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Radon is the invisible ghost of the home performance world. It's odorless, colorless, and the second leading cause of lung cancer in the United States.2 But unlike aesthetic flaws you can spot during a walkthrough, radon requires testing to detect. Nearly 1 in 15 U.S. homes has radon levels above the EPA action threshold of 4.0 pCi/L, yet most real estate listings don't include testing history, and many buyers don't think to ask until after closing. Understanding radon risk early helps you plan for testing and, if needed, mitigation.1
If you're wondering which states carry the highest radon risk, when to test during a real estate transaction, or what mitigation actually costs, this guide provides the most comprehensive state-by-state radon risk analysis available for 2026. More importantly, it shows you how to use Pearl's Safety Pillar as a discovery framework rather than a verdict.
What You'll Learn
How geology determines which states carry the highest risk (and why new homes aren't automatically safer)
When and how to test during your home-buying timeline
What mitigation costs in 2026, and why it's a value-add, not a dealbreaker
The analysis below draws on 2026 data from the EPA, American Lung Association, World Population Review, and Pearl's evaluation of home performance systems across 92 million single-family homes.1,2,3,5 Pearl doesn't test for radon; certified professionals do that, but we help buyers understand what to ask for and where to look.
Geology dictates physics in the home performance world. The 2026 data confirms a "Granite Belt" stretching from Pennsylvania through New England, where uranium-rich bedrock drives higher indoor concentrations.1,5 While national averages provide a baseline, these rankings serve as a trigger for professional testing, as local "hot spots" can occur in any state. The table below shows states with the highest and lowest average radon levels based on 2026 data.
| Rank | State | Avg. Level (pCi/L) | 2026 EPA Risk Zone | Geological Driver |
|---|---|---|---|---|
| 1 | Alaska | 10.7 | Zone 1 (High) | Volcanic + glacial deposits |
| 2 | South Dakota | 9.6 | Zone 1 (High) | Pierre Shale formation |
| 3 | Pennsylvania | 8.6 | Zone 1 (High) | Reading Prong granite |
| 4 | Ohio | 7.8 | Zone 1 (High) | Appalachian shale deposits |
| 5 | Washington | 7.5 | Zone 1 (High) | Cascade volcanic rock |
| 6 | Kentucky | 7.3 | Zone 1 (High) | Mississippian limestone |
| 7 | Montana | 7.2 | Zone 1 (High) | High altitude + thin soil |
| 8 | Idaho | 7.0 | Zone 1 (High) | Snake River Plain basalt |
| 9 | Colorado | 6.9 | Zone 1 (High) | Granite + uranium mining history |
| 10 | Iowa* | 6.8 | Zone 1 (High) | Glacial till over limestone |
| ... | ... | ... | ... | ... |
| 41 | Texas | 1.8 | Zone 3 (Low) | Sedimentary coastal plain |
| 42 | Louisiana | 1.7 | Zone 3 (Low) | Mississippi River delta deposits |
| 43 | Mississippi | 1.5 | Zone 3 (Low) | Deep sedimentary layers |
| 44 | Florida | 1.2 | Zone 3 (Low) | Limestone (except Central phosphate belt) |
| 45 | California | 1.0 | Zone 3 (Low) | Coastal sedimentary (Sierra foothills higher) |
| 46 | Arizona | 0.9 | Zone 3 (Low) | Desert sandstone |
| 47 | New Mexico | 0.8 | Zone 3 (Low) | Desert sedimentary |
| 48 | Nevada | 0.6 | Zone 3 (Low) | Basin and Range geology |
| 49 | Hawaii | 0.4 | Zone 3 (Low) | Volcanic basalt (porous, ventilated) |
*note: Rankings based on World Population Review 2026 data. Note: Iowa state health officials report an average of 8.5 pCi/L, which would place Iowa among the top 4 states nationally. Regional variation and sampling methodology can affect state averages.
31 states currently show average exposure levels above the EPA's 4.0 pCi/L action level.1 This is not a niche problem; it's a mainstream home-buying consideration.
