Normal Radon Levels: What Homeowners Need to Know
“Normal radon levels” is a phrase that hides a more useful question — what level of indoor radon is acceptable, and how does any given home compare? The U.S. national indoor average is about 1.3 pCi/L per EPA. Colorado’s Front Range, which sits in EPA Radon Zone 1 — the highest predicted radon band — measures meaningfully higher, with statewide indoor averages closer to 2 to 4 pCi/L per Colorado Department of Public Health and Environment data. EPA’s recommended action level is 4 pCi/L; the World Health Organization sets a lower reference of 2.7 pCi/L. This guide explains the numbers, how to read them in context, and how Front Range geology shapes what “normal” means in a Denver, Boulder, or Colorado Springs basement. This article summarizes EPA, WHO, CDC, and CDPHE guidance current as of 2026; see our methodology page for sourcing — radon is a Group 1 human carcinogen and there is no level that is truly safe; the action levels are response thresholds, not biological safety guarantees.
What is the normal radon level in a U.S. home?
EPA reports the U.S. indoor radon average at approximately 1.3 picocuries per liter (pCi/L). The outdoor average is roughly 0.4 pCi/L. EPA recommends taking action — i.e., mitigation — at indoor levels of 4 pCi/L or higher, and considering action between 2 and 4 pCi/L. The World Health Organization, drawing on a broader meta-analysis of lung cancer risk, recommends a reference level of 2.7 pCi/L (100 Bq/m³).
The “no truly safe level” framing comes from the linear no-threshold model used by EPA and WHO to extrapolate risk from radon exposure. Radon-induced lung cancer risk increases with exposure, with no clear threshold below which risk drops to zero. EPA attributes roughly 21,000 U.S. lung cancer deaths per year to indoor radon, making it the second-leading cause of lung cancer after smoking.
What is normal for Front Range Colorado?
Colorado sits almost entirely in EPA Radon Zone 1, the highest of three predicted-radon-potential zones. The reason is geology — the granite and uranium-bearing rock under the Front Range produces radon gas continuously, and the gas migrates up through soil into building basements and crawlspaces. CDPHE indoor radon data shows that roughly 1 in 2 Colorado homes test above the EPA action level of 4 pCi/L, with statewide indoor averages in the 2 to 4 pCi/L band — meaningfully higher than the national 1.3 pCi/L average.
Within the Front Range, county-level variation is real:
- Boulder, Jefferson, El Paso, Douglas, and Larimer counties test particularly high because of the underlying geology
- Eastern Plains counties test lower on average but still well above the national baseline
- Foothills neighborhoods with substantial granite outcropping test on the upper end of the band
None of these numbers makes a high reading “normal” in the sense of acceptable — a 6 pCi/L reading is “normal” for Front Range geography in the sense that it is common, but it is also above the EPA action level and warrants mitigation. The Front Range radon testing pillar covers Colorado-specific context.
Understanding pCi/L and Bq/m³
Two units appear in radon reporting: picocuries per liter (pCi/L) in the United States and becquerels per cubic meter (Bq/m³) internationally. The conversion is approximately 1 pCi/L = 37 Bq/m³. WHO’s reference level of 100 Bq/m³ corresponds to about 2.7 pCi/L. EPA’s action level of 4 pCi/L corresponds to about 148 Bq/m³. Most U.S. consumer reports use pCi/L; international guidance uses Bq/m³.
How indoor radon levels are measured
The two main test categories are short-term and long-term:
Short-term tests (2 to 7 days)
Charcoal canisters and short-term electret ion chambers are deployed for 48 hours to a week, mailed to a lab, and produce a single reading. EPA recommends a short-term test as a screening step for real estate transactions or fast decisions. Closed-house conditions are required during the test — windows and exterior doors closed except for normal entry and exit.
Long-term tests (90 days to 1 year)
Alpha-track detectors are deployed for 3 months to a year and produce a time-weighted average. Long-term tests better reflect annual indoor radon exposure because radon levels fluctuate with weather, season, and home use.
Continuous radon monitors (CRM)
Electronic CRMs sample radon hourly and display a running average. NRPP-listed professional CRMs are used in real estate transactions; consumer-grade monitors (Airthings, Ecosense, Corentium) are increasingly common in Colorado homes for ongoing tracking. See our radon monitor explainer for the consumer-grade landscape.
What level should trigger mitigation?
EPA’s published action level is 4 pCi/L. EPA also recommends considering mitigation between 2 and 4 pCi/L. WHO’s lower reference level of 2.7 pCi/L reflects the more conservative international position. Colorado mitigation contractors will install systems at any level above 2 pCi/L on homeowner request, and many Front Range homeowners choose to mitigate above 2.7 pCi/L given the geological baseline.
