Radon Map: Test Results by State vs EPA's 1993 Zones

North Dakota, Nebraska, South Dakota and Iowa top the radon map at 58.5 percent or more of tests high

In CDC's lab data for 2008 to 2017, North Dakota leads at 59.3 percent of tests at or above 4 pCi/L, and Nebraska, South Dakota and Iowa sit at 58.5 percent. The 47-jurisdiction figure is 35.7 percent.

pCi/L: Picocuries per liter of air, the US unit for radon. EPA recommends fixing a home at 4 pCi/L (150 Bq/m3) or more and considering a fix between 2 and 4.

CDC's Environmental Public Health Tracking Network collects radon test counts from laboratories and splits them into three bands: under 2 pCi/L, 2 to under 4, and 4 or higher. KBR pulled every county row from the Tracking Network API for the latest 10-year lab window, 2008 to 2017, and summed the counts to states. That gives 2,013,544 pre-mitigation tests, 718,943 of them at or above 4 pCi/L.

The top ten run from North Dakota (59.3 percent) through Ohio (52.3 percent), Colorado (47.0 percent), Wisconsin (46.9 percent), Montana (45.5 percent), Wyoming (44.3 percent) and Kentucky (43.7 percent). The Gulf Coast sits at the bottom: Louisiana 2.2 percent, Texas 10.2 percent. Massachusetts sent the most tests, 234,152, with 23.8 percent high.

EPA's 1993 zones sort counties in the right order, but 15.3 percent of Zone 3 tests still hit 4

Matching 2,013,541 county-level lab tests to EPA zones, the share of high results falls from 42.4 percent in Zone 1 to 15.3 percent in Zone 3. The map works as a ranking and fails as an all-clear.

The EPA Map of Radon Zones was built in 1993 from indoor measurements, geology, aerial radioactivity, soils and foundation types. Its legend describes Zone 1 as counties where average indoor levels may be greater than 4 pCi/L, Zone 2 as between 2 and 4, and Zone 3 as under 2. EPA says the map should not be used to decide whether a home needs testing. Its current county list sorts 3,144 counties into 1,074 in Zone 1, 1,034 in Zone 2 and 1,036 in Zone 3.

High results in Zone 1 counties were 2.8 times as common as in Zone 3. But 26,315 tests from Zone 3 counties read 4 pCi/L or higher. Among the 158 Zone 3 counties with at least 100 tests, the median county share was 9.6 percent, and 19 counties reached 25 percent or more. The reverse miss is rare: 5 of 676 Zone 1 counties with 100 or more tests came in under 15 percent.

Testing tracks the map. Zone 1 counties produced 60.5 percent of all lab tests with 26.3 percent of residents: 14.23 tests per 1,000 people over ten years, against 1.37 in Zone 3. Fewer tests in low zones means fewer high results get found there, which is one reason the low zones look quiet.

Seven states with almost no Zone 1 residents test like high-radon states

Utah, New Hampshire, Oregon, Nevada, Florida, North Carolina and Vermont each have under 10 percent of residents in Zone 1, yet 26.3 percent of their 216,364 lab tests read 4 pCi/L or higher.

Set each state's share of high tests beside the share of its people the 1993 map placed in Zone 1, and two groups stand out. Utah (41.9 percent of tests high, 4.1 percent of residents in Zone 1) and New Hampshire (35.0 percent, 3.7 percent) sit above or just below the 47-jurisdiction figure of 35.7 percent. Oregon, Florida and Vermont have no Zone 1 county at all and still return more than 20 percent high results.

The opposite pattern appears in Connecticut and Maryland. Connecticut places 62.6 percent of residents in Zone 1 but only 26.5 percent of its lab tests were high. Connecticut's own program data tell a different story: 44.9 percent of 31,488 state-reported tests from 2014 to 2019 were at or above 4. Two sources, two answers, which is why this report treats single-state gaps as leads, not verdicts.

Florida shows how a county can beat its zone. Collier County, in Zone 3, logged 20,308 lab tests with 25.9 percent at or above 4 pCi/L; Lee County logged 11,873 with 25.1 percent.