Zone 3 (Low Risk) is not a "Safe Zone." Modern building science shows that airtight homes can trap radon even in low-potential soil. A post-2015 home in Florida with inadequate mechanical ventilation (an Operations feature) can trap radon gas, creating a Safety concern that exceeds 4.0 pCi/L.8
High-altitude and northern-tier states dominate the highest risk categories due to granitic bedrock composition and the "stack effect" during heating months (see Section 7 for seasonal variation).5
Radon isn't a sign of poor home maintenance or neglect. It's a consequence of where your home sits on the periodic table. Radon is created when uranium in rock and soil naturally breaks down over time. The gas seeps upward through the ground and enters homes through foundation cracks, sump pits, and any point of contact with soil.5
The "Granite Belt" stretching from Pennsylvania (Reading Prong) through New England contains uranium-rich granite bedrock from an ancient mountain formation (the Appalachian range).5 Similarly, western states with volcanic geology (Washington, Idaho, Alaska) and northern states with glacial deposits (Iowa, Wisconsin, Minnesota) show elevated risk because glaciers scraped uranium-bearing rock into the topsoil.1
| Geological Feature | States Most Affected | Why Radon Is Higher | Typical pCi/L Range | Why It Matters To HomeOwners |
|---|---|---|---|---|
| Granite/Gneiss Bedrock | PA (statewide); NH, VT, MA, ME (localized highlands) | Uranium naturally present in crystalline igneous rock | 6.0–10.0+ | High radon levels occur even in newer, well-maintained homes. Mitigation is common and expected. |
| Shale Formations | OH, KY, WV, NY, SD (9.6) | Organic-rich shale contains uranium and radium | 5.0–8.0 | Appalachian counties consistently exceed EPA limits. Budget for testing and potential mitigation. |
| Glacial Till Deposits | WI, MN, IA, IL, MI | Glaciers scraped uranium-bearing rock into soil | 5.0–9.0 | Risk is regional, not localized. Test even if your neighborhood seems low-risk. |
| Volcanic Ash/Basalt | WA, OR, ID, AK | Volcanic rock contains radium and uranium isotopes | 6.0–10.0+ | Some of the nation's highest readings. Mitigation systems are standard, like sump pumps in flood zones. |
| Phosphate-Rich Soil | FL (Central), NC | Phosphate mining exposes uranium deposits | 3.0–7.0 (localized) | Florida's low state average hides high-risk pockets. ZIP code matters more than state classification. |
| High Altitude + Thin Soil | CO, WY, MT, NM (Los Alamos, Santa Fe) | Less soil buffer between bedrock and foundation | 5.0–9.0 | Thin soil provides less filtration. Stack effect* intensifies in winter. Year-round ventilation critical. |
| Limestone/Karst | Midwest, Southeast | Porous limestone allows gas to migrate easily | 2.0–6.0 | High variability; neighboring homes can differ by 3x. Always test, even in Zone 3 areas. |
*see Seasonal Variation: Why Winter Tests Are Higher section
Radon is a consequence of physics, not behavior. You can't "maintain your way out" of a geological risk. This isn't a judgment on the seller or the home; it's the periodic table asserting itself.5
Even "low-risk" states have localized hot spots. Florida is mostly Zone 3, but Central Florida's phosphate mining belt shows Zone 1 characteristics. California's coastal regions are low-risk, but the Sierra foothills with granite bedrock can exceed 4.0 pCi/L.1
Hawaii ranks as the lowest-risk state (0.4 pCi/L average), but specific neighborhoods in volcanic regions can exceed 4.0 pCi/L.1 Florida is mostly low-risk (Zone 3), but Central Florida's phosphate belt shows Zone 1 characteristics. Radon isn't distributed evenly across a state; it pools where geology creates pathways.
| State | High-Risk Counties/Regions | Low-Risk Counties/Regions | Variation Factor |
|---|---|---|---|
| Ohio | Appalachian counties (Zone 1: 8.0+ pCi/L) | Lake Erie shoreline counties (Zone 3: <2.0 pCi/L) | 4x difference |
| California | Granite-rich Sierra Nevada foothills (Zone 1) | Coastal sedimentary plains (Zone 3) | 5x difference |
| Illinois | Northern glacial till region (Zone 1: 7.0+ pCi/L) | Southern sedimentary plains (Zone 3: <2.0 pCi/L) | 3.5x difference |
| Florida | Central phosphate belt (Zone 1: 5.0–7.0 pCi/L) | Southern coastal areas (Zone 3: <1.0 pCi/L) | 6x difference |
| Pennsylvania | Reading Prong granite (Zone 1: 10.0+ pCi/L) | Western sedimentary basins (Zone 2: 3.0–5.0 pCi/L) | 3x difference |
Never rely on state-level data alone. A Zone 3 state can have Zone 1 counties, and a Zone 1 state can have pockets of low risk.5
Geography matters more than ZIP code reputation. A home at the base of a hill (where soil gas accumulates) can have 3x the radon of a home at the top of the same hill.8
Home age correlates with radon risk, but not always in the direction buyers expect. Modern tight homes can actually trap radon more effectively than leaky older homes, a phenomenon building scientists call the "tight home paradox."8
| Construction Era | Key Characteristic | Radon Risk Profile | Primary Vulnerability | What to Look For |
|---|---|---|---|---|
| Pre-1990 | Porous foundations, natural air leakage | Moderate-High | Cracks in basement slab, unsealed sump pits, no radon-resistant features | Foundation condition, existing mitigation |
| 1990-2000 | Early radon awareness, inconsistent building practices | Moderate | Passive radon systems (vent pipe with no fan) | Check if passive system was activated |
| 2000-2015 | Tighter building envelopes, voluntary radon-resistant construction | Moderate | Passive systems never activated, vapor barriers installed incorrectly | Verify fan installation on vent pipe |
| 2015-Present | High-performance air sealing, modern energy codes | Lower but Rising | Inadequate mechanical ventilation (ERV/HRV undersized or absent) | Confirm ventilation system capacity |
Modern energy codes require airtight construction (less than 3 air changes per hour under test conditions, per the 2015 International Residential Code). While this saves energy and improves comfort, it also eliminates the natural air leakage that once diluted radon.8 A brand-new home in Iowa can have higher radon levels than a leaky 1950s home in the same neighborhood if mechanical ventilation is inadequate.
