Iowa radon map, by county
Iowa has the highest indoor radon levels in the United States, and it is not close. Every one of its 99 counties is an EPA Zone 1 county. This map shows what licensed Iowa testers actually recorded in 2023: 24,029 tests, with the sample size for every county shown beside its reading.
24,029
recorded by licensed Iowa testers
3.85 pCi/L
the middle county, of 99
36.5%
of tests, the EPA action level
99
80 with enough tests to colour
Two Iowa numbers that disagree, and why
You will see it written everywhere, including on our own Iowa directory page, that 71.6% of Iowa homes are above the action level. That comes from the Iowa Radon Survey, which tested a random sample of Iowa houses. This map says 36.5%. Both are right, and the gap between them is the most useful thing on this page.
A random survey asks "what is in Iowa's housing stock". This dashboard records the tests people actually chose to run, which skews heavily toward real estate transactions and includes retests of houses that have already been fixed. Volunteered tests are not a random sample of anything. So treat the 71.6% as the answer to "how common is radon in Iowa homes", and this map as the answer to "what are testers finding right now, county by county".
We show both rather than the more dramatic one. A page that quietly picks whichever number sells better is a page a realtor should not forward.
Read this before you use a number from this map
A county is a very large place. Iowa counties run to hundreds of square miles. The spread between two houses on the same street is routinely wider than the spread between two counties, so a county median tells you how common a problem is in the area and nothing at all about your own foundation.
19 of the 99 counties had fewer than 30 tests in 2023. They appear in grey. Several of them have the highest medians in the state, which is exactly why they are not coloured: a striking number from twenty three houses is a hint, not a finding.
This is one year, not the whole archive. Iowa's dashboard holds tests from 2000 to 2024. This map is the 2023 slice. A single recent year is a fairer picture of what testers are finding today than a twenty year pool, but it is a smaller sample.
The highest counties in Iowa
Ranked by median reading, and restricted to counties with at least 30 tests in 2023. Sample size sits beside every row, because it decides whether the row means anything.
| County | Tests | Median pCi/L | At or above 4.0 | Highest single |
|---|---|---|---|---|
| Worth | 31 | 9 | 67.7% | 34.1 |
| Pocahontas | 41 | 7.8 | 70.7% | 26.2 |
| Mitchell | 70 | 7.4 | 81.4% | 51.1 |
| Montgomery | 38 | 7.2 | 60.5% | 58.8 |
| Guthrie | 53 | 7.1 | 67.9% | 27 |
| Cass | 60 | 7.05 | 70% | 34.4 |
| Lyon | 82 | 6.9 | 75.6% | 49.3 |
| Sioux | 160 | 6.85 | 65.6% | 40.5 |
| Cherokee | 70 | 6.55 | 68.6% | 46.7 |
| Hancock | 46 | 6.55 | 65.2% | 34.6 |
| Crawford | 43 | 6.2 | 60.5% | 30.5 |
| Poweshiek | 144 | 6.1 | 68.8% | 119.2 |
| Grundy | 128 | 6 | 63.3% | 56.1 |
| Calhoun | 33 | 5.9 | 69.7% | 27.7 |
| Mahaska | 87 | 5.9 | 62.1% | 41.7 |
The most-tested counties
Where Iowa has the most data, and therefore the rows to trust most.
| County | Tests | Median pCi/L | At or above 4.0 |
|---|---|---|---|
| Johnson | 3,727 | 1.7 | 22.3% |
| Polk | 3,245 | 3.4 | 45% |
| Linn | 2,662 | 1.8 | 21.4% |
| Black Hawk | 1,233 | 2.4 | 33.9% |
| Dubuque | 938 | 2.25 | 30.7% |
| Scott | 938 | 2.8 | 35.3% |
| Warren | 731 | 1.2 | 21.1% |
| Story | 529 | 3.15 | 41.8% |
| Des Moines | 527 | 1.8 | 27.3% |
| Dallas | 523 | 3 | 40.5% |
What the numbers mean
The EPA acts at 4.0 pCi/L and recommends you consider acting between 2.0 and 4.0. The World Health Organization's reference level is lower still, 100 Bq/m³, about 2.7 pCi/L. All of these are action levels, not safety guarantees. No reading is known to be harmless, which is why the advice everywhere is to test your own house rather than to look up your county and stop.
In Iowa that advice carries more weight than almost anywhere. The typical county median here is 3.85 pCi/L, which is at or near the level the EPA says to act on before you have looked at a single individual house.
If your reading is high
It is a fixable problem. Mitigation is a pipe and a fan venting soil gas above the roofline, usually a day's work. The distinction worth knowing before you start calling is that measuring and mitigating are separate certifications, and plenty of companies hold only the first. Our Iowa directory lists every certified company in the state and splits them by which they are actually certified to do.
Iowa also requires the seller of a home to complete a written disclosure that asks about radon directly: whether the property has been tested, what the results were, and whether a mitigation system was installed. A disclosure is not a test. If nobody has ever tested the house, the honest answer on that form is that nobody knows.
Source. Radon test data published by the Iowa Department of Health and Human Services on its public radon dashboard, 2023 tests, aggregated to county. Boundaries are US Census Bureau 2023 cartographic county boundaries, public domain. The 71.6% figure is from the Iowa Radon Survey. Iowa's own rules keep individual addresses and owner names confidential to the certified tester, so no address-level Iowa data exists publicly and none is reproduced here.
This page is an educational summary of public records. It is not a health assessment of any individual property, and no reading here should be used to price a house or to decide against testing one.