State programs' newer counts run through 2023, and 7 of 9 steady reporters came in lower

22 states sent annual test counts to CDC for at least one year from 2014 to 2023: 3,891,265 tests, 30.1 percent at or above 4 pCi/L. Only 11 reported 2023.

The second CDC series comes from state radon programs rather than labs, one year at a time. It is newer but thinner: 11 states reported 2023 (128,740 tests, 33.4 percent high), and the mix of states changes almost every year, so a yearly national line would mostly track who reported. KBR therefore compares each state with itself.

Nine states reported every year from 2014 to 2023. In 7 of them (Colorado, Florida, Minnesota, Nebraska, Oregon, Utah, Vermont) the 2019 to 2023 share is below the state's 2008 to 2017 lab share. Nebraska moved from 58.5 percent in lab data to 40.3 percent in 2019 to 2023 program data. The two series come from different collectors, so the gap is a prompt for state-level checks, not proof that homes got safer.

Two outliers look like definition differences: New Jersey reported 950,679 tests with 11.4 percent high, and Rhode Island 1,756 tests with 2.3 percent high against 23.8 percent in Rhode Island's lab data.

16 states make sellers hand over a radon notice or test results; 15 have no seller rule

KBR classified every radon law in ELI's 2026 database for 50 states and DC. 16 require a radon-specific notice, pamphlet or test results at sale, 7 require radon-resistant features in all new homes, and 15 license or certify mitigators themselves.

The Environmental Law Institute radon excerpt lists 155 state law entries enacted through June 2026. KBR read each and asked three questions: does a seller have to tell a buyer about radon, must new homes be built radon-resistant, and who may install a mitigation system. Minnesota's Radon Awareness Act is typical of the strongest seller rules: a written disclosure, a radon warning statement and a state publication, in force since January 1, 2014.

Only Illinois, Iowa, Minnesota and Nebraska have the strongest form of all three rules; 48.8 percent of their 377,848 lab tests were high. 12 states have none of the three, including Wyoming (44.3 percent high) and Idaho (41.5 percent). Across the no-rule states with lab data, 28.8 percent of 151,353 tests were high. Of the ten states with the highest share of high tests, 6 have a radon-specific seller rule.

What 4 pCi/L means next to the 1.3 national average and EPA's 21,000 estimate

EPA's 4 pCi/L action level is about three times its 1.3 pCi/L estimate of the average US home, and ten times the 0.4 pCi/L outdoor average. The 21,000 deaths figure is a model built on that average, not a count.

EPA estimates that radon causes about 21,000 lung cancer deaths a year, about 2,900 among people who never smoked. At 1.3 pCi/L, EPA's risk table gives a never smoker about a 2 in 1,000 lifetime chance of lung cancer and a smoker about 20 in 1,000. The World Health Organization's 2009 handbook set a lower reference level of 2.7 pCi/L where possible; EPA kept 4 pCi/L and points to its existing advice to consider fixing from 2 to 4.

The national average itself comes from a residential survey completed in 1991. EPA's FAQ still cites that 1991 survey, which is why test-result shares such as those above are the most current state picture available, with their selection bias.

Testing, finished basements and what mitigation costs, from agency guidance

CDC and EPA describe short-term tests of 2 to 90 days and long-term tests of more than 90 days. Minnesota's health department puts typical mitigation systems at $1,500 to $3,000.

CDC advises testing when a home has never been tested, when buying or selling, before and after renovations, and before anyone starts spending more time in a basement or lower level, such as when a basement becomes a bedroom or office. The test goes in the lowest lived-in level. For homeowners planning a lower-level remodel, that puts the test before drywall goes up; see the remodel timeline for where it fits.

EPA does not publish a dollar figure; it says most homes can be fixed for about the same cost as other common home repairs and that some systems cut radon by up to 99 percent. The Minnesota Department of Health gives a range of $1,500 to $3,000. Twenty jurisdictions require a state credential or national certification for mitigators; elsewhere, the questions for choosing a contractor apply, and the contractor directory lists remodelers by city.

State radon laws: disclosure at sale, new construction and mitigator credentials (through June 2026)

Checked annually, when ELI updates its database. Last checked 2026-11-02. Entries are neutral records of public data; each links to its source record.