High-performance air sealing creates a near-hermetic seal. Radon has nowhere to escape.
Passive radon systems (gravel layer + vent pipe) are often installed but never activated with a fan.5
ERV/HRV systems may be sized for energy code compliance but not for radon dilution in Zone 1 geologies.8
Newer isn't safer by default. A 2020 home with no mechanical ventilation in a Zone 1 county is a higher risk than a 1980 home with a functioning mitigation system.
Pre-1990 homes face different risks: Foundation cracks, unsealed sump basins, and lack of vapor barriers. But their "leakiness" can accidentally dilute radon.
The sweet spot: Homes built 2000-2015 with activated passive systems (fan installed) often perform best, tight enough for efficiency, ventilated enough for safety.5
The EPA threshold of 4.0 pCi/L is the industry standard for intervention.2,5 Understanding what radon levels mean for long-term health helps contextualize testing results.
| Radon Level (pCi/L) | Risk Comparison (Non-Smoker) | Long-Term Lung Cancer Risk | EPA Recommended Action |
|---|---|---|---|
| 0.4 pCi/L | Average outdoor air | Baseline (1 in 1,000) | None (natural background) |
| 2.0 pCi/L | Above outdoor average | 7 in 1,000 over lifetime | Consider mitigation (optional) |
| 4.0 pCi/L | EPA Action Level | 14 in 1,000 over lifetime | Fix the home |
| 8.0 pCi/L | 2x EPA action level | 30 in 1,000 over lifetime | Immediate mitigation required |
| 10.0 pCi/L | Equivalent to 200 chest X-rays/year | 40 in 1,000 over lifetime | Emergency mitigation |
| 20.0 pCi/L | 5x EPA action level | 110 in 1,000 over lifetime | Do not occupy; immediate remediation |
Every additional 2.7 pCi/L of radon exposure raises the risk of developing lung cancer by 10%.3 This is cumulative exposure, living in a 6.0 pCi/L home for 30 years significantly increases risk.
