Roughly one in four St. Louis County homes that has been tested came back at or above the EPA action level. Here is what the actual Minnesota Department of Health data says, what the numbers mean, and why a county average still cannot tell you about your house.
If you are asking whether radon is a real problem in Duluth, the short answer is yes, but the honest answer has more shape to it than most pages will give you. St. Louis County is not the worst county in Minnesota for radon. It is not close to the worst. It is also nowhere near clean, and the data has limits that matter a great deal for how you should read it.
The Minnesota Department of Health publishes county-level radon test results through the Minnesota Public Health Data Access portal. In the published dataset covering test results from 2010 through 2020, 27% of properties tested in St. Louis County came back at or above 4.0 pCi/L, the EPA action level, and 56.6% came back at or above 2.0 pCi/L, the level at which the EPA says a homeowner should consider fixing the house. The county median was 2.1 pCi/L.
For comparison, the statewide Minnesota figures in the same dataset are 40.3% at or above 4.0 pCi/L and 71.2% at or above 2.0. So St. Louis County runs meaningfully below the Minnesota average. That is a genuine and slightly unusual thing to be able to say on a page like this, and it is true. It is also not a reason to skip testing, for reasons covered below.
The one-line version: more than half of the St. Louis County homes in the MDH dataset were above 2 pCi/L and more than a quarter were above 4. Those are not long odds. A test costs a fraction of a mitigation system and takes 48 hours. Call (218) 520-9679 or request a test.
Radon concentration is reported in picocuries per liter of air, abbreviated pCi/L. A curie is a unit of radioactivity, and a picocurie is one trillionth of one. So the number on your test report is not a measure of how much gas is in the air by volume — it is a measure of how much radioactive decay is happening in a liter of the air you are breathing.
Practically, you can treat it as a straightforward risk scale. One pCi/L represents roughly two radioactive decay events per minute in every liter of air in the room. At 4 pCi/L that is about eight per minute, per liter, continuously, in a house you breathe in for thousands of hours a year. The decay products are solid particles that lodge in lung tissue, which is why radon is a lung problem and not a general-toxicity problem.
Two thresholds matter. The EPA action level is 4.0 pCi/L — at or above that, the EPA says fix the house as soon as practical. The EPA also recommends that homeowners consider mitigating anywhere between 2.0 and 4.0 pCi/L. There is no threshold below which radon is considered harmless; 4.0 is a practical action point, not a safety line. The outdoor background level is around 0.4 pCi/L, which is the realistic floor for what any indoor system can achieve.
Here are the published MDH figures for St. Louis County alongside its neighbors and the state as a whole. All of these come from the same Minnesota Public Health Data Access radon dataset covering 2010–2020.
| Area | % at or above 4.0 pCi/L | % at or above 2.0 pCi/L | Median | 95th percentile |
|---|---|---|---|---|
| St. Louis County | 27.0% | 56.6% | 2.1 pCi/L | 10.8 pCi/L |
| Carlton County | 41.7% | 72.9% | 3.1 pCi/L | 14.1 pCi/L |
| Lake County | 25.4% | 47.8% | 1.8 pCi/L | 8.5 pCi/L |
| Cook County | 40.6% | 66.0% | 2.9 pCi/L | 16.5 pCi/L |
| Itasca County | 30.8% | 62.4% | 2.5 pCi/L | 11.6 pCi/L |
| Minnesota statewide | 40.3% | 71.2% | 3.1 pCi/L | 11.7 pCi/L |
Two things jump out. First, the immediate region is not uniform — Carlton County, which starts about fifteen minutes down the road in Esko and Cloquet, sits at 41.7% above the action level, well over St. Louis County's 27%. Second, look at the 95th percentile column. Even in the county with the mildest profile on this list, one home in twenty tested above 8.5 pCi/L. In St. Louis County the 95th percentile is 10.8. High readings are not rare here. They are just not the majority.
MDH's own summary of the statewide pattern notes that counties in southern and western Minnesota have the highest share of properties over 4 pCi/L, which the county data bears out — the worst counties in the state sit near 75–80%. Northeastern Minnesota is on the lower end of a state that has one of the worst radon profiles in the country. That is a relative statement, not an absolute one.