State-by-state findings

1. 35.7 percent of 2,013,544 pre-mitigation radon tests reported by labs to CDC for 2008 to 2017 read 4 pCi/L or higher, across 46 states and DC.^[1]
2. North Dakota had the highest share of high lab results, 59.3 percent of 6,607 tests, 2008 to 2017.^[1]
3. Nebraska (58.5 percent of 42,782), South Dakota (58.5 percent) and Iowa (58.5 percent of 95,245) follow, 2008 to 2017.^[1]
4. Louisiana had the lowest share, 2.2 percent of 499 lab tests, followed by DC at 8.7 percent and Texas at 10.2 percent, 2008 to 2017.^[1]
5. In EPA Zone 1 counties, 42.4 percent of 1,217,840 lab tests read 4 pCi/L or higher, 2008 to 2017.^[1][4]
6. In EPA Zone 3 counties, where the 1993 map predicts averages under 2 pCi/L, 15.3 percent of 172,195 lab tests still read 4 pCi/L or higher.^[1][4]
7. 19 of 158 Zone 3 counties with 100 or more lab tests had 25 percent or more of tests at or above 4 pCi/L; King George County, Virginia, led at 45.5 percent.^[1][4]
8. Zone 1 counties produced 60.5 percent of lab tests while holding 26.3 percent of residents in the 47 jurisdictions with data; Zone 3 produced 8.6 percent of tests for 38.8 percent of residents.^[1][4][5]
9. Utah has 4.1 percent of residents in Zone 1 but 41.9 percent of its 28,342 lab tests read 4 pCi/L or higher.^[1][4][5]
10. Oregon has no Zone 1 county, yet 25.4 percent of 23,951 lab tests read 4 pCi/L or higher, 2008 to 2017.^[1][4]
11. EPA puts 26.2 percent of US residents (89,540,604 people in 2025) in Zone 1 counties.^[4][5]
12. 16 of 51 jurisdictions require a radon-specific notice, pamphlet or test results at sale; 15 have no seller radon rule in ELI's database through June 2026.^[20]
13. 7 jurisdictions (Connecticut, DC, Illinois, Iowa, Maine, Minnesota, Nebraska) require radon-resistant features in all new homes; 6 more require them only in Zone 1 or named areas.^[20]
14. Wyoming ranks ninth for high lab results (44.3 percent) and has none of the three radon laws tracked here.^[1][20]
15. EPA estimates radon causes about 21,000 lung cancer deaths a year in the US, about 2,900 of them among people who never smoked.^[7]

Rankings and ratios we derived

Download the calculations (CSV)

Executive summary

• Across 2,013,544 pre-mitigation radon tests that labs reported to CDC for 2008 to 2017, 35.7 percent read at or above EPA's 4 pCi/L action level (718,943 tests, 46 states and DC).
• North Dakota (59.3 percent), Nebraska (58.5 percent), South Dakota (58.5 percent) and Iowa (58.5 percent) post the highest shares; Louisiana (2.2 percent) and DC (8.7 percent) the lowest.
• EPA's 1993 zones put counties in the right order: 42.4 percent of tests were high in Zone 1 counties, 28.2 percent in Zone 2 and 15.3 percent in Zone 3.
• The low zone is not a safe zone: in 19 of 158 Zone 3 counties with at least 100 tests, a quarter or more of results were 4 pCi/L or higher.
• Testing follows the map more than the risk: Zone 1 counties hold 26.3 percent of residents but produced 60.5 percent of tests, 10.4 times the per-person testing rate of Zone 3.
• Seven states with under 10 percent of residents in Zone 1 (Utah, New Hampshire, Oregon, Nevada, Florida, North Carolina, Vermont) returned 26.3 percent high results across 216,364 tests.
• State law, through June 2026: 16 states require a radon notice or test results at sale, 20 list radon on a general seller form and 15 have no seller rule; 7 require radon-resistant features in all new homes.
• The newest CDC counts are state-program data for 2023 from 11 states: 33.4 percent of 128,740 tests were at or above 4 pCi/L.