Radon levels fluctuate seasonally. A "pass" in the summer (2.5 pCi/L) should be re-verified during the winter heating season, when levels can spike to 5.0+ pCi/L (see Section 7).8
There is no "safe" level of radon exposure, but 4.0 pCi/L is the practical threshold where mitigation becomes cost-effective and health-protective.2,5
Radon levels fluctuate based on how your home breathes. In winter, the "stack effect" (warm indoor air rising and escaping through attics and upper-story leaks) creates negative pressure at the foundation level. This negative pressure pulls soil gases, including radon, up through foundation cracks like a chimney draft.8
Winter: Warm air inside the home rises and escapes through ceiling/attic leaks
Pressure differential: Creates a vacuum at the foundation level
Soil gas intrusion: Negative pressure pulls radon through cracks, sump pits, utility penetrations
Amplification: Furnace operation and closed windows intensify the effect8
In summer, reduced temperature differential means less stack effect. Additionally:
Open windows dilute indoor radon
AC systems can create a slight positive pressure (pushing air out, not pulling soil gas in)
Result: Summer tests can underestimate annual exposure by 50-70% in cold climates7,8
| Season | Typical Level Variation | Why Levels Change | Best Testing Window |
|---|---|---|---|
| Winter (Heating Season) | 2-3x higher than summer | Stack effect + closed windows + furnace operation | October–March (ideal) |
| Summer (Cooling Season) | Lower (but not "safe") | Reduced stack effect, open windows, AC creates slight positive pressure | June–August (acceptable for screening) |
| Spring/Fall (Shoulder Seasons) | Moderate, highly variable | Depends on daily HVAC usage | March–May, September–October |
Winter tests are 2-3x higher than summer in heating-dominated climates (Zone 1 states like PA, OH, IA, CO).8
If you're buying a home in a Zone 1 state and the radon test was conducted in summer, it's worth understanding that winter levels can be significantly higher due to seasonal physics. Many buyers choose to retest during heating season to see the full range.8
Testing for radon is inexpensive, non-invasive, and essential in Zone 1 and Zone 2 counties. But timing matters. Radon levels fluctuate seasonally. If a home was tested during summer months, it's worth understanding that winter readings can be 2-3x higher due to the stack effect. This isn't a problem with the test; it's physics. If you're buying during winter or in a Zone 1 state, consider retesting during the heating season to understand the full range of exposure.8
| Test Type | Duration | Best For | Cost (2026) | Accuracy | Limitations |
|---|---|---|---|---|---|
| Short-term Charcoal Canister | 2-7 days | Quick screening | $15-$30 (DIY) | Moderate | Can miss seasonal spikes; requires closed-house conditions |
| Continuous Electronic Monitor | 48 hours minimum | Home inspection period | $100-$200 (rental) | High | Requires calibration; sensitive to air currents |
| Long-term Alpha Track Detector | 90+ days | Annual average | $30-$50 (DIY) | Very High | Too slow for real estate transactions |
| Professional Lab Analysis | 2-4 days | Closing contingency, legal disputes | $150-$300 | Very High | Gold standard; lab-certified results for negotiations |
DIY charcoal tests ($20) are fine for screening, but for real estate transactions, use a certified professional to avoid disputes.6,7
Closed-house conditions are mandatory. Open windows during testing invalidates results and can lead to artificially low readings.5
Test in winter if possible. Summer tests underestimate annual exposure by 40-70% in cold climates.8
In 2026, radon mitigation remains one of the most cost-effective health upgrades available. If a home in a high-risk zone lacks a system, it represents an opportunity to improve the home's safety profile rather than a dealbreaker. A properly installed system can lower levels by up to 99%.4
Most mitigation systems cost $50-$100 per year to operate (equivalent to running a single LED lightbulb 24/7).4 Fans are designed to run continuously and typically last 5-10 years before replacement ($150-$300).
| System Type | Best Foundation Type | How It Works | Avg. Cost (2026) | Post-Mitigation Level |
|---|---|---|---|---|
| Sub-Slab Depressurization (Active) | Basement or slab-on-grade | PVC pipe + fan pulls radon from under foundation | $1,200–$2,500 | <1.0 pCi/L (99% reduction) |
| Crawl Space Encapsulation + Ventilation | Dirt/gravel crawl space | Seal floor with vapor barrier + active ventilation | $2,000–$5,000 | <2.0 pCi/L |
| Heat Recovery Ventilator (HRV) | Tight, modern construction | Dilutes indoor radon with fresh air exchange | $1,500–$2,500 | <2.5 pCi/L (60-80% reduction) |
| Active Soil Suction (Sump Pit) | Homes with existing sump | Fan attached to sump cover pulls radon away | $800–$1,300 | <1.5 pCi/L |
| Passive System Activation | New construction with pre-installed pipe | Add fan to existing passive pipe | $300–$600 | <2.0 pCi/L |
Most mitigation systems can be installed in a single day by a certified professional (no major construction required).6
Sub-slab depressurization is the gold standard for basements and slab homes, 99% reduction is typical.4
A documented mitigation system is an asset. Upload installation records and post-mitigation test results to the Pearl Registry to protect your home's Safety Pillar score.
Installing a mitigation system is step one. Verifying it works long-term is step two. Mitigation systems are mechanical devices; they require occasional maintenance and periodic retesting to ensure continued performance.5
| Monitoring Task | Frequency | Why It Matters | How to Check |
|---|---|---|---|
| Initial Post-Mitigation Test | Within 30 days of activation | EPA protocol; verifies system achieved <4.0 pCi/L (ideally <2.0 pCi/L) | Certified professional test (include results in Pearl Registry) |
| Visual System Check | Monthly | Ensures fan is running and PVC pipe is intact | Look for vibration on PVC pipe; check manometer gauge (shows suction) |
| Radon Retest | Every 2 years | Detects system degradation or new foundation cracks | Short-term test (2-4 days); more frequent if major renovations |
| Fan Replacement | Every 5-10 years | Fans wear out; replacement costs $150-$300 | Schedule when fan noise increases or manometer shows reduced suction |
| Post-Renovation Retest | Immediately after major foundation work | HVAC upgrades, foundation repairs, or room additions can alter system effectiveness | Always retest after: basement finishing, HVAC replacement, foundation crack repair |
Retest within 30 days of system activation to confirm it's working (EPA protocol).5
Retest every 2 years or immediately after foundation work, HVAC upgrades, or room additions.5
Log all results in the Pearl Registry to maintain your Safety Pillar score and protect resale value.
