means they may be between 2 and 4 pCi/L; Zone 3 means they may be less than 2 pCi/L. Source: U.S. Environmental Protection Agency, EPA Map of Radon Zones, page updated October 14, 2025.
3. 3EPA developed the nationwide radon-zone model in 1993 using indoor radon measurements, geology, aerial radioactivity, soil parameters and foundation types. Source: U.S. Environmental Protection Agency, EPA Map of Radon Zones, verified July 24, 2026.
4. 4Iowa and North Dakota are the only states in which every EPA source record is Zone 1: 99 of 99 records in Iowa and 53 of 53 in North Dakota. Source: Cedar Rapids Radon Mitigation Research calculation from EPA's September 2025 county file, recomputed July 24, 2026.
5. 5Twelve states and the District of Columbia have zero Zone 1 source records: Alaska, Arizona, Arkansas, Delaware, Florida, Hawaii, Louisiana, Mississippi, Oklahoma, Oregon, Texas, Vermont and the District of Columbia. Source: Cedar Rapids Radon Mitigation Research calculation from EPA's September 2025 county file, recomputed July 24, 2026.
6. 6Of EPA's 3,144 source records, 3,127 directly match a 2025 Census county or county-equivalent name and 17 retain a legacy or otherwise unmatched geography label. Source: Cedar Rapids Radon Mitigation Research crosswalk of the EPA September 2025 file and the deduplicated 50-state-and-DC comparison set built from the U.S. Census Bureau 2025 County Adjacency File, completed July 24, 2026.
7. 7EPA states that homes with elevated radon have been found in all three zones and that the map should not determine whether an individual home is tested. Source: U.S. Environmental Protection Agency, EPA Map of Radon Zones, verified July 24, 2026.
8. 8EPA recommends fixing a home at 4 pCi/L or higher and considering action between 2 and 4 pCi/L; EPA also states that there is no known safe level of radon exposure. Source: U.S. EPA action-level guidance, page updated November 12, 2025.
9. 9EPA reports an average indoor radon concentration of about 1.3 pCi/L in U.S. homes and an average outdoor concentration of about 0.4 pCi/L. Source: U.S. EPA action-level guidance, verified July 24, 2026.
10. 10EPA's 2003 risk assessment estimated about 21,000 U.S. lung-cancer deaths per year from indoor radon, with an uncertainty range of 8,000 to 45,000; this is a modeled risk estimate, not a counted annual death toll. Source: U.S. EPA, Health Risk of Radon and Assessment of Risks from Radon in Homes (EPA 402-R-03-003), verified July 24, 2026.
11. 11EPA estimates that nearly 1 in 15 U.S. homes has a radon level that should be reduced. Source: U.S. EPA, Radon in Homes, Schools and Buildings, verified July 24, 2026.
12. 12As of June 30, 2026, CDC reports state- and county-level radon testing data for most of the continental United States from some Tracking Program-funded states and several national testing laboratories; EPA's supplemental page identifies six national laboratories. Sources: CDC Radon Testing and U.S. EPA supplemental information.
13. 13CDC's county measures can include the number of buildings tested, the number and percentage of tests by radon range, and median and maximum pre-mitigation levels in tested buildings. Source: CDC National Environmental Public Health Tracking Program, page dated June 30, 2026.
14. 14Iowa HHS reports that the Iowa Radon Survey found 71.6% of Iowa homes above EPA's 4 pCi/L action level, the largest percentage of any state; the public page does not state the survey year, sample size or sampling method. Source: Iowa Department of Health and Human Services, Radon Resources, verified July 24, 2026.
15. 15Linn County, Iowa, is EPA Radon Zone 1 and has current Census GEOID 19113; that is a county-level potential classification, not a measured county average and not a result for an individual building. Sources: EPA September 2025 county file and the deduplicated 50-state-and-DC comparison set built from the U.S. Census Bureau 2025 County Adjacency File, verified July 24, 2026.

What do radon levels by county show in the EPA file?

EPA's September 2025 download assigns Zone 1, 2 or 3 to 3,144 county and county-equivalent source records. Each row below preserves the EPA classification and identifies whether the source name directly matches a current 2025 Census geography; the zone is a potential category, not a measured county average.

EPA radon zones describe predicted county-level potential. They are not measured averages, and no zone value describes the radon concentration inside a particular building. Testing is the only way to establish a building's concentration.

How to read a county record

• EPA radon zone — a category, not a measured number of picocuries. Zone 1 is the highest predicted potential and Zone 3 the lowest.
• pCi/L — picocuries per liter, the unit used for radon concentration in air in the United States. EPA recommends action at 4 pCi/L or higher and consideration of action from 2 to 4 pCi/L.
• Predicted average range — EPA's wording for the zone. It describes the model's county-level potential, not a particular building.
• Source record — one row in EPA's file. It is not a count of homes, tests, people or current Census geographies.
• Geographic scope — EPA's source records and the state summary cover the 50 states and the District of Columbia. Puerto Rico and the Island Areas are outside this release.
• Current Census geography — the 2025 Census name attached only where the state-and-name match is direct.
• Verification tier — every published classification record in this release is ★, meaning it was extracted from or read directly against the named primary source.

Table 1. What is the national distribution of EPA radon zones?

Source: U.S. Environmental Protection Agency, EPA Map of Radon Zones, county file identified by EPA as September 2025. Distribution recomputed by Cedar Rapids Radon Mitigation Research on July 24, 2026.
One count note: EPA's current page identifies a September 2025 download with 3,144 Zone 1, 2 or 3 source records; an older EPA fact sheet says the map assigned each of 3,141 counties to a zone. This page uses the reproducible row count from the released file.

Table 2. What is the EPA radon zone for every source county and county equivalent?

All 3,144 published source records. The downloadable CSV and JSON carry the full 27-field schema, source URLs, verification date and geography notes. Select your state: