Radon Testing in Longmont, CO | InspectandTest
Radon is the leading cause of lung cancer among nonsmokers, and Longmont sits in one of the highest-risk regions for it. Radon testing in Longmont is the only way to know your home’s level, because the gas is colorless, odorless, and undetectable without a measurement. Longmont is in Boulder County, which falls within EPA Radon Zone 1, the agency’s highest designation, and many local homes test above the 4 pCi/L action level. This guide explains how testing works, why local geology and housing drive elevated readings, what it costs, and the Colorado disclosure rules that apply. This summarizes EPA and CDPHE guidance current as of 2026; consult a certified professional for testing decisions.
Radon testing in Longmont: what to know
Radon is a radioactive gas produced as uranium in soil and rock breaks down. It enters homes through foundation cracks, sump pits, and gaps around pipes, then concentrates in the lowest occupied levels. The EPA recommends action at or above 4 pCi/L, and CDPHE, which runs Colorado’s radon program and offers discounted test kits to residents, uses the same threshold.
Testing comes in two forms. A short-term test of two to seven days gives a quick screening result, useful for a real-estate transaction under closed-house conditions. A long-term test of more than 90 days captures seasonal variation and gives a truer annual average, which is better for a home you already own and live in.
Radon is not a marginal concern in Boulder County. EPA and CDPHE both flag Colorado as a state where indoor radon frequently exceeds the action level, and the Zone 1 designation covering Longmont reflects the highest predicted average indoor levels in the country. Because the gas comes from the soil and rock beneath each individual home, results vary dramatically from house to house. That is the single most important point for a Longmont homeowner: you cannot rely on a neighbor’s reading, the age of the home, or a builder’s assurance. Only a test of your own home tells you your level, and the only way to find a problem is to look for it.
Short-term versus long-term radon tests in Longmont
The right test depends on your situation. A short-term test is fast and inexpensive, making it the standard for real-estate transactions that must conclude within the contract timeline. It runs under closed-house conditions, meaning windows and exterior doors stay shut except for normal entry, which keeps fresh air from artificially lowering the reading. The tradeoff is that a few days cannot capture how radon shifts with weather and season.
A long-term test, running more than 90 days and ideally through the heating season, smooths out those swings and gives a result much closer to your true year-round exposure. For a Longmont home you plan to live in, this is the more meaningful number, since radon exposure is a long-term health risk measured over years rather than days. Many homeowners run a short-term test first for a quick read, then confirm with a long-term test or a professional measurement if the initial result is near or above 4 pCi/L. Retesting is also wise after major foundation work or after a mitigation system is installed.
Why Longmont homes have high radon
The Front Range geology beneath Boulder County contains uranium-bearing soils, which is the root cause of the Zone 1 designation. Radon levels vary house to house even on the same block, so a neighbor’s low result tells you nothing about your own home.
Longmont’s housing patterns shape the risk. The older downtown core, including Old Town Longmont, has pre-WWII homes with foundations that crack and open over time, letting soil gas in. The large newer subdivisions built from the 1990s through the 2010s, such as those around Prospect and Southmoor Park, are sealed tightly for energy efficiency, which reduces natural air exchange and lets radon accumulate. Finished basements, common in both eras, place living space in the lowest, highest-radon level of the home. Winter makes readings peak, because closed-up houses during the heating season trap radon at its highest concentrations.
If a test comes back high, the remedy is a mitigation system that vents the gas before it builds up indoors. The mold concern in flood-affected Longmont basements often overlaps with the same damp conditions, so some owners address both; our guide to mold inspection in Longmont covers that side of the wet-basement problem.
What a Longmont radon test covers
A professional radon test generally includes:
- Closed-house conditions set up before and during a short-term test, with windows and exterior doors kept shut except for normal use.
- A continuous radon monitor or test kit placed in the lowest livable level, away from drafts and exterior walls.
- A measurement period of two to seven days for short-term tests, longer for an annual average.
- A written report stating the average pCi/L and whether it meets or exceeds the 4 pCi/L action level.
- Guidance on next steps, including retesting or mitigation if the level is elevated.
Continuous monitors used by professionals also log tampering and hourly fluctuations, which charcoal kits cannot do, and that record matters in a sale. For how the equipment works, see our radon testing guide for Front Range homeowners.
What to do if your Longmont radon test is high
A result at or above 4 pCi/L is common in Boulder County and is not cause for panic, but it does call for action. The standard fix is a radon mitigation system, most often active sub-slab depressurization, which uses a fan and a vent pipe to draw the gas out from beneath the foundation before it can enter the home. A properly installed system reliably brings most homes well below the action level, and a follow-up test confirms it worked.
