You cannot guess at radon, and you cannot borrow your neighbor's result. The only way to know what your house is doing is to measure it — correctly, in the right spot, under the right conditions.
A radon test measures how much radioactive gas has accumulated inside your home, reported in picocuries per liter of air (pCi/L). It is the first step in every radon decision. Nobody should sell you a mitigation system before a number exists, and nobody can tell you that you do not need one without a number either.
Minnesota's average indoor level is about 4 pCi/L, more than three times the national average of 1.3. Two out of every five Minnesota homes tested come back at a level the Department of Health considers a major health risk. Duluth's thin, rocky soil over fractured gabbro bedrock and its enormous stock of hillside basements make it a place where high readings are routine, not exotic.
The single most common mistake homeowners make is assuming radon is a neighborhood-level problem. It is not. It is a house-level problem, and the difference between two homes can be dramatic.
Radon entry depends on three things specific to one building: the geology directly under that foundation, the openings in that particular slab and wall assembly, and the pressure difference that building creates between its interior and the soil. Two identical houses built by the same crew in the same year on the same Duluth street can easily read 2 pCi/L and 12 pCi/L. One has a poured slab over clean gravel with a sealed sump; the other has hollow block walls on a seam of fractured rock, an open sump pit, and a stronger stack effect because it is a taller, leakier house. Same street, same soil map, wildly different air.
The takeaway: a test costs a tiny fraction of a mitigation system and settles the question in 48 hours. If you have never tested, or you tested more than two years ago, call (218) 520-9679 and get an actual number.
There are two fundamentally different kinds of measurement, and they answer different questions. Short-term tells you what the house is doing right now. Long-term tells you what the house does on average across a year.
| Short-term test | Long-term test | |
|---|---|---|
| Duration | 2 to 7 days (48 hours minimum) | 90 days or more, ideally a full year |
| What it is good for | A fast screening number, real estate transactions, confirming a new mitigation system worked | Establishing your true annual average exposure before spending money |
| Accuracy | Accurate for the window it covers, but a snapshot — weather, wind, and season all move it | Averages out daily and seasonal swings; the more reliable picture of actual risk |
| Closed-house conditions | Required, starting 12 hours before the test | Not required — live normally |
| When to choose it | You are buying, selling, in a hurry, or just need to know if there is a problem at all | You are not under deadline, or a short-term result landed in the ambiguous 2–4 range |
In practice most Duluth homeowners start with a short-term test. If it comes back clearly high, you do not need more data — you need a system, and our radon mitigation page covers what that involves and costs. If it comes back borderline, a long-term test is the honest next move.
Activated charcoal. A canister or packet that adsorbs radon over 2 to 7 days, then gets sealed and mailed to a lab. Inexpensive, and the basis of most hardware-store kits. It gives you one number and nothing else, and it is sensitive to humidity and to mishandling in the mail.
Alpha track detectors. A plastic film in a small chamber that records physical damage tracks from alpha particles. These are the standard long-term device, left in place for 90 days to a year, then sent to a lab for counting.
Continuous radon monitors (CRM). A professional electronic instrument that samples the air on a rolling basis and stores a reading every hour. This is what a licensed measurement professional brings to your house, and the difference is not subtle. A CRM gives you the hourly curve, not just an average — so you can see the overnight peak, the effect of a windy afternoon, and whether the number was stable or erratic. It logs temperature, humidity, and barometric pressure, which is how the report shows that conditions were valid. And it has tamper detection: motion sensors and power-interruption logging that reveal whether a window got opened or the unit was moved mid-test. In a real estate deal, where the seller has a financial interest in a low number, that tamper record is the entire reason a CRM is the standard.
A short-term test is only valid if the house is held in closed-house conditions. Break these and the result is meaningless — usually artificially low, which is the worst way to be wrong.
Placement is the other half of a valid test. The device belongs on the lowest level of the home that is occupied living space, or that could reasonably become occupied living space. An unfinished Duluth basement counts, because basements in this city get finished all the time. Within that level, the device sits 20 inches to 6 feet above the floor, at least 3 feet from exterior walls, and at least 1 foot from windows, in a spot where it will not be bumped, covered, or moved.
If the lowest level is a dirt-floor or partial crawl space rather than a basement, placement gets more complicated and the fix is different too — see crawl space radon systems for how those buildings are measured and treated.
This matters more here than almost anywhere. When you heat a house, warm air rises and escapes through the upper floors and attic bypasses, and replacement air gets drawn in low — a large share of it straight out of the soil through the foundation. That is the stack effect, and its strength scales with the temperature difference between inside and outside. Duluth's heating season runs a large part of the year, which means the stack effect is running a large part of the year.
The consequence is that winter readings in Duluth run substantially higher than summer ones in the same house. A summer test that comes back at 3.2 pCi/L is not an all-clear. That same house may well sit above 6 in January, and January is when your family is inside it with the windows shut.
There are two reasonable responses. Test during the heating season, roughly October through April, when a short-term result reflects your worst-case conditions. Or, if you tested in summer and got anything above about 2, run a 90-day-plus long-term test that spans the cold months and gives you a true annual average. What you should not do is take a warm-weather 3.2 as a final answer.
