Radon Risk by State: High-Risk Zones Explained

The United States Environmental Protection Agency divides the country into radon potential zones based on predicted average indoor radon levels. While this map is a useful starting point for understanding regional risk, it has important limitations — and understanding those limitations is as important as knowing the zones themselves.

EPA Radon Zone Map Overview

The EPA’s Map of Radon Zones assigns each county in the United States to one of three categories:

• Zone 1 (Red): Predicted average indoor radon levels above 4 pCi/L — the EPA’s action level
• Zone 2 (Orange): Predicted average indoor radon levels between 2 and 4 pCi/L
• Zone 3 (Yellow): Predicted average indoor radon levels below 2 pCi/L

The map is built from state geological surveys, aerial radiometric surveys, soil measurements, and indoor radon measurement databases. It was designed to help state radon