Pricing for sub-slab depressurization (the standard mitigation method) typically runs $1,500 to $3,500 on the Front Range for a single-fan system, with most jobs completing in a day. Post-mitigation indoor levels usually drop to 1 to 2 pCi/L, sometimes below 1 pCi/L for newer construction with effective sealing.
Seasonal and structural variation in “normal”
Indoor radon levels are not constant. They vary with:
- Season — winter readings are typically higher because homes are tighter and heated air rises, drawing soil gas up through the foundation
- Weather — barometric pressure drops during storms can pull more radon into the home
- HVAC operation — exhaust fans, dryers, and combustion appliances depressurize the home and increase soil gas entry
- Foundation type — basements and crawlspaces typically test higher than slab-on-grade because of proximity to the soil source
- Construction details — sump pits, exposed dirt floors, and unsealed slab cracks raise readings
A single short-term winter reading at 5 pCi/L and a long-term annual average at 3 pCi/L can describe the same home. The annual exposure number is the right baseline for cancer-risk extrapolation; the short-term reading is the right trigger for a real estate decision.
Common misconceptions about “normal” radon
“My neighbor tested fine, so I am fine too”
Radon varies block to block, house to house, and even between rooms in the same home. The neighbor’s reading is not predictive.
“My home is new, so it does not need testing”
New construction in Radon Zone 1 sometimes includes a passive radon vent stack, but passive systems do not guarantee low indoor levels. EPA recommends testing every new home regardless of design.
“I had it tested 10 years ago and it was fine”
Soil settling, foundation cracks, HVAC changes, and additions can all change indoor radon levels over time. EPA recommends retesting every 2 years and after major renovations.
How radon enters a home
Radon is a colorless, odorless, tasteless gas produced by the radioactive decay of uranium in soil, rock, and groundwater. The gas migrates upward through soil pores and enters buildings through:
- Cracks in concrete slabs — typically the largest entry pathway in Colorado homes with basements
- Sump pits and drainage openings — direct soil-to-air paths if not sealed
- Slab penetrations — gaps around supply pipes, drains, and electrical entries
- Crawlspace floors — bare-earth crawlspaces let soil gas migrate freely upward
- Construction joints — where the slab meets the foundation wall
- Wall voids — hollow block walls can act as conduits from soil gas to occupied levels
- Well water — radon dissolved in groundwater can off-gas into indoor air when water is heated or aerated (a smaller contributor in most municipal systems)
The negative pressure differential between a heated, occupied home and the soil underneath creates the driving force — the home effectively pulls soil gas upward through any unsealed path. This is why winter readings in Colorado tend to run higher and why tighter, more energy-efficient homes can paradoxically show higher indoor radon than older, drafty homes on the same lot.
Front Range geology in more detail
Colorado’s high baseline indoor radon traces to two geologic features. The first is the granite and felsic igneous rock under the Front Range, which contains comparatively high uranium concentrations. The second is the post-mountain alluvial fan and sedimentary deposits across the foothills and plains, which preserve and transmit radon gas through soil. Together they produce indoor radon readings that run well above the national average across most of the Front Range corridor.
County-level CDPHE data shows particular concentration in Boulder County (where granite outcropping is common), Jefferson County (foothills and front-range bedrock), Larimer County, El Paso County, and Douglas County. Eastern Plains counties test somewhat lower on the geologic profile but still meaningfully above the national 1.3 pCi/L average. There is no zip code on the Front Range that can be reliably assumed to have low indoor radon without testing.
When to call a professional
For a fast real estate decision, a NRPP-listed (National Radon Proficiency Program) professional running a 48-hour continuous radon monitor under closed-house conditions is the standard. For a long-term homeowner baseline, a 90-day to one-year alpha-track or consumer CRM gives the more useful annual average. For mitigation, a NRPP-listed mitigation contractor designs and installs the system. CDPHE maintains state-level resources for Colorado homeowners.
References
- EPA radon overview and action level — U.S. Environmental Protection Agency
- EPA Map of Radon Zones — U.S. Environmental Protection Agency
- CDPHE Colorado radon program — Colorado Department of Public Health and Environment
- CDC radon and health — Centers for Disease Control and Prevention
Front Range homeowners who want a connection to a NRPP-listed radon professional for testing or mitigation can reach out through our contact page. Colorado geology makes radon a near-universal concern in Zone 1 — and a confirmed reading from a vetted professional is the right starting point for any mitigation decision.