Average Radon Levels: What Homeowners Need to Know
The U.S. national average indoor radon level is 1.3 pCi/L per EPA — well below the agency’s 4 pCi/L action level but above outdoor air, which averages 0.4 pCi/L. Averages vary widely by geography because radon comes from soil gas, and soil gas concentration depends on local geology. Colorado’s Front Range averages 2 to 4 pCi/L per CDPHE survey data because of granite bedrock and uranium-bearing sedimentary deposits, with many neighborhoods averaging above EPA’s action level. This guide summarizes EPA, CDC, and CDPHE guidance current as of 2026 on what average radon levels look like, how to interpret a specific home’s reading against the average, and when averages don’t matter. Our editorial methodology page explains the sources and limits of this guide.
What Are Average Radon Levels in the U.S.?
EPA’s published figures based on the National Residential Radon Survey and ongoing monitoring data:
- U.S. national average indoor: 1.3 pCi/L
- Outdoor air typical: 0.4 pCi/L
- EPA action level (mitigation recommended): 4 pCi/L
- WHO recommendation: 2.7 pCi/L (100 Bq/m³)
The national average obscures wide regional variation. EPA’s Map of Radon Zones divides U.S. counties into three zones based on predicted average indoor levels:
- Zone 1: Predicted average above 4 pCi/L
- Zone 2: Predicted average 2 to 4 pCi/L
- Zone 3: Predicted average below 2 pCi/L
Zone 1 counties cover much of the Mountain West, Upper Midwest, Appalachian region, and parts of the Northeast. Zone 3 counties cluster along the Gulf Coast and parts of the Pacific Coast. The map is a planning tool — individual home readings within any zone can vary dramatically from the predicted average.
Colorado Average Radon Levels
Colorado is one of the highest-radon states. CDPHE survey data places statewide indoor averages at 2 to 4 pCi/L, with significant neighborhood-level variation. Most Front Range counties — Denver, Boulder, Jefferson, Douglas, Arapahoe, Adams, Broomfield, Larimer, Weld, El Paso — sit in EPA Zone 1.
Roughly half of tested Colorado homes register at or above the 4 pCi/L action level. That’s substantially higher than the U.S. national pattern, where roughly 1 in 15 homes reads above 4 pCi/L. The driver is geology: granite bedrock in the foothills and uranium-bearing sedimentary deposits across the Denver Basin produce sustained radon release into soil gas, which enters homes through foundation cracks, sump pits, and slab penetrations.
For Front Range homeowners, the practical implication: don’t assume your home is “below average” because you live in a low-radon-news area. Local geology overrides national averages, and the national average is a poor predictor of your home’s reading. Always test. The radon testing pillar covers Front Range geology and testing protocols in more detail.
Why Averages Don’t Tell You About Your Home
Radon concentration in a specific home depends on factors that local averages can’t capture:
- Soil composition immediately under the foundation. Uranium content in the underlying soil varies on a parcel-by-parcel basis.
- Foundation type and condition. Slab-on-grade vs. crawl space vs. full basement; cracks, sump pits, slab joints.
- House construction. Energy-efficient tight construction can trap radon; older construction with more air leakage often shows lower readings.
- HVAC system. Stack effect, exhaust fan operation, fresh-air make-up units.
- Occupant behavior. Windows open or closed, fan use, time spent in lower levels.
Two homes on the same street, built in the same year by the same builder, can register 2 pCi/L and 12 pCi/L. The only way to know is to test.
How Averages Are Calculated
The published averages come from large-scale residential testing surveys. EPA’s National Residential Radon Survey, conducted in the late 1980s, measured radon in roughly 6,000 homes across the U.S. and produced the 1.3 pCi/L national average that’s still referenced today. State health departments and CDPHE have run subsequent state-level surveys to refine local averages.
Caveats on the averages:
- Most surveys use short-term tests in the lowest livable level — which may overstate or understate annual whole-home exposure.
- Self-reported testing data (CDPHE’s free test kit program, for example) is biased toward homeowners who suspect a problem.
- National averages have updated slowly because comprehensive resurvey is expensive.
