By Jake Lusby, owner of Air Sense Environmental (Illinois IEMA Mitigation Professional, license RNM20232346). Data compilation: Air Sense Environmental editorial team. Published: October 25, 2026.
We reviewed the public radon test records that the Missouri Department of Health and Senior Services (DHSS) and the Illinois Emergency Management Agency and Office of Homeland Security (IEMA-OHS) publish for 11 St. Louis-area counties. Of 44,141 valid residential radon test results, 14,252 (32.3%) were at or above 4.0 picocuries per liter (pCi/L), the level at which the U.S. Environmental Protection Agency (EPA) recommends fixing a home. That is about 1 in 3 tests. No Air Sense Environmental customer data is used in this study.
Download the data: county summary (CSV) and Illinois mitigation-system split (CSV).
Key findings
- About 1 in 3 recorded tests reached the EPA action level. Across 11 counties, 14,252 of 44,141 valid results (32.3%) were at or above 4.0 pCi/L.
- Most tests landed at or above the “consider fixing” line. 62.5% of valid results (27,572) were at or above 2.0 pCi/L. EPA says to consider fixing between 2 and 4 pCi/L.
- Jefferson County, Missouri had the highest share at or above 4.0 pCi/L: 43.3% of 1,541 valid DHSS results. Monroe County, Illinois was highest on the Illinois side at 39.4% of 1,929 valid IEMA-OHS results.
- St. Louis City had the lowest share: 17.6% of 777 valid results, still close to 1 in 6 tests.
- High readings were not rare outliers. 11.0% of valid results (4,846) were at or above 8.0 pCi/L, double the EPA action level.
- Neighbors can differ. County medians ranged from 1.7 pCi/L (St. Louis City) to 3.4 pCi/L (Jefferson County). A county figure cannot tell you your own home’s level. Only a test can.
Why this matters
EPA estimates radon is responsible for about 21,000 lung cancer deaths in the U.S. each year and is the number one cause of lung cancer among non-smokers. EPA recommends fixing a home at 4 pCi/L or higher and considering a fix between 2 and 4 pCi/L. The average indoor radon level is about 1.3 pCi/L. Source: EPA, Health Risk of Radon.
Results by county
Counties are sorted by the share of valid tests at or above 4.0 pCi/L. “Valid tests” are the records each state marks as valid, after the filters described in the method section.
| County | State | Valid tests | At or above 4.0 | % at or above 4.0 | % 2.0 to under 4.0 | % at or above 8.0 | Median (pCi/L) |
|---|---|---|---|---|---|---|---|
| Jefferson County | MO | 1,541 | 668 | 43.3% | 25.0% | 21.0% | 3.4 |
| Monroe County | IL | 1,929 | 760 | 39.4% | 30.7% | 14.8% | 3.1 |
| Jersey County | IL | 247 | 92 | 37.2% | 30.0% | 14.6% | 2.9 |
| St. Clair County | IL | 13,246 | 4,695 | 35.4% | 28.4% | 13.2% | 2.8 |
| Clinton County | IL | 815 | 270 | 33.1% | 31.8% | 11.7% | 2.7 |
| Madison County | IL | 12,773 | 3,972 | 31.1% | 33.4% | 8.8% | 2.7 |
| Macoupin County | IL | 940 | 285 | 30.3% | 30.1% | 10.3% | 2.5 |
| St. Charles County | MO | 3,871 | 1,134 | 29.3% | 29.0% | 9.9% | 2.4 |
| Franklin County | MO | 483 | 136 | 28.2% | 28.0% | 9.1% | 2.2 |
| St. Louis County | MO | 7,519 | 2,103 | 28.0% | 29.6% | 9.1% | 2.3 |
| St. Louis City | MO | 777 | 137 | 17.6% | 27.5% | 3.9% | 1.7 |
Missouri and Illinois totals
| Region | Source | Valid tests | % at or above 4.0 | % at or above 2.0 | % at or above 8.0 |
|---|---|---|---|---|---|
| Missouri side (St. Louis City, St. Louis County, St. Charles, Jefferson, Franklin) | MO DHSS | 14,191 | 29.4% | 58.2% | 10.3% |
| Illinois side (Madison, St. Clair, Monroe, Clinton, Jersey, Macoupin) | IEMA-OHS | 29,950 | 33.6% | 64.5% | 11.3% |
| All 11 counties | Both | 44,141 | 32.3% | 62.5% | 11.0% |
For context, 10,627 of 31,719 valid Missouri DHSS results statewide (33.5%) were at or above 4.0 pCi/L. The two states collect data differently (see the method section), so comparisons between counties in the same state are more reliable than comparisons across the river.
