Radon Levels in Northern Colorado
Point at a map of Colorado’s radon zones and Northern Colorado lights up in the highest-risk color. Both Larimer County and Weld County, the two counties that make up this region, carry an EPA Radon Zone 1 designation, the most elevated of the three categories the federal government uses. That single fact shapes how homeowners here should think about testing, buying, selling, and renting. This guide explains what Zone 1 actually means, why the geology under Fort Collins, Greeley, and the towns between them produces it, and why the zone map still cannot tell you what is happening inside your specific house.
What EPA Radon Zone 1 means
The U.S. Environmental Protection Agency divides every county in the country into one of three radon zones based on predicted indoor levels. Zone 1 counties have the highest radon potential: a predicted average indoor screening level greater than 4.0 pCi/L (picocuries per liter of air). Zone 2 counties fall between 2.0 and 4.0, and Zone 3 counties sit below 2.0.
Larimer and Weld are both Zone 1. You can confirm this on the EPA Map of Radon Zones, which shades Colorado county by county. The number 4.0 matters because it is also the EPA action level: the agency advises fixing a home when a radon test reads 4.0 pCi/L or higher, and suggests considering action when a result lands between 2.0 and 4.0. You can read the EPA’s own explanation of that threshold on its action level page.
One important limit: the zone map was built for planning and code decisions, not to grade an individual property. A Zone 1 label means the county’s predicted average clears the action level. It does not mean every home is high, and it does not mean any given home is safe. It means the odds justify testing.
The statewide picture from CDPHE
The Colorado Department of Public Health and Environment (CDPHE) has documented how common elevated radon is across the state. According to CDPHE, about half of Colorado homes have radon levels above the 4.0 pCi/L action level. You can see that statement in the department’s own press release.
That “about half” figure is a Colorado statewide number. It is not a Larimer County figure or a Weld County figure, and it would be a mistake to repeat it as one. What ties it to Northern Colorado is context: the state as a whole runs high, and this region carries the highest zone classification within that state. Put those two facts side by side and the practical conclusion is simple. In a Zone 1 region inside a high-radon state, testing is the reasonable default rather than the exception.
We deliberately do not publish a “percent of homes above 4.0” number for Larimer or Weld specifically, and you should be skeptical of any local page that does. Those county-level percentages are not authoritatively verified. The honest, defensible facts are the Zone 1 designation and the CDPHE statewide figure, and those are enough to justify a test on any home here.
Why the geology drives radon here
Radon is a naturally occurring radioactive gas produced by the breakdown of uranium in soil and rock. It seeps up through the ground and enters buildings through foundation cracks, construction joints, gaps around pipes, sump pits, and crawl space floors. Because a house is often slightly depressurized relative to the soil beneath it, it can draw that gas indoors and hold it.
Northern Colorado’s soils and underlying geology contain enough uranium-bearing material to push the predicted indoor average above the action level, which is exactly what earns Larimer and Weld their Zone 1 status. This is a regional characteristic of the ground itself, not a defect in any particular builder’s work or any single neighborhood. It is also why radon is not seasonal folklore or a rare event here: the source is the earth under the whole area.
Geology explains the elevated potential. It does not, however, deliver a reading for your address. That is the next and most important point.
Why only a test reveals your home’s level
Two houses on the same block can post very different radon numbers. Soil composition shifts across short distances. Foundation type matters: a full basement, a slab-on-grade, and a crawl space each interact with soil gas differently. Cracks, the presence of a sump, how tightly the home is sealed, and how its heating and ventilation move air all change the result. New construction and older housing can both read high or low.
Because of that variation, the zone map and the statewide statistics can tell you the regional risk is high, but they cannot tell you your number. The only way to know a specific home’s level is to measure it. A short-term or long-term test returns a value in pCi/L that you compare directly to the 4.0 action level. Our radon testing service page walks through how measurement works and how we connect you with an independent, state-licensed radon professional to carry it out. Once you have a result in hand, Radon Test Results Explained helps you interpret it and decide whether mitigation is warranted.
When a result becomes a trigger
A radon test that comes back at or above 4.0 pCi/L is the common trigger for action. In Northern Colorado, that trigger shows up in several situations at once. A homeowner testing for general health reasons uses it to decide on mitigation. During a home sale, Colorado’s disclosure law puts radon squarely on the table, and a high result can shape negotiations. For rentals, a landlord’s radon duties turn on documented measurements. All of those threads run through the same 4.0 threshold and the same underlying Zone 1 reality.
Colorado also licenses the people who do this work. Radon measurement and radon mitigation are both state-licensed professions in Colorado, and that license belongs to the professional, never to a matching service like this one. Before any work begins, you can verify a contractor’s credential on the state’s DORA license lookup. The broader legal framework, including sales and lease disclosure rules, is laid out in our Colorado Radon Law Guide.
Northern Colorado, city by city
Every launch city in this region sits inside EPA Zone 1, on one or both county sides. That includes fast-growing new-build towns and older established cores alike, because the zone classification follows the ground, not the age of the housing.
Fort Collins, the Larimer County seat, spans a historic Old Town core and newer south-side and Harmony Corridor development, all within Zone 1. You can read the local detail on the Fort Collins radon page. Greeley, the Weld County seat, mixes early-1900s bungalows and Victorians downtown with 1990s and 2000s subdivisions to the west, also entirely Zone 1; see the Greeley radon page for specifics. The same designation covers Loveland, Windsor, Wellington, Berthoud, Timnath, and Johnstown. The full set is on our locations hub.
What to do with this
The takeaway is short. Northern Colorado is EPA Radon Zone 1, Colorado as a whole runs about half of homes above the action level per CDPHE, geology explains the elevated potential, and only a test reveals your specific number. If you own, are buying, are selling, or are renting a home in Larimer or Weld County, the next step is a measurement.
When you are ready, contact us and we will connect you with an independent, state-licensed radon professional. If a test comes back elevated, our radon mitigation service page explains what fixing the problem involves. There is no cost to you for the match, and you can read exactly how we make money so you know where we fit.