The highest-radon US counties (and what they have in common)

The highest-radon regions in the United States are a small, repeatable set: the Reading Prong of eastern Pennsylvania and the wider Appalachian uranium belt, the glacial-till plains of Iowa and the upper Midwest, the Colorado Front Range, the Black Hills, and the granitic terrain of northern New England. What they share — uranium-rich bedrock, high soil permeability, cold-climate building practices, and a high prevalence of basements — explains why the same belts keep showing up as EPA Zone 1.

Why the same regions keep showing up

The EPA Map of Radon Zones (EPA-402-R-93-071, 1993) divides every US county into three predicted-average tiers: Zone 1 (predicted indoor average above 4 pCi/L), Zone 2 (2–4 pCi/L), and Zone 3 (below 2 pCi/L). State health departments have, in the decades since, layered their own short-term and long-term survey data on top of that 1993 baseline. When you compare the two layers — federal prediction and state measurement — the same belts dominate the upper end of both. That is not coincidence. It is a direct readout of the underlying geology.

Radon is the decay product of radium-226, which is itself a decay product of uranium-238. Wherever the bedrock or surficial deposits carry above-average uranium, the soil gas above them carries above-average radon. Whether that radon ends up inside a house depends on how easily it can move through the ground, how strongly the house pulls air upward from the soil, and how much of the building envelope sits underground. Those four variables — bedrock, permeability, climate-driven stack effect, and basement prevalence — are the framework this guide hangs on.

The four common factors

1. Uranium-rich bedrock

Some rock types simply contain more uranium than others. Granites, particularly the high-silica peraluminous granites found in the Reading Prong and parts of New England, can carry uranium concentrations several times the crustal average. Black shales — organic-rich sedimentary rocks deposited under low-oxygen conditions — concentrate uranium during burial; the Chattanooga Shale of the southern Appalachians and similar units across the eastern interior are classic examples. Phosphate-rich rocks and certain metamorphic terrains derived from uranium-bearing parents round out the list. The USGS has published radon-potential studies that map these units in detail.

2. High soil permeability

Uranium in the bedrock is only half the story. The radon gas it produces has to physically migrate from the source rock, through the soil column, and into the gravel layer beneath a foundation slab before it can be drawn indoors. Coarse glacial till, fractured crystalline bedrock, karst limestone, and gravelly outwash all let soil gas move easily; tight clays and saturated soils resist it. This is why Iowa — sitting on deep, permeable, uranium-bearing glacial till deposited by the last continental ice sheets — produces such extreme readings even though its bedrock uranium content is unremarkable on its own.

3. Cold-climate building practices and the stack effect

Houses in cold-winter regions are kept closed and heated for half the year. Warm indoor air rises, leaks out of the upper envelope, and creates a slight negative pressure at the foundation. That negative pressure actively pulls soil gas — and any radon in it — up through cracks, utility penetrations, and the slab-to-wall joint. The taller the warm column and the colder the outside air, the stronger the pull. This is why the same uranium concentration produces a much higher indoor reading in a Minneapolis basement in January than in a slab-on-grade Florida ranch.

4. Basement prevalence

A basement is, in radon terms, a room built directly into the soil-gas reservoir. Regions where basements are the default — roughly the northern half of the country, from New England across the upper Midwest and into the Great Plains — concentrate exposure pathways far more than slab-on-grade regions. When EPA's National Residential Radon Survey (EPA 402-R-92-011) measured the country in the early 1990s, the strongest geographic patterns tracked the basement belt as much as the geology belt.

The geologic belts widely documented as Zone 1

The Reading Prong (eastern Pennsylvania, northern New Jersey, southern New York)

The Reading Prong is a band of Precambrian metamorphic rock that runs roughly from Reading, Pennsylvania, northeast across the Lehigh Valley and into northern New Jersey and the lower Hudson Valley. Its gneisses and granites carry uranium concentrations well above the regional average. Berks, Lehigh, Northampton, and Bucks counties in Pennsylvania, along with Warren and Sussex counties in northern New Jersey, are widely documented Zone 1 areas in both the EPA map and Pennsylvania DEP survey data. It is also the origin of the modern US radon program (see callout below).

The Iowa glacial till plain

Iowa is the single most uniformly high-radon state in the country. Almost every county is classified Zone 1. The Iowa Department of Public Health Radon Program has long published a state-wide indoor radon average widely cited at approximately 8.5 pCi/L — more than twice the EPA action level. The cause is not unusual bedrock; it is the deep, permeable, uranium-bearing glacial till deposited across the state by Pleistocene ice sheets, combined with near-universal basements and bitterly cold winters that maximise the stack effect.

The Colorado Front Range

The eastern slope of the Rockies — Boulder, Larimer, Jefferson, Douglas, and El Paso counties — sits on uranium-rich granitic and metamorphic bedrock, with fractured rock that vents soil gas efficiently. The Colorado Department of Public Health & Environment Radon Program lists most Front Range counties as Zone 1, and the combination of high elevation, dry permeable soils, and cold winters produces indoor concentrations that routinely exceed 4 pCi/L in basement-level living spaces.

The Black Hills and the upper Midwest

South Dakota's Black Hills are built on uplifted Precambrian crystalline rock with documented uranium occurrences. Across the wider upper Midwest — Minnesota, Wisconsin, North and South Dakota, Nebraska — glacial till over fractured Precambrian basement produces a broad swath of Zone 1 and Zone 2 counties. Minnesota in particular publishes state survey results showing more than 40% of homes above the EPA action level.