Mitigation systems require minimal maintenance (fan replacement every 5-10 years, visual checks monthly).4
Radon risk is often the most overlooked component of home safety. While a buyer can inspect a foundation or check an HVAC system, they cannot see or smell radon during a home tour. Instead of viewing this as a hidden threat, Pearl provides the framework for discovery. Every home in America already has a Pearl SCORE™ based on publicly available data. While Pearl doesn't assess radon risk or predict exposure levels, that's what professional testing is for; we do surface documented safety features that may already be protecting the home.
Search any address in the Pearl Home Performance Registry™ to see:
Whether radon mitigation systems are documented in public records or homeowner disclosures
What other Safety Pillar features (air quality systems, water filtration, etc.) are already in place
How the home's documented safety features compare to those of others in the area
What's already been invested in the home's health and safety infrastructure
| Pearl Tool | Function | Radon-Specific Utility | Buyer Value |
|---|---|---|---|
| Pearl SCORE™ | Summarizes 5-pillar home performance (Safety, Comfort, Operations, Resilience, Energy) | Shows whether radon mitigation systems are documented in public records or homeowner disclosures | Turns "Should I test?" into "Yes, here's why" |
| Safety Pillar Metrics | Focuses on indoor air quality, radon, lead, carbon monoxide, mold | Shows whether mitigation systems are documented in public records; reflects EPA Zone risk patterns | Surfaces what listings often don't include |
| Pearl Home Performance Registry™ | Permanent digital home record stored on the blockchain | Stores radon test results, mitigation system documentation, retest history | Protects resale value for future buyers |
Pearl SCORE doesn't assess radon risk or tell you when to test. That depends on your region's geology and professional testing protocols. What Pearl does is surface documented radon mitigation systems and other safety features that may already be addressing radon, so you can see what investments exist before you make an offer.
Claiming a home in the Pearl Home Performance Registry™ allows you to correct the public record regarding your home's safety. If you install a system, future buyers will see it.
Radon is geological, not optional, and it varies dramatically by state, county, and even neighborhood elevation. The question isn't whether your target home has radon considerations. The question is: what does the public record already show, and should you test?
Every home in America already has a Pearl SCORE™ based on publicly available data. Search any address in the Pearl Home Performance Registry to see how the home's Safety Pillar compares to others in the area.
Search the Pearl Registry by Address →
[1] World Population Review. (2026). "Radon Levels by State: 2026 Data." Retrieved from worldpopulationreview.com
[2] American Lung Association. (January 2026). "National Radon Action Month Guidance." Retrieved from lung.org
[3] Iowa Environmental Council. (2026). "2026 Radon Fact Sheet." Retrieved from iaenvironment.org
[4] Angi/HomeGuide. (2026). "National Average Radon Mitigation Costs." Retrieved from homeguide.com
[5] U.S. Environmental Protection Agency. (2026). "A Citizen's Guide to Radon." EPA 402/K-12/002. Retrieved from epa.gov/radon
[6] National Radon Safety Board (NRSB). (2026). "Certified Radon Professional Directory." Retrieved from nrsb.org
[7] American Association of Radon Scientists and Technologists (AARST). (2026). "Radon Measurement Standards." Retrieved from aarst.org
[8] Building Science Corporation. (2025). "Stack Effect and Indoor Air Quality in Cold Climates." Retrieved from buildingscience.com
This article was produced by Pearl for informational and educational purposes. Radon levels and geological data are based on EPA Zone classifications and third-party research as of 2026. Pearl SCORE™ identifies geological and regional conditions associated with radon risk, helping you understand when professional testing is recommended. Certified radon professionals conduct the testing; licensed contractors install mitigation systems. All testing should be conducted by EPA-certified professionals. Pearl SCORE™ identifies conditions associated with radon risk but does not diagnose specific radon levels in any individual home. Information is accurate as of the publication date; radon levels, mitigation costs, and testing protocols may vary by location.
