Radon in Colorado: What Denver Homeowners Need to Know

Key fact: About 50% of Colorado homes tested above the EPA action level of 4.0 pCi/L, compared to roughly 6% nationally. The Denver metro is in EPA Zone 1 — the highest-risk designation.

Why Colorado Has So Much Radon

Radon comes from uranium. As uranium naturally decays in soil and rock, it produces radon gas — which seeps upward through the ground and into buildings through cracks, openings, and pressure differentials.

Colorado's Front Range sits on geology that's particularly rich in uranium-bearing formations. The Denver Basin — the sedimentary formation underlying most of the metro area — contains elevated uranium concentrations compared to national averages. Add to that the granitic soils of the foothills and mountains, and you have persistent radon production throughout the region.

Altitude plays a role too. At 5,280+ feet, Denver homes sit closer to uranium-producing bedrock than lower-elevation areas. The Palmer Divide (Castle Rock, Parker) and the foothills (Arvada, Highlands Ranch) test even higher because they're at greater elevation with more direct exposure to uranium-bearing geology.

Colorado's EPA Zone 1 Designation

The EPA divides the country into radon zones based on predicted average indoor radon levels:

• Zone 1 — predicted average above 4 pCi/L (highest potential)
• Zone 2 — predicted average 2–4 pCi/L
• Zone 3 — predicted average below 2 pCi/L

Most of the Denver metro — including Jefferson County (Arvada, Lakewood), Douglas County (Highlands Ranch, Castle Rock, Parker), and Arapahoe County (Aurora, Centennial) — is designated Zone 1. This doesn't mean every home tests high, but it means the baseline risk is elevated and testing is strongly recommended.

What the Numbers Mean

Radon is measured in picocuries per liter of air (pCi/L). Here's how to interpret your test results:

Level	Interpretation	Recommended Action
Below 2.0 pCi/L	Low — well below action level	No action needed; retest every 2 years
2.0–4.0 pCi/L	Moderate — below EPA action level	Consider mitigation; retest periodically
4.0–8.0 pCi/L	Elevated — at or above action level	Mitigation recommended
Above 8.0 pCi/L	High	Mitigation strongly recommended
Above 20 pCi/L	Very high	Prioritize mitigation

Note: The EPA action level is 4.0 pCi/L. We guarantee our systems achieve below 2.0 pCi/L — a more protective standard.

Why Basements Are the Risk

Radon enters homes primarily through the foundation — cracks in concrete slabs, gaps around pipes, and pressure differentials that draw soil gas indoors. Basements have the most foundation exposure and the least air exchange with the outdoors, so radon accumulates there first and at higher concentrations.

Finished basements are a particular concern: bedrooms, home offices, and playrooms in finished basements expose occupants to the highest radon concentrations in the home for extended periods. A test taken in the finished basement will almost always read higher than one taken on the main floor.

Testing: What You Need to Know

Short-term tests (2–7 days) using charcoal canisters are the most common method. They're inexpensive (under $30 at hardware stores) and accurate enough for initial screening. For real estate transactions, a short-term test is standard.

Long-term tests (90+ days) give a more accurate picture of annual average exposure. If you're making a mitigation decision based on health (rather than a transaction deadline), a long-term test is more reliable.

Test placement matters: place the test in the lowest livable level of your home — a finished basement if you have one, or the first floor if you don't. Test during closed-house conditions (windows and doors closed, except for normal entry and exit).

When to Mitigate

The EPA recommends mitigation at 4.0 pCi/L or above. Many professionals — and 5280 Radon specifically — recommend action at 2.0+ pCi/L for maximum safety margin, particularly in homes with children, pregnant women, or occupants with elevated health risk.

For real estate transactions, the buyer or lender typically specifies a threshold. Most transactions require mitigation if levels are at or above 4.0 pCi/L.

How Mitigation Works

The standard treatment is sub-slab depressurization (SSD): a fan system that draws radon from beneath the foundation and exhausts it above the roofline before it can enter the living space. Installation takes 4–6 hours for most homes. The system runs continuously on 10–20 watts of power.

See our residential mitigation page for the full process, or our pricing page for what it costs.