The Appalachian valleys (eastern KY, WV, TN, western VA, western NC)

The folded and faulted carbonates, shales, and metamorphics of the Appalachian valleys host significant uranium-bearing units — black shales, phosphatic limestones, and locally enriched metamorphic rocks. The valley topography channels groundwater and concentrates soil gas in particular landscape positions. Many counties across eastern Kentucky, West Virginia, eastern Tennessee, and the western edges of Virginia and North Carolina are classified Zone 1 or Zone 2.

Northern New England (Maine, New Hampshire, Vermont)

The granitic terrain of northern New England — the source of New Hampshire's nickname — is uranium-rich, basements are near-universal, and winters are long. Maine, New Hampshire, and Vermont have substantial Zone 1 coverage in the EPA map, and these states also have unusually high radon-in-water concentrations from private wells drilled into the same granites, which adds an additional indoor exposure pathway.

Regional reference table

What the rankings do not tell you

Two cautions deserve as much space as the maps themselves.

House-to-house variance is enormous. Even within a single Zone 1 county, two adjacent houses can read 1 pCi/L and 20 pCi/L. Soil-gas concentrations vary on the scale of metres because fractures, soil texture, and water-table depth vary on that scale; foundation construction, slab cracks, sump configuration, and HVAC behaviour vary even more. A county-average classification is a useful prior, not a substitute for testing the specific building you live in.

Living in a Zone 3 county does not mean you can skip testing. Zone 3 means the predicted county-wide indoor average is below 2 pCi/L. Homes well above 4 pCi/L still show up in Zone 3 counties — sometimes in clusters tied to a single rock unit or soil type. The EPA, the US Surgeon General, and every state radon program recommend testing every home regardless of zone. See our DIY radon testing guide for the cheapest defensible way to do that.

What residents of high-radon regions should do differently

If your county sits in one of the belts above, three behaviours change.

Default to long-term testing. A 90-day alpha-track or electret test integrates across weather, season, and HVAC variation in a way a short-term charcoal test cannot. In a known Zone 1 region, the short-term test is useful only as a quick screen; the long-term test is the number you should make decisions with. Our short-vs-long-term testing guide walks through the trade-offs.

Retest on a shorter cadence. AARST guidance recommends retesting every two years in known elevated areas, plus after any major renovation, HVAC change, foundation work, or sump-pump installation. See when to retest after renovation for the trigger list.

Treat mitigation as routine, not exceptional. In Iowa, the Front Range, the Reading Prong, and the basement-rich upper Midwest, a working sub-slab depressurisation system is a normal part of the housing stock — comparable to a sump pump or a radon-resistant new-construction package. Budget for it the way you budget for a water heater, and don't be surprised when an inspection turns up an existing system that needs servicing rather than first-time installation.

Why we don't publish a precise county leaderboard

You may have noticed this guide does not contain a "top 10 US counties by pCi/L." That is deliberate, and worth explaining.

The 1993 EPA Map of Radon Zones is a qualitative classification, not a measured leaderboard — it sorts every county into one of three predicted-average tiers based on the geology, aerial radiometric surveys, and indoor measurements available at the time. State surveys are richer, but their methodologies differ: some are short-term, some long-term; some are volunteer-submitted, some statistically sampled; some are population-weighted, some are not. The highest published averages are state-wide, not county-level. Stitching those incompatible numbers into a precise national county ranking would require inventing numbers we cannot back up — and that is the line we will not cross.

What we will tell you, with confidence, is which belts and counties are widely documented as Zone 1 across both the federal map and the relevant state programs. That is the framing of this guide, of every county-level dossier we sell, and of our sources page.

The bottom line

The geography of US radon is not a mystery and it is not random. Uranium-rich bedrock, permeable soils, cold winters, and basements — wherever those four factors line up, the radon problem follows. The Reading Prong, Iowa, the Front Range, the Black Hills and upper Midwest, the Appalachian valleys, and northern New England are the regions where all four factors line up most often. If you live in one of them, the question is not whether to test — it is how recently you last did, and whether your number warrants a sub-slab depressurisation system. And if you want the numbers and recommendations specific to your county on one printable page, that is what we sell for $15. For context on the lung-cancer arithmetic that makes all of this matter in the first place, see our guide to the radon lung-cancer risk numbers.

Sources

• EPA — Map of Radon Zones, EPA-402-R-93-071 (1993). epa.gov/radon/find-information-about-local-radon-zones-and-state-contact-information
• EPA — National Residential Radon Survey, EPA 402-R-92-011 (1992).
• Iowa Department of Public Health — Radon Program (state-wide indoor radon average ≈8.5 pCi/L).
• Pennsylvania Department of Environmental Protection — Radon Division (Reading Prong survey data).
• Colorado Department of Public Health & Environment — Radon Program (Front Range county classifications).
• US Geological Survey — National Geologic Map and radon-potential studies (geologic units bearing elevated uranium).
• US Surgeon General — Health Advisory on Radon (2005).

Related guides

• EPA radon zones explained — what Zone 1, 2, and 3 actually mean
• Short-term vs long-term radon testing — which to use when
• Sub-slab depressurisation explained — how a mitigation system actually works
• The radon lung-cancer risk numbers, in plain English