Illinois tests in homes with and without a reported mitigation system
| Group (Illinois, 6 counties) | Valid tests | At or above 4.0 | % at or above 4.0 |
|---|---|---|---|
| No mitigation system reported | 26,186 | 9,510 | 36.3% |
| Mitigation system reported | 2,542 | 161 | 6.3% |
Read this table carefully. These are different homes, not before-and-after pairs, and the system field is self-reported in licensee records. The pattern is consistent with mitigation lowering radon levels, but it does not measure how well any one system or contractor performs.
What to do with this information
- Test your own home. Radon varies from house to house, even on the same street.
- If your result is 4.0 pCi/L or higher, EPA recommends fixing the home. Between 2.0 and 4.0 pCi/L, EPA says to consider fixing.
- After any mitigation, retest to confirm the result. Air Sense Environmental recommends a follow-up test after mitigation.
- In Illinois, use an IEMA-licensed radon professional. You can check a license on IEMA’s public license lookup. In Missouri, ask any contractor what credentials, testing method, and written terms they provide.
Want to know your number? Air Sense Environmental can arrange radon testing for your home. Call 314-664-9807. If a test comes back high, see our radon mitigation page.
Method
Missouri. Missouri Department of Health and Senior Services (DHSS), Environmental Public Health Tracking, “Residential testing for radon in Missouri” map service, layer 0 “Test Result by Location.” The service describes the data as residential testing originally authored with data from 2005 through 2017. We downloaded every record for St. Louis City and St. Louis, St. Charles, Jefferson, and Franklin counties (14,721 records), kept records marked Valid_Test = YES (14,191), and counted results at or above 2.0, 4.0, and 8.0 pCi/L and county medians. Source: MO DHSS EPHT radon residential map service.
Illinois. Illinois Emergency Management Agency and Office of Homeland Security (IEMA-OHS), Public Radon Dashboard (linked from the IEMA-OHS radon in Illinois page), layer “Individual Location Results” (Radon_Full). IEMA-OHS says its radon data comes from annual reports submitted by licensed measurement and mitigation professionals. We counted records for Madison, St. Clair, Monroe, Clinton, Jersey, and Macoupin counties, kept valid tests (lValidTest = 1 and a result from 0 to under 1,000 pCi/L; 29,950 of 33,911 records), and computed the same thresholds and medians. Source: IEMA-OHS Radon_Full feature service.
Cross-checks. Missouri counts at or above 4.0 pCi/L matched independent server-side count queries for all five Missouri counties, and county averages matched the DHSS county summary layer. Illinois valid-test counts and counts at or above 4.0 pCi/L for Madison and St. Clair counties matched server-side counts. Figures were re-checked against both live services on October 25, 2026.
Privacy. The Illinois layer contains street addresses. We did not store or publish any address-level data. Only county totals appear here.
Thresholds follow EPA guidance (EPA, Health Risk of Radon).
Limitations
- County results cannot predict an individual home’s radon level. Only a test of that home can.
- These are tests people chose to run, not a random sample of homes. Homes tested during real-estate transactions, or because of a concern, may differ from homes never tested.
- The Illinois layer may include more than one test per address (for example several rooms, or tests after mitigation). As a sensitivity check, keeping only the highest no-system result per mapped location raised the Illinois share at or above 4.0 pCi/L to roughly 44%. That figure is approximate, because mapped locations may not equal unique homes, so it is not used as a headline.
- The Illinois layer’s date fields are not machine-readable, so the test period for that layer cannot be stated from the data. IEMA-OHS describes its map data as measurements for calendar years 2003 to 2019. The layer was last edited March 12, 2026.
- Missouri data covers test years 2005 to 2017 and is not current.
- The two states use different collection programs, so cross-state comparisons are approximate.
- Short-term and long-term tests are mixed together. Test type is not reliably coded.
Sources
- Missouri Department of Health and Senior Services, Environmental Public Health Tracking: Residential testing for radon in Missouri, layer 0, Test Result by Location
- Illinois Emergency Management Agency and Office of Homeland Security: Radon in Illinois and Public Radon Dashboard, Individual Location Results (Radon_Full)
- U.S. Environmental Protection Agency: Health Risk of Radon
Download and cite the data
County summary CSV: one row per county with record counts, valid tests, counts and shares at each threshold, medians, the source URL, and a method note. Illinois mitigation-system split CSV: valid tests and results at or above 4.0 pCi/L by reported mitigation system, per county.
Suggested citation: Air Sense Environmental, “St. Louis-area radon test results by county,” October 25, 2026, https://airsenseenvironmental.com/st-louis-radon-test-results-by-county/. Reporters with questions about the method can reach Jake Lusby at jake@airsenseenvironmental.com or 314-664-9807. More press resources are in our Media Room.