We would rather flag the limits than let you over-read the table above.
Radon comes from uranium decaying in rock and soil. Whether it reaches your lungs depends on the geology beneath the foundation and on how hard the building pulls on the ground. Duluth has some specific conditions worth understanding.
The Duluth Complex. The city sits on gabbro and basalt bedrock with a thin, rocky layer of glacial soil over it. Thin soil means less material between the source rock and your slab, and fractured bedrock close to the surface acts as a direct conduit — soil gas travels through fracture networks far more readily than through deep, dense clay.
Hillside foundations. Duluth is built up a steep slope above the lake, and a very large share of homes here have walkout, daylight, or partially buried basements cut into rock and fill. That geometry produces an unusual amount of contact between living space and earth, often on multiple levels of the same house, and often with retaining-wall style construction that was never detailed for air sealing.
Old housing stock. Much of Duluth predates 1950. Hollow-block and fieldstone foundation walls, dirt-floor and partial crawl spaces, open sump pits, and unsealed floor-wall joints are all common here. A hollow block wall is particularly effective at collecting soil gas along its whole length and releasing it into the basement.
A long heating season and the stack effect. When a house is heated, warm air rises and leaks out through the upper floors and attic bypasses, and replacement air gets drawn in low — a substantial share of it directly out of the soil. That pressure difference is the stack effect, and its strength scales with the indoor-outdoor temperature gap. Duluth runs a heating season most of the year, so this mechanism is active far more of the time than it is in most of the country.
This is the single most important thing on this page. Radon is a building-level problem, not a neighborhood-level one. Two homes built in the same year by the same crew on the same Duluth block routinely test at 2 pCi/L and 12 pCi/L.
The reason is that three independent variables all have to line up: the rock and soil directly under that specific footprint, the number and size of openings in that specific slab and wall assembly, and the pressure that specific building generates. Change any one and the result moves. A poured slab over clean gravel with a sealed sump behaves nothing like a hollow-block foundation over a fracture seam with an open pit. A tall, leaky house pulls harder than a tight rambler.
Which is why a county percentage — even a well-collected one — is background context and nothing more. It tells you the problem is worth taking seriously in this area. It cannot tell you what is in your basement right now. Only a measurement does that, and radon testing in Duluth covers how to do one correctly.
Because the stack effect drives entry, the same Duluth house can read very differently in January than in July. Heating-season readings run substantially higher. A summer test at 3.2 pCi/L is not an all-clear for that house in winter, and winter is exactly when your family is inside with the windows shut.
Two reasonable approaches. Run a short-term test during the heating season, roughly October through April, so the number reflects worst-case conditions. Or, if you tested in warm weather and got anything above about 2, follow up with a 90-day-or-longer test spanning the cold months to establish a true annual average. What you should not do is treat a July 3.2 as a final answer.
| Result | How to read it | What to do |
|---|---|---|
| Under 2.0 pCi/L | Low. Below the range where EPA suggests any action. Roughly 43% of tested St. Louis County homes land here. | Retest in two years, and sooner after foundation work, a new HVAC system, or finishing the basement. |
| 2.0 – 4.0 pCi/L | The gray zone. Below the action level, inside the range EPA says to consider fixing. | Run a long-term test through the heating season. If the annual average holds above 2, mitigation is a defensible choice — more so with a finished basement or kids downstairs. |
| 4.0 – 8.0 pCi/L | At or above the EPA action level. About 27% of tested homes in the county are at 4.0 or higher. | Mitigate. A standard sub-slab system handles this range routinely and typically brings a house under 2. |
| 8.0 – 20 pCi/L | High. In St. Louis County the 95th percentile is 10.8, so this is the top few percent of results. | Mitigate promptly. Fan sizing, suction point count, and diagnostics start to matter more as the number climbs. |
| Above 20 pCi/L | Very high, and uncommon, but it happens here. | Treat it as a priority. Confirm with a second measurement, then mitigate. Expect the design to need more than one suction point. |
Radon is the leading cause of lung cancer among people who have never smoked and the second leading cause overall. The EPA estimates it is responsible for about 21,000 lung cancer deaths a year in the United States, roughly 2,900 of them among people who never smoked. The EPA also publishes an uncertainty range on that estimate of 8,000 to 45,000, which is worth knowing, because it tells you the science establishes a real and serious effect while still being honest about the width of the confidence interval.
