What radon number means I should fix my home?

The EPA action level is 4.0 pCi/L. At or above that reading, the EPA advises fixing the home. Between 2.0 and 4.0 pCi/L, the EPA suggests you consider action, since no level of radon is risk-free. Northern Colorado sits in EPA Radon Zone 1, so elevated results are common, and the only way to know your specific home is to test it.

Why can a single short-term radon test mislead me?

Radon rises and falls day to day with weather, air pressure, and how open the house is. A short-term test of two to seven days captures a narrow window. A result just above or below 4.0 pCi/L can flip on a retest. That is why a borderline short-term reading usually calls for a second test or a long-term test before you decide.

How do I test for radon in winter versus summer?

Radon often reads higher in winter because homes stay closed and heating systems pull air through the foundation. Short-term tests use closed-house conditions: keep windows and outside doors shut except for normal entry for 12 hours before and during the test. Long-term tests run more than 90 days and average out seasonal swings, giving a more reliable year-round picture.

My result was elevated. What is the next step?

Confirm the reading, then get a mitigation quote from an independent, Colorado-licensed radon professional. NoCo Radon Pros is a free matching service, not a contractor, and holds no radon license. We connect you with a state-licensed professional and you can verify any license on the DORA lookup. A standard sub-slab system in Colorado commonly runs about $1,500.

Do I need to retest after installing a mitigation system?

Yes. A post-mitigation test confirms the system brought the level down, ideally below 4.0 pCi/L and as low as reasonably achievable. The Colorado-licensed radon professional you are matched with can advise on timing and placement, and periodic retesting afterward is smart because levels can shift over a home's life.

Radon Test Results Explained

That number on your radon report, written as pCi/L, is the single figure that decides whether a Northern Colorado home needs attention, yet most homeowners in Larimer and Weld Counties see it for the first time on an inspection sheet with no idea what counts as high. This guide walks through how to read the result, what the EPA action level means, why one quick test can point you the wrong way, and what to do if the reading comes back elevated.

What pCi/L actually measures

Radon is measured in picocuries per liter of air, abbreviated pCi/L. It is a count of radioactive decay happening in the air you breathe, so a higher number means more radon and more exposure over time. There is no visible sign of it in a house, which is why a lab or digital monitor result is the only way to know what a home holds.

Two things make this reading matter in our area. Both Larimer County and Weld County fall inside EPA Radon Zone 1, the highest-potential category, where the predicted average indoor screening level is greater than 4.0 pCi/L. And statewide, CDPHE reports that about half of Colorado homes have elevated radon levels. That is a Colorado-wide figure, not a Larimer or Weld count, but it tells you elevated results here are common rather than rare. For how our region compares, see the Northern Colorado radon levels guide.

The 4.0 pCi/L action level

The reference point on almost every report is the EPA action level of 4.0 pCi/L. The EPA advises fixing a home at or above 4.0 pCi/L. Between 2.0 and 4.0 pCi/L, the EPA suggests you consider taking action, because no amount of radon is completely without risk. Below 2.0 pCi/L, most homeowners simply plan to retest down the road.

Here is a plain way to read your own number:

At or above 4.0 pCi/L: the EPA recommends mitigation. This is the clear case for getting a quote.
2.0 to 4.0 pCi/L: a judgment call. Many Colorado homeowners choose to reduce it anyway, especially in a Zone 1 county.
Under 2.0 pCi/L: low for now. Retest every few years and after major foundation or HVAC changes.

Why the concern at all? Radon is the second leading cause of lung cancer in the United States after smoking, and the leading cause among people who have never smoked, per CDPHE. That single health fact is the reason the action level exists. It is not medical advice, and if you have health questions the right people to ask are your doctor and the state resources, but it explains why a reading over 4.0 is worth acting on.

Short-term versus long-term tests

Not all results carry the same weight, and the type of test behind your number changes how much you should trust it.

A short-term test runs from two to seven days. It is fast and it is the standard tool during a real estate transaction because deadlines are tight. The tradeoff is that it captures only a narrow slice of time. Radon moves up and down with the weather, with air pressure, and with how much the house is opened and closed. A two-day test during a cold, sealed-up week can read very differently from one during a mild, breezy stretch.

A long-term test runs more than 90 days. Because it spans changing conditions, it averages out the swings and gives a truer picture of what a household actually breathes across a year. If you are not under a transaction clock and you want the most reliable answer, a long-term test is the better instrument.

This is where a single short test can mislead you. Suppose a two-day kit reads 3.8 pCi/L and you decide the home is fine. A follow-up test the next month could easily land at 4.6. The reverse happens too: a scary 5.0 on a closed-up winter test may settle lower over a longer window. A borderline short-term result near the 4.0 line is a signal to test again, not a final verdict. Learning how to run a reliable test is covered in the radon testing service overview.

Seasonal swings and closed-house conditions

Two factors move radon numbers more than any others: the season and how the house is sealed during the test.

Radon often reads higher in winter. Homes stay shut, and heating systems create a slight vacuum that can pull soil gas up through the foundation. Summer readings on the same house can look lower simply because windows are open and air moves freely. Neither number is wrong, but they answer slightly different questions.

To keep a short-term test honest, it uses closed-house conditions. Keep windows and exterior doors shut, except for normal entry and exit, for 12 hours before the test starts and throughout the test. Run heating or air conditioning as usual, but skip whole-house fans and open windows. Place the device in the lowest lived-in level of the home, away from drafts, exterior walls, and high humidity. Skipping these steps is one of the most common reasons a result comes back artificially low and gives false comfort.

When to retest

A few situations call for another test rather than a one-and-done reading:

Borderline results near 4.0 pCi/L. Confirm with a second short-term test or a long-term test before deciding.
Conflicting results. If two tests disagree, lean on the higher reading or the longer test.
After a mitigation system goes in. A post-mitigation test verifies the system actually lowered the level.
After big home changes. New foundation work, a basement finish, HVAC replacement, or major weatherization can all shift radon, so retest afterward.
On a schedule. Even a low home is worth a retest every few years, since levels are not fixed for the life of a house.

If your result is elevated

When a confirmed reading sits at or above 4.0 pCi/L, the practical next step is a mitigation quote. The good news is that radon is fixable. A standard active sub-slab system pulls soil gas from beneath the foundation and vents it above the roofline, and in Colorado most homes run about $1,000 to $2,500, with roughly $1,500 common for a standard sub-slab system. Crawl spaces and larger jobs can cost more. You can see the full breakdown in the radon mitigation cost guide and how systems work on the radon mitigation service page.

One point on licensing that matters in Colorado. Since July 1, 2022, anyone performing radon measurement or mitigation for pay must hold a state license through DORA, created under House Bill 21-1195. NoCo Radon Pros is a free matching service. We are not a contractor and we hold no Colorado radon license. That license belongs to the independent professional you are matched with. We connect you with a Colorado-licensed radon professional, and you can confirm any license yourself on the DORA license lookup.

If you are reading a result because you are buying, selling, or renting, radon also ties into Colorado disclosure law. The full picture lives in the Colorado radon law guide. When you are ready to turn an elevated number into a plan, get matched with a state-licensed professional for a mitigation quote at no cost to you.