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Reproduce this analysis

Every figure comes from the files below. The scripts that pulled and computed them are kept with the report and can be re-run.

Data quality and exclusions

• Lab file: 8,790 county-level cells (county x three concentration bands); 807 cells read No Events and were counted as zero. No cells carried a suppression flag.
• Hawaii, Kansas, Mississippi and New Jersey have no lab data in the 2008 to 2017 window and are blank on the tile map; Kansas and New Jersey appear in the state-program table.
• EPA zone list: 3,144 counties. 56 rows needed a name fix before matching Census names: 41 Virginia independent cities, eight Connecticut legacy counties (matched to Vintage 2021 population), Doña Ana (accent), Shannon County SD (now Oglala Lakota), and four Alaska areas renamed or split since 1993.
• 4 EPA rows have no current Census county (Yellowstone National Park, and the former Virginia cities of Bedford, Clifton Forge and South Boston) and carry zero population. Hoonah-Angoon, Alaska, split from a Zone 3 area, was assigned Zone 3 for population only.
• County join: 2,661 counties had lab tests; one (Valdez-Cordova, Alaska, 3 tests) has no current FIPS match and is left out of zone totals only.
• State-program data: states enter and leave the series by year (11 states in 2023, 19 in 2014), so only within-state comparisons are made; NJ and RI values look like definition differences and are flagged, not dropped.

Methodology

Source selection: state test shares come only from CDC's Environmental Public Health Tracking Network API, the federal compilation of radon test counts. Zones come from EPA's county spreadsheet of the 1993 map. Population comes from Census Bureau county estimates. Health and action-level figures come from EPA and CDC pages. State laws come from the Environmental Law Institute's 2026 radon excerpt, with Minnesota and Illinois checked against official pages.

Retrieval: the keyless API was called with the request format used in CDC's own EPHTrackR package, county geography, concentration-level stratification (under 2, 2 to under 4, 4 or higher). Lab data (measure 866) were pulled for the latest 10-year window, 2008 to 2017; state-program data (measure 743) for each year 2014 to 2023. The keyless tier allows few requests a minute, so calls were spaced and retried.

Calculations: state and zone shares are sums of county counts at or above 4 pCi/L divided by sums of all county counts, never averages of county percents. Zones were joined to CDC counties by FIPS after matching EPA county names to Census names; renamed, split and merged counties are handled as described in the data-quality notes. Tests per 1,000 residents divide ten years of tests by the July 1, 2025 population of the same counties.

Registry: each of the 155 ELI entries was read and coded for three questions: seller disclosure (radon-specific notice or test results; radon on a general form; none), radon-resistant new construction for homes (statewide; Zone 1 or named areas; voluntary or limited; none) and mitigator credentials (state credential; national certification required by law; contractor license; testers only; none). School, child care and funding laws were not coded. The registry is checked against ELI's annual update; last checked 2026-11-02.

Conflicts: where lab and state-program data disagree for a state (Connecticut, Rhode Island, New Jersey), both are shown and neither is preferred. The EPA zone web page date (August 2026) and spreadsheet posting (September 2025) do not change the 1993 zones.

Limitations: tests are self-selected and concentrated in real estate transactions and high-radon areas; repeat tests of one building may count more than once; four states lack lab data; the lab series stops at 2017. None of this measures the radon level of any specific home.

Last updated 2026-11-02.

This report presents published data with its limitations. It is not legal, financial or investment advice.

Sources, ranked by quality

Tier 1: primary and official sources (25)