The EPA action level is 4.0 pCi/L. At or above that, fix the house. The EPA also recommends that homeowners consider mitigation anywhere from 2.0 to 4.0, because there is no level of radon that is genuinely safe. Radon risk is cumulative — it is a function of concentration multiplied by years of exposure, which is why a moderate number in a house you plan to live in for twenty years deserves more attention than the same number in a house you are leaving next spring.
| Result | What it means | Reasonable next step |
|---|---|---|
| Under 2.0 pCi/L | Low. Below the level where EPA suggests action of any kind. | Retest in two years, or sooner after any major foundation, HVAC, or basement work. |
| 2.0 – 4.0 pCi/L | The gray zone. Below the action level but above what EPA calls acceptable without thought. | Run a long-term test through the heating season. If the annual average holds above 2, mitigation is a defensible choice, especially with children or a finished basement. |
| 4.0 – 8.0 pCi/L | At or above the EPA action level. Very common in Duluth. | Mitigate. A standard sub-slab system handles this range routinely and typically brings the house under 2. |
| Above 8.0 pCi/L | High. Worth acting on promptly rather than over the next season. | Mitigate soon, and expect the system design to matter — fan sizing, suction point count, and diagnostics all get more important as the number climbs. |
Radon is not a one-and-done measurement, because the things that drive it change. Retest every two years as a baseline. Also retest specifically after any of the following: installation of a mitigation system (a confirmation test at least 24 hours after the fan starts is mandatory, not optional); major foundation work such as underpinning, slab replacement, or new footing drains; a new furnace, air handler, or HVAC system, since these change how the house moves air; and finishing a basement, which converts an unoccupied space into one where your family spends hours a day.
Anyone who performs radon testing in a building they do not own or lease must be licensed by the Minnesota Department of Health. That covers home inspectors, real estate testers, and measurement professionals. National industry certification alone is not sufficient in Minnesota. Ask for the MDH license number before anyone places a device in your house.
If your test is tied to a purchase agreement, the timing and documentation rules are stricter and the deadlines are real — our real estate radon testing page covers inspection windows, the Minnesota Radon Awareness Act disclosure, and how results get handled at the table. For anything else, get in touch and we will get a device placed.
A short-term test requires a minimum of 48 hours and commonly runs two to seven days. You also need to hold closed-house conditions for 12 hours before it starts, so plan on a three-day commitment overall. Long-term tests using alpha track detectors run 90 days at minimum and ideally a full year. If you are on a real estate timeline, a 48-hour continuous monitor test with same-day results at pickup is the standard approach, since it fits inside a typical inspection contingency window.
A charcoal kit from a hardware store can give you a usable screening number if you follow the instructions exactly, hold closed-house conditions, place it correctly, and mail it promptly. What it cannot give you is hourly data, temperature and humidity logging, or tamper detection. It produces one average with no way to verify the conditions during the test. For a personal screening that is often fine. For a real estate transaction or for deciding whether to spend a couple thousand dollars on a system, a professional continuous radon monitor is the better instrument.
Yes. Radon levels are determined by your specific foundation, the soil and rock directly beneath it, and the pressure your particular house creates. Two identical homes side by side on the same Duluth street routinely test at 2 and 12 pCi/L. Differences in slab construction, whether the walls are hollow block or poured concrete, whether the sump pit is covered, and how leaky the upper floors are all change the result. A neighbor's number tells you something about the area's geology and nothing reliable about your house.
Winter, if you can. Duluth's long heating season keeps the stack effect running, which pulls soil gas into the house, so heating-season readings run substantially higher than summer ones. A short-term test between October and April reflects the conditions your family actually lives in with the windows closed. If you test in summer and get anything above roughly 2 pCi/L, treat it as incomplete and follow up with a long-term test that spans the cold months rather than assuming the low number is your real average.
That is the gray zone. It is below the EPA action level of 4.0 but inside the range where the EPA recommends you consider mitigation, because no radon level is truly safe and the risk accumulates over years. The sensible move is a long-term test running 90 days or more through the heating season to find your annual average. If that average stays above 2, mitigation is a reasonable decision, particularly if the basement is finished, children spend time down there, or you plan to stay in the home long term.
On the lowest level of the home that is occupied living space or could reasonably become living space, which in most Duluth homes means the basement even if it is currently unfinished. Position it 20 inches to 6 feet above the floor, at least 3 feet from any exterior wall, and at least 1 foot from a window. Keep it out of drafts from supply registers, fireplaces, and dryers, and never place it in a bathroom, kitchen, or laundry room, where humidity and exhaust fans distort the reading.
Yes. Minnesota law requires anyone performing radon testing in a building they do not own or lease to be licensed by the Minnesota Department of Health. This applies to home inspectors and measurement professionals alike, and national industry certification by itself does not satisfy the requirement. You can test your own home yourself with a store-bought kit without a license, but the moment someone else is doing the testing for you, ask for their MDH license number before they place a device.
A 48-hour test tells you where you stand, and the result comes to you in writing. If it comes back high, we will walk you through exactly what a system for your foundation would involve.