Is 4.0 pCi/L dangerous?

4.0 pCi/L is the EPA action level, the point at or above which the EPA recommends installing a mitigation system. It is a guideline for action, not a medical diagnosis, and it does not tell you anything about a specific person's health. The practical takeaway is straightforward: confirm the reading with a follow-up test, then reduce the level if it holds.

What if my result is between 2 and 4 pCi/L?

Between 2 to 4 pCi/L, the EPA suggests you consider fixing the home, since there is no known safe level of radon. Many Wisconsin homeowners in this range run a longer test first to see where the yearly average settles, then decide. A radon professional can help you weigh the number against your home and your budget.

Retesting makes sense on a schedule and after change. A common practice is to test again every two years, and also after major renovations, foundation work, or buying a home, because those can shift how radon enters and moves. If you install a mitigation system, a post-mitigation test confirms the fan brought the level down.

Is a short-term test accurate enough?

A short-term test is accurate for the window it covers, but it is a snapshot, and radon rises and falls with the weather and the season. The EPA recommends confirming a short-term result at or above 4.0 pCi/L with a follow-up test before you pay for a system. For a truer yearly figure, a long-term test runs 90 days or more.

My two tests disagree. Which one do I trust?

Two readings can differ because radon moves with weather, season, and how the house was closed up during each test. Rather than pick one, look at whether either lands at or above 4.0 pCi/L and how far apart they are. When results conflict near the action level, a longer test gives the steadier annual average, and a radon professional can interpret both.

Radon Test Results Explained

You have a number in front of you, printed in pCi/L on a lab report or shown on a monitor, and now you are trying to work out whether it is fine, borderline, or a problem. That is the question this guide answers for Wisconsin homeowners. A radon result is a single figure with a lot riding on it, and the hard part is not reading the number but knowing what the EPA guidance says to do at that number. Below, each range is laid out factually, along with why a follow-up test matters and when to test again. Badger State Radon is a free matching service, not a lab or a contractor, so this page explains the reading and, if it comes back high, connects you with an independent local radon professional.

What pCi/L means

Radon results come back in picocuries per liter of air, written pCi/L. It is a measure of how much radioactive decay is happening in a liter of the air inside your home: the higher the number, the more radon is present. It is not a percentage and it is not something you could ever notice by smell or feel, since radon has no color, odor, or taste. Radon exists at low levels in outdoor air everywhere and concentrates once it seeps into an enclosed space through foundation cracks, sump openings, and, in some areas, well water. Because the gas is invisible, the number on your report is the only readout you get, which is why the unit and the thresholds tied to it matter. If you want the mechanics of how the kit or monitor produced that figure, the radon testing page walks through short-term, long-term, and continuous devices.

The 4.0 action level and the 2 to 4 range

The EPA sets its action level at 4.0 pCi/L. At or above that number, the EPA recommends reducing the level with a mitigation system, and between 2 to 4 pCi/L it recommends that you consider a fix, because there is no known safe level of radon. The EPA explains what the action level means and how it is meant to be used in its guidance on the radon action level. As background on why the threshold exists at all: radon is the leading cause of lung cancer among people who do not smoke and is linked to about 21,000 lung cancer deaths a year in the United States, according to the EPA. Here is how the common ranges read in practice:

Below 2 pCi/L: a relatively low result. The EPA does not call for a fix at this level, though no amount of radon is considered risk-free, so periodic retesting is still reasonable.
2 to 4 pCi/L: the consider-a-fix range. Many homeowners run a longer test here to see where the yearly average settles before deciding whether to install a system.
At or above 4.0 pCi/L: the EPA recommends reducing the level. The usual next move is to confirm the reading and then plan a mitigation system.
Well above 4.0 pCi/L: a very high reading warrants prompt attention. The recommended response is the same, a mitigation system, but there is less reason to wait on a long follow-up before acting.

Where a number lands against these ranges is what drives the decision, not how the reading felt. For the statewide context, including how about one in 10 Wisconsin homes tests above the action level, see the Wisconsin radon guide.

Short-term vs long-term results

One reading rarely settles the question on its own. A single short-term test, the kind most homeowners start with, is a snapshot of a few days, and radon rises and falls with the weather, the season, and how tightly the house was closed up during the test. That is why the EPA recommends confirming a short-term result at or above 4.0 pCi/L with a follow-up test before you pay for a system, a step described in its radon standards of practice. A long-term test, which runs 90 days or more, captures a better annual average and is the steadier figure if you want to know how the house behaves across the year rather than across a week. Timing matters when you compare readings: winter is peak testing season in Wisconsin, because a closed-up home with the heat running tends to draw and hold the most radon, so a January number and a July number from the same house can differ for reasons that have nothing to do with a bad test. If your short-term number is high and you are weighing the confirmation route, a second short-term test placed under closed-building conditions is the faster confirmation, while a long-term test is the more complete one. Neither is wrong; they answer slightly different questions.

When to retest

A radon result is a reading of one house at one time, so it is worth refreshing. A common practice is to test again every two years even if an earlier result was low, because soil conditions, foundation settling, and changes in how the home is used can move the number over time. Retest sooner after any major renovation or foundation work, after you finish or start living in a basement level, and after buying a home, since the reading a previous owner reported may be years old or may have been run under different conditions. If you have already installed a mitigation system, a post-mitigation test is how you confirm the fan actually brought the level down, and periodic checks afterward confirm it stays down. Testing is inexpensive in Wisconsin, with regional Radon Information Center kits running about $15, so retesting is rarely a budget question.

What a high result does and does not mean

A result at or above the action level means one specific thing: your home tested high enough that the EPA recommends reducing it, and the recommended fix is a mitigation system. It is a reading about a building, and it points to a building solution. The radon mitigation page explains how the common sub-slab approach vents the gas from under the foundation, and the radon mitigation cost in Wisconsin guide lays out what a system typically runs, generally $1,000 to $2,000 for a contractor-installed setup.

A high result does not mean the things people often fear when they see the number. It is not a medical diagnosis, and it says nothing about any one person's health, symptoms, or odds of illness. A radon test measures air, not people, and Badger State Radon does not and cannot translate a reading into a personal risk figure. If you have health questions, those belong with a doctor. What the number can do is trigger the practical next step, which is confirming the reading and, if it holds, getting the level down.

Next steps

If your result sits at or above 4.0 pCi/L, or in the 2 to 4 pCi/L range and you have decided to act, the next step is a mitigation professional. Wisconsin does not license radon contractors, so the market is open, and the professionals you are matched with work locally and can hold the voluntary national credentials from the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB). Badger State Radon does not test, does not mitigate, and does not hold any radon certification. Tell us your city or county and where your test landed, and we connect you with an independent local radon professional who can confirm the reading and quote a system. You are never obligated to hire anyone.