Cost should not be a reason to delay. A typical Longmont mitigation system is a one-time expense in the low thousands of dollars, modest against both the home’s value and the health risk radon poses over years of exposure. If your test came back high during a sale, you have options: many Colorado deals resolve radon by having the seller install or credit a system before closing. Homes with a wet basement from Longmont’s flood history may need a system that also addresses the sump, so plan for that. For the full picture of mitigation, see our guide to radon mitigation in Longmont. The key is to act on a high reading rather than set it aside, since radon is a leading cause of lung cancer and the fix is well established.
Radon testing cost in Longmont
A DIY short-term kit costs roughly $15 to $40, and CDPHE periodically offers discounted kits to Colorado residents. A professional short-term test with a continuous monitor, typically arranged during a real-estate transaction, generally runs from about $125 to $250. Bundling radon into a full home inspection often lowers the per-service cost.
For a home you own, the discounted DIY long-term kit is a cost-effective way to get an accurate annual average. For a purchase, the professional monitor earns its cost through tamper logging and closed-house verification.
Set the cost against what is at stake. Radon testing is among the least expensive steps in protecting a household’s health, and the EPA identifies radon as a leading cause of lung cancer after smoking. A test that costs from a few dollars to a couple hundred can reveal a hazard that, left unmeasured, exposes a family for years. For a buyer, the cost is trivial against the price of a Longmont home, and testing during the transaction means a high result can become the seller’s expense to address. There is no scenario where skipping the test saves meaningful money, because the only way to know your home’s level, and therefore whether you need to act, is to measure it.
Longmont regulations and disclosure
Colorado requires radon disclosure in residential real-estate transactions and includes radon on the Seller’s Property Disclosure; CDPHE administers the state program. Sellers must share known radon information and any prior test results or mitigation systems. Colorado does not license home inspectors, so for radon professionals look for certification through a recognized national radon program rather than a state permit.
The disclosure rule means a high reading need not end a deal, but it must be disclosed. Many Longmont transactions resolve radon by having the seller install or credit a mitigation system before closing.
This framework protects buyers and benefits sellers alike. Because radon must be disclosed and is routinely tested during the inspection window, a buyer who orders the test is unlikely to be blindsided by a hidden radon problem after closing. A seller with a recent test and, where needed, a mitigation system already installed removes a common sticking point and signals a responsibly maintained home. For any Longmont homeowner, the message is the same: radon is a known, measurable, and fixable issue, and the disclosure rules exist to ensure it is handled openly rather than ignored, especially in a Zone 1 county where elevated readings are common.
Neighborhoods and areas served
Radon testers serving Longmont commonly cover Old Town Longmont, Prospect, Southmoor Park, and the newer subdivisions across the city. Service typically extends to adjacent Boulder County and nearby communities, including Boulder, Loveland, Firestone, and Frederick. Because radon varies so much home to home, a local tester will not assume a result from the neighborhood; every property needs its own measurement.
The variation surprises many Longmont homeowners. A newer home in a Prospect or Southmoor Park subdivision can test higher than a pre-WWII house in Old Town, or the reverse, depending entirely on the soil, the foundation, and how the home exchanges air. New construction is not automatically safe; the tight building envelopes built for energy efficiency can hold radon in if the home was not roughed for passive venting. A finished basement raises exposure regardless of the home’s age, because it puts living space in the lowest level where radon concentrates. None of this can be predicted from the curb, which is why testing your specific home is the only reliable path. Homes with a damp basement from Longmont’s flood history may also have foundation cracks that double as radon entry points, another reason to measure rather than assume.
How to choose a Longmont radon tester
Look for certification through a recognized national radon proficiency program, and ask whether the tester uses a continuous radon monitor for transactions, since the tamper log and closed-house verification protect both buyer and seller.
Confirm the report states the average in pCi/L against the 4 pCi/L action level and includes clear next steps. Ask whether the tester handles or refers mitigation, so you have a path forward if the level is high. If you are comparing consumer devices first, our guide on whether radon detectors work is a useful starting point.
References
- Health Risk of Radon — U.S. Environmental Protection Agency
- Colorado Radon Program and Test Kits — Colorado Department of Public Health and Environment
- EPA Map of Radon Zones — U.S. Environmental Protection Agency
- About Radon and Health — Centers for Disease Control and Prevention
Longmont homeowners and buyers who want a certified radon test with results they can act on can reach out through our contact page to connect with a vetted Boulder County professional.