Radon in Colorado

Colorado sits on uranium-bearing granite and the soils weathered from it, and the state health department estimates that about half of Colorado homes have radon above the EPA's 4 pCi/L action level. 53 of the state's 64 counties are EPA Radon Zone 1. This guide covers predicted levels for the Front Range and the rest of the state, why test results and averages differ, Colorado's rules and assistance programs, and how to test and fix a home.

Predicted (model estimate). County and city values below are long-term area averages from the 2001-2021 National Radon Exposure Model, not readings from any home. See the full Colorado radon page for every county.

Front Range counties at a glance

Most Coloradans live along the Front Range, from Fort Collins to Pueblo. Every county in the table is EPA Zone 1. The statewide population-weighted predicted basement average is 2.6 pCi/L, and 94.9% of Colorado ZIP codes are predicted at or above 2 pCi/L in the basement.

Highest predicted counties statewide

Outside the metro areas, the mountain and southern counties carry the highest long-term predictions: Pueblo County (4.2), Chaffee County (4.1), Teller County (4.0), Gunnison County (3.9), Jackson County (3.7), Montrose County (3.7), Phillips County (3.7), Fremont County (3.6).

Largest Colorado cities

• Denver: 2.1 pCi/L predicted basement mean (pop. 716,577)
• Colorado Springs: 2.4 pCi/L predicted basement mean (pop. 488,664)
• Aurora: 2.3 pCi/L predicted basement mean (pop. 395,052)
• Fort Collins: 3.0 pCi/L predicted basement mean (pop. 170,376)
• Lakewood: 2.6 pCi/L predicted basement mean (pop. 155,961)
• Thornton: 2.1 pCi/L predicted basement mean (pop. 144,922)
• Arvada: 2.3 pCi/L predicted basement mean (pop. 121,414)
• Westminster: 2.2 pCi/L predicted basement mean (pop. 114,875)
• Greeley: 3.0 pCi/L predicted basement mean (pop. 112,609)
• Pueblo: 3.8 pCi/L predicted basement mean (pop. 111,077)

Why Colorado has so much radon

Radon comes from the natural breakdown of uranium in soil and rock; CDPHE describes it moving from uranium-bearing granite deposits in the soil toward the atmosphere, and granite and the sediments eroded from it underlie much of the state. The gas moves up through the soil, and a house sits right in its path. CDPHE puts it simply: a home is usually warmer and at lower pressure than the surrounding soil, so soil gas, including radon, moves in through the gap between the slab and the walls, cracks, sump openings, construction joints, plumbing penetrations and crawlspaces.

Colorado homes add two multipliers. Basements are common along the Front Range, and basements are where radon is highest. And the long heating season, with cold nights and big indoor-outdoor temperature differences, maximizes the stack effect that pulls soil gas into a house. CDPHE also stresses that the age and type of a home do not predict its radon: new and old, slab and basement, all can be high.

Why "half of homes" and a 2.6 pCi/L average can both be true

CDPHE's estimate that about half of Colorado homes are above 4 pCi/L comes from radon test results. The predicted values on this site come from a national model of each area's long-term average across seasons and floors. The two measure different things. Most home tests are short-term, are taken in the lowest level of the house with windows closed, and are often run in the heating season, all of which push a single result above a year-round average. Radon is also highly skewed: a modest area average still includes many homes far above 4. The practical takeaway is the same either way - in Colorado, testing is the only way to know, and a high result is common.

Testing a Colorado home

CDPHE recommends that every home be tested - single-family houses, manufactured homes and apartments. Do-it-yourself short-term kits are sold at home improvement stores for about $10 to $50, according to CDPHE, and the department also sells discounted kits through its radon program. Its instructions match national practice:

• Close exterior windows and doors 12 hours before the test and keep them closed during it; normal entry and exit is fine and the furnace can run normally.
• Place the kit on the lowest level you live on (a basement counts even if unfinished), 2 to 7 feet above the floor, away from exterior walls, vents, kitchens, bathrooms, laundry rooms and crawlspaces.
• Run it at least 48 hours, record the start and stop times and the serial number, and ship it to the lab within a day.
• For a long-term picture, a 91-day-to-one-year test averages out swings from temperature and barometric pressure.

CDPHE says it is always a good time to test as long as you can keep the house closed, and that some people find that easier in winter - see why winter is the best time to test. For kit types and placement in more detail, see how to test for radon.

Colorado rules worth knowing

• Real estate and rentals. CDPHE publishes "Radon and real estate transactions in Colorado," a brochure it lists as required for real estate transactions and rental agreements. Buyers are encouraged to test during the inspection period with a licensed measurement professional. See radon and real estate.
• Schools and child care. Colorado regulations require schools and child care facilities to test for radon and keep the results available on request. They do not require mitigation.
• New construction. Some Colorado jurisdictions require radon-resistant new construction; CDPHE keeps a list. A passive system built in this way can usually be converted to an active, fan-driven system cheaply if a test comes back high.

Fixing a high result in Colorado

If a confirmed result is 4 pCi/L or higher - or between 2 and 4 and you want it lower - the standard fix is active sub-slab depressurization: a pipe through the basement slab, a continuously running fan, and an exhaust above the roofline. Crawlspace homes get a sealed membrane with suction underneath. Front Range basements on gravel usually mitigate well with one suction point; dense clay soils can need more suction points and better diagnostics. Our mitigation guide explains the system and what a good bid includes.

What it costs in Colorado, from published sources: Garfield County, Colorado cites $1,500 – $2,500 as the average cost of mitigation. Compare other states in radon mitigation cost by state.

Use a mitigator certified by NRPP or NRSB and ask for their certification number. Start with the EPA find-a-professional page, get two or three written bids, and retest within 30 days of the fan starting.

Low-income assistance: CDPHE's Low Income Radon Mitigation Assistance (LIRMA) program helps qualifying homeowners pay for a system, with separate applications for homeowners and participating contractors. CDPHE's 2024 rulemaking notice shows assistance limited to a maximum of $1,500 per homeowner, and nearly $950,000 paid to about 493 low-income homeowners since 2018. CDPHE also points homeowners to Colorado Department of Local Affairs home rehabilitation programs, HUD rehabilitation programs and USDA rural repair loans and grants.

Sources

• CDPHE - Radon (Colorado radon program home)
• CDPHE - Understanding radon
• CDPHE - Testing your home for radon
• CDPHE - Radon and real estate
• CDPHE - Low Income Radon Mitigation Assistance (LIRMA)
• Colorado Environmental Public Health Tracking - Radon data
• Garfield County, Colorado - State funding assists low-income households with radon mitigation