• [1] CDC National Environmental Public Health Tracking Network. Radon Tests from Labs: Number of Pre-mitigation Radon Tests by Radon Level over a 10-year Period (measure 866), county x concentration, 2008 to 2017. 2008 to 2017. Accessed 2026-11-02.
• [2] CDC National Environmental Public Health Tracking Network. Radon Tests from States: Annual Number of Pre-mitigation Radon Tests by Radon Level (measure 743), county x concentration, 2014 to 2023. 2014 to 2023. Accessed 2026-11-02.
• [3] U.S. Environmental Protection Agency. The EPA Map of Radon Zones (web page, last updated August 17, 2026). 1993 map. Accessed 2026-11-02.
• [4] U.S. Environmental Protection Agency. Text Version: Radon Zones Map (county spreadsheet, September 2025 posting). 1993 zones. Accessed 2026-11-02.
• [5] U.S. Census Bureau. County Population Totals: Vintage 2025 (co-est2025-alldata). July 1, 2025. Accessed 2026-11-02.
• [6] U.S. Census Bureau. County Population Totals: Vintage 2021 (co-est2021-alldata), used for Connecticut's eight legacy counties. July 1, 2021. Accessed 2026-11-02.
• [7] U.S. Environmental Protection Agency. Health Risk of Radon. EPA 2003 risk assessment. Accessed 2026-11-02.
• [8] U.S. Environmental Protection Agency. What is the average level of radon found in homes in the U.S.?. 1991 national residential survey. Accessed 2026-11-02.
• [9] U.S. Environmental Protection Agency. What is EPA's Action Level for Radon and What Does it Mean?. current guidance. Accessed 2026-11-02.
• [10] U.S. Environmental Protection Agency. Does the 2009 World Health Organization's recommendation of a lower radon action level affect the EPA's radon action level?. 2009 WHO handbook. Accessed 2026-11-02.
• [11] U.S. Environmental Protection Agency. A Citizen's Guide to Radon (December 2016). 2016. Accessed 2026-11-02.
• [12] U.S. Environmental Protection Agency. How much can a radon mitigation system cost?. current guidance. Accessed 2026-11-02.
• [13] U.S. Environmental Protection Agency. Overview of EPA's State and Tribal Indoor Radon Grants Program: Activities Conducted During Fiscal Year 2025 (July 2026). October 1, 2024 to September 30, 2025. Accessed 2026-11-02.
• [14] Centers for Disease Control and Prevention. Testing Your Home for Radon (August 26, 2026). current guidance. Accessed 2026-11-02.
• [15] Centers for Disease Control and Prevention. Radon and Your Health (June 18, 2026). current guidance. Accessed 2026-11-02.
• [16] Minnesota Department of Health. Radon Mitigation Systems. current guidance. Accessed 2026-11-02.
• [17] Minnesota Department of Health. Radon in Homes. current. Accessed 2026-11-02.
• [18] Minnesota Office of the Revisor of Statutes. Minnesota Statutes 144.496, Minnesota Radon Awareness Act (2026 Minnesota Statutes). effective January 1, 2014. Accessed 2026-11-02.
• [19] Illinois Emergency Management Agency and Office of Homeland Security. Radon: Real Estate. current. Accessed 2026-11-02.
• [21] U.S. Environmental Protection Agency. The National Radon Action Plan: A Strategy for Saving Lives. NRAP 2021 to 2025. Accessed 2026-11-02.
• [22] U.S. Environmental Protection Agency. State and Tribal Indoor Radon Grants (SIRG) Program and Resources. FY2022 to 2026 strategic plan. Accessed 2026-11-02.
• [23] U.S. Environmental Protection Agency, Region 8. EPA encourages Rocky Mountain and Great Plains region residents to Test Your Nest. news release. Accessed 2026-11-02.
• [24] Centers for Disease Control and Prevention (CDCgov on GitHub). EPHTrackR: R package for the Tracking Network API (source code). current. Accessed 2026-11-02.
• [27] U.S. Environmental Protection Agency. How often should I test/retest my home for radon?. current guidance. Accessed 2026-11-02.
• [28] Illinois Emergency Management Agency and Office of Homeland Security. Radon (program page). current. Accessed 2026-11-02.

Tier 2: trade bodies, market research, company data (2)

• [20] Environmental Law Institute. Database of State Indoor Air Quality Laws, Database Excerpt: Radon Laws (Summer 2026, laws enacted through June 2026). through June 2026. Accessed 2026-11-02.
• [26] University of Georgia Extension Radon Program. Georgia Radon Map. tests 1990 to 2024. Accessed 2026-11-02.

Tier 3: journalism and expert commentary (1)

• [25] Mapscaping. US Radon Zones Map: Find High-Risk Counties by State (May 4, 2026). 2026. Accessed 2026-11-02.