What that risk looks like at the individual level is more moderate than the headline number suggests, and we would rather give you the actual figures than a scare. Per EPA's published risk tables, if 1,000 people who never smoked were each exposed to 4 pCi/L over a lifetime, about 7 would be expected to develop fatal lung cancer from it. At 2 pCi/L that drops to about 4. For the general population, the figure at 4 pCi/L is about 23 per 1,000.
The picture changes sharply with smoking. For 1,000 lifetime smokers exposed to 4 pCi/L, the estimate is about 62 — roughly nine times the never-smoker figure. Radon and tobacco smoke do not simply add together; they compound. If anyone in the household smokes, or the house has a history of indoor smoking, a moderate radon number deserves more weight than it otherwise would.
Two honest framings follow from this. Radon exposure is not an emergency — nothing bad happens because you spent one more winter above 4 pCi/L. It is a cumulative, decades-long exposure, which means the number matters most in a house you intend to live in for a long time. And at the same time, it is one of the very few serious health risks in a home that can be permanently removed for a known, modest, one-time cost. Those two facts together are the whole argument.
Whichever band you land in, Minnesota law requires that anyone performing radon testing or mitigation in a building they do not own or lease be licensed by the Minnesota Department of Health. Ask for the license number before anyone places a device or picks up a drill.
It is a real and common problem here, though St. Louis County is not among Minnesota's worst. In the Minnesota Department of Health's published county dataset covering 2010 through 2020, 27% of tested properties in St. Louis County came back at or above the EPA action level of 4.0 pCi/L, and 56.6% were at or above 2.0. The statewide figures are 40.3% and 71.2%, so the county runs below the Minnesota average in a state that has one of the highest radon profiles in the country. More than a quarter of tested homes above the action level is still a serious base rate.
In the MDH county dataset for 2010 through 2020, the median result for St. Louis County was 2.1 pCi/L and the 95th percentile was 10.8 pCi/L, meaning one in twenty tested properties came back above 10.8. Averages for radon are reported in more than one way because the distribution is skewed by a small number of very high readings, so the median and the percentile spread describe the county better than a single average does. There is no published MDH figure specific to the city of Duluth.
Not publicly, and this matters. The Minnesota Department of Health publishes radon test results at the county level. St. Louis County stretches from the Lake Superior shore to the Canadian border, so a countywide percentage blends Duluth's hillside neighborhoods with a very large and geologically varied rural area. Any site presenting a Duluth-specific or ZIP-code-specific radon average should be asked to show its source. The only number that describes your home is a test in your home.
Because radon entry is determined by three things that are specific to one building: the rock and soil directly under that foundation, the openings in that particular slab and wall assembly, and how hard that building pulls air out of the ground. Two identical homes on the same Duluth street routinely test at 2 and 12 pCi/L. A poured slab over gravel with a sealed sump behaves nothing like a hollow-block foundation over a fracture seam with an open pit, even when the houses look the same from the curb.
Substantially, and more so in Duluth than in most places. Heating a house makes warm air rise and escape through the upper floors, which lowers the pressure at the foundation and draws soil gas in. That is the stack effect, and its strength scales with the indoor-outdoor temperature difference. Duluth's heating season runs most of the year, so winter readings commonly come back well above summer readings in the same house. If you test in warm weather and get anything above roughly 2 pCi/L, treat it as incomplete and follow up with a long-term test through the cold months.
Per the EPA's published risk tables, out of 1,000 people who never smoked and were exposed to 4 pCi/L over a lifetime, about 7 would be expected to die of lung cancer caused by radon. At 2 pCi/L the figure is about 4. For lifetime smokers at 4 pCi/L the estimate rises to about 62 per 1,000, because radon and tobacco smoke compound rather than simply add. Nationally the EPA attributes about 21,000 lung cancer deaths a year to radon. It is a cumulative exposure risk, not an acute one, which is why the number matters most in a home you plan to occupy for many years.
County statistics are context. A 48-hour test in your own basement is an answer, and you get it in writing. If it comes back high we will tell you exactly what a system for your foundation would cost, with no obligation.