Radon is a naturally occurring radioactive gas — invisible, odorless, tasteless. It comes from the decay of uranium in the soil and rock beneath your home, and it's measured in units of picocuries per liter of air (pCi/L). Outdoor air averages around 0.4 pCi/L. Indoor air in Colorado averages much higher, and roughly half of Colorado homes test above the EPA's action level of 4.0 pCi/L.[1]
This page is the plain-language introduction. We'll cover what radon actually is, where it comes from, how it's measured, and why it matters. The companion pages in this section go deeper on the Colorado-specific geology, how radon enters homes, what the health risks are, and what your specific test number means.
The short version
- Radon is a radioactive gas produced by the natural decay of uranium in soil and rock.
- It's invisible and odorless. You can't tell it's there without a test.
- It accumulates in lower levels of homes — basements first, then living areas above.
- It's the #1 cause of lung cancer in non-smokers and the #2 cause overall (after smoking).[2]
- The EPA action level is 4.0 pCi/L. CDPHE recommends mitigation at or above this level.[1][3]
- Roughly half of Colorado homes test above the action level.
Where radon comes from
Radon is part of a chain of radioactive decay that starts with uranium-238, an element that exists naturally in trace amounts in most rocks and soils. Over billions of years, uranium-238 slowly decays through a series of other radioactive elements:
- Uranium-238 (half-life ~4.5 billion years) decays into Thorium-234.
- Through a series of intermediate steps, the chain produces Radium-226.
- Radium-226 (half-life ~1,600 years) decays into Radon-222.
- Radon-222 (half-life ~3.8 days) is the gas we test for. It decays further into "radon progeny" — short-lived radioactive particles that are actually what causes the lung damage.
The chain matters because uranium is everywhere in trace amounts, so radon is everywhere in trace amounts. Some rocks and soils have more uranium than others. Colorado's geology — particularly the Front Range — has more than most.[4]
How radon is measured
In the United States, radon is measured in picocuries per liter of air (pCi/L). A picocurie is a measure of radioactivity — specifically, the rate at which radon atoms are decaying in a given volume of air.
In most of the world outside the U.S., radon is measured in becquerels per cubic meter (Bq/m³). The conversion: 1 pCi/L ≈ 37 Bq/m³.
| Unit | Equivalent | Used by |
|---|---|---|
| 4.0 pCi/L | ~148 Bq/m³ | EPA action level (U.S.) |
| 2.7 pCi/L | 100 Bq/m³ | WHO reference level (international) |
| 0.4 pCi/L | ~15 Bq/m³ | Outdoor air average (U.S.) |
| 1.3 pCi/L | ~48 Bq/m³ | U.S. indoor average |
The U.S. EPA action level (4.0 pCi/L) is higher than the WHO's recommended action level (2.7 pCi/L). Both are based on cancer risk modeling; they differ in how much risk each agency deems acceptable.[2][5]
Why radon accumulates indoors
The air pressure inside a home is typically slightly lower than the air pressure in the soil beneath the foundation. That small pressure difference, driven by the stack effect (warm indoor air rising and creating suction at lower levels) and by HVAC systems, pulls soil gas up through small openings in the foundation: slab cracks, the floor-wall joint, sump pits, plumbing penetrations, and unsealed crawlspace gaps.
The radon-laden soil gas enters the lower levels of the home and accumulates there. Lower levels (basements, crawlspaces, ground floors of slab homes) test highest. Upper floors test lower because the gas mixes with outdoor air through normal ventilation as it rises.
Why Colorado specifically
Colorado has elevated indoor radon for two reasons:
- The geology. The Front Range and Colorado Plateau contain uranium-bearing granites and shales. Pikes Peak granite, in particular, has uranium-bearing accessory minerals. Uranium in the bedrock means radium in the soil, which means radon in the gas beneath your foundation.[4]
- The housing stock. Most Colorado homes have full or partial basements. Basements concentrate radon. Add high baseline soil gas to homes designed to capture it, and you get the state's high indoor radon prevalence.
The health connection
The radon gas itself isn't the threat — it's the radon progeny, short-lived radioactive particles produced when radon-222 decays. When you breathe air containing radon, the progeny attach to lung tissue and emit alpha radiation, which damages cells and can lead to lung cancer over years of exposure.
EPA estimates that radon causes roughly 21,000 lung cancer deaths per year in the United States. About 2,900 of those are among people who never smoked. Among current and former smokers, the risk is multiplied by the synergy between smoking and radon — they're not additive; they're compounding.[2]
What to do next
The only way to know your home's level is to test. Three options for Colorado:
- A short-term DIY kit ($15–$40, El Paso County Public Health Lab or retail).
- A long-term DIY kit ($30–$60, more accurate annual average).
- A professional continuous monitor ($150–$300, standard for real estate transactions).
If your test comes back at or above 4.0 pCi/L, the standard response is mitigation. Colorado has state-level radon contractor licensing through DORA, so verification is straightforward. How to choose a contractor →