What Is Radon and Why Does It Matter in Colorado?

Radon is a naturally occurring radioactive gas produced as uranium breaks down in soil and rock. It has no colour, no smell and no taste, which is the entire problem: a house can hold a level the EPA would act on for decades and nobody inside it would ever notice.

In Colorado that matters more than almost anywhere else in the country. CDPHE estimates roughly 1 in 2 Colorado homes have elevated radon, and El Paso County Public Health reports over 40% of county homes tested between 2005 and 2023 came back elevated. Radon is the second leading cause of lung cancer in the United States.

Why Colorado geology produces so much of it

The Front Range sits on and beside the Pikes Peak granite batholith, a vast body of coarse granitic rock that carries trace uranium throughout. Uranium decays through a chain that produces radium, and radium decays into radon.

The granite is only half the story. Over millions of years it weathered and washed eastward, forming the Dawson Arkose — a gravelly, highly permeable sandstone that covers much of El Paso and Douglas Counties. So a house here sits on uranium-bearing parent material, and it sits on ground porous enough for soil gas to travel through easily. Coarse, permeable soil is exactly what lets radon reach a foundation from metres away rather than seeping slowly from directly underneath.

That is why the EPA designates El Paso County, along with Teller, Douglas, Denver and Pueblo Counties, as Radon Zone 1 — its highest of three tiers, meaning the predicted average indoor screening level for the county exceeds 4.0 pCi/L.

How it actually gets into a house

Radon does not seep in passively. It is pulled in. A heated building sits at slightly lower air pressure than the ground beneath it, because warm indoor air rises and escapes at the top, and replacement air is drawn in at the bottom. That replacement air comes through every crack, joint and penetration in the lowest level.

The usual entry points are the cove joint where the basement floor meets the wall, shrinkage cracks in the slab, gaps around plumbing and utility penetrations, hollow block wall cores, and open sump pits. A sump pit without a sealed lid is effectively a hole straight into the ground.

This is also why the effect is strongest in winter. The colder it is outside, the bigger the pressure difference, and the harder the house pulls. A Colorado home often reads higher in January than the same house reads in July.

What a number means

Radon is measured in picocuries per litre, written pCi/L. Two thresholds matter:

• The EPA recommends taking action at 4.0 pCi/L or higher
• The EPA recommends considering action between 2 and 4 pCi/L
• What every reading means, band by band, with a pCi/L to Bq/m³ converter

For context, the average indoor level in the United States is about 1.3 pCi/L and the average outdoors is roughly 0.4 pCi/L. If you already have a result in hand, our radon levels chart explains what any reading means and converts between pCi/L and Bq/m³.

One point deserves to be stated plainly: no level of radon is proven safe. 4.0 is an action level, not a safety line. A lower reading means lower exposure, not none. Anyone describing a result as “safe” is saying something the EPA does not.

Things that do not predict your level

• A new house. Modern construction is more tightly sealed, exchanges less air with outdoors, and holds whatever gets in for longer. New homes test elevated routinely
• A neighbour's result. Radon varies house to house on the same street, because it depends on your specific foundation, your soil contact and how your building moves air
• A slab instead of a basement. A concrete slab is not a radon barrier. Control joints, plumbing penetrations and the perimeter gap all pass soil gas
• How the house feels. There is no smell, no stuffiness, no symptom. Air that feels fine tells you nothing

What to do about it

Testing is the only way to find out, and it is cheap relative to what it settles. A professional radon test is a flat $150 and takes about 48 hours. When it makes most sense to test covers the timing.

If the result comes back at or above the action level, an active mitigation system reduces it — the EPA states these systems can lower levels by up to 99%. CDPHE puts a typical residential system in Colorado at $1,000 to $2,000.

And if you are buying or selling, radon now carries specific disclosure duties in Colorado. What sellers, buyers and landlords have to disclose covers Senate Bill 23-206.