Treat the averages as orientation, not as a substitute for testing your specific home.
Average Levels by Region
Approximate regional averages based on published EPA and state data:
- Mountain West (Colorado, Wyoming, Montana, Idaho): 2 to 4 pCi/L
- Upper Midwest (Minnesota, Iowa, Wisconsin, North Dakota): 2 to 4 pCi/L
- Appalachian region (Pennsylvania, West Virginia, Tennessee): 2 to 4 pCi/L
- Northeast (Maine, New Hampshire, Vermont): 2 to 4 pCi/L
- Great Plains (Kansas, Nebraska, South Dakota): 1.5 to 3 pCi/L
- Pacific Coast (California, Oregon, Washington): 0.5 to 1.5 pCi/L
- Gulf Coast (Florida, Louisiana, Texas coast): 0.5 to 1.5 pCi/L
For comparison with EPA action levels and WHO recommendations, see the what is a safe radon level guide.
How to Compare Your Reading to the Average
If your home tests at:
- Below 1.3 pCi/L: Below U.S. average. EPA recommends retesting every 2 years.
- 1.3 to 2 pCi/L: Around U.S. average, below EPA action level and WHO recommendation. Retest every 2 years.
- 2 to 4 pCi/L: Above U.S. average, above WHO recommendation, below EPA action level. EPA describes this as a “consider mitigation” range.
- 4 to 8 pCi/L: Well above U.S. average and EPA action level. Confirm with second test, then mitigate.
- Above 8 pCi/L: Far above U.S. average. EPA recommends mitigation without waiting for a second test.
Front Range Colorado homeowners should expect readings shifted higher than the U.S. average. A 3 pCi/L reading on the Front Range is unremarkable; the same reading in a Pacific Coast metro would stand out.
Mitigation Lowers Concentration Below Average
Active soil depressurization (ASD) mitigation typically reduces indoor radon below 2 pCi/L, often into the 0.5 to 1.5 pCi/L range. That’s at or below the U.S. national average — well below EPA’s action level and the WHO recommendation. Front Range mitigation cost typically runs $1,500 to $3,500 for a single-family home.
Post-mitigation retesting within 30 days verifies the system has reduced concentrations to acceptable levels. Continued retesting every 2 years catches system degradation. A continuous radon monitor running through and after installation provides ongoing confirmation.
What Drives Variability Around the Average
Even within a single home, radon varies:
Seasonal
Winter typically 1.5x to 3x higher than summer because closed-house conditions reduce ventilation and stack effect pulls soil gas in. EPA recommends long-term testing (90+ days) to average across seasonal swings.
Floor Level
Basement readings often 2x to 3x higher than ground-floor readings; upper floors generally lowest. The action-level decision typically applies to the lowest livable level.
Day-to-Day
Weather (low-pressure systems drive higher readings), HVAC operation, and occupant behavior (windows, fans) all produce day-to-day swings of 50 percent or more.
These factors are why a single 48-hour short-term test can produce a borderline reading that warrants long-term confirmation. Continuous monitors smooth this variability and produce more representative annual averages.
The “Average” Doesn’t Trigger Action
A common misconception: my home reads at the U.S. national average, so I’m fine. EPA’s action level is 4 pCi/L, the average is 1.3 pCi/L, and the WHO recommendation is 2.7 pCi/L — all three are different reference points. A home at 2 pCi/L is “above average” but below the EPA action threshold; a home at 1.3 pCi/L is at average and well below any action threshold. The action levels matter for mitigation decisions; the averages are orientation.
References
- EPA radon overview and national average data — U.S. Environmental Protection Agency
- EPA Citizen’s Guide to Radon — U.S. Environmental Protection Agency
- CDC radon health information — Centers for Disease Control and Prevention
- CDPHE Colorado radon program and survey data — Colorado Department of Public Health and Environment
- EPA Map of Radon Zones — U.S. Environmental Protection Agency
Front Range homeowners who’ve tested above the U.S. average and want a connection to a certified radon mitigation professional can reach out through our contact page for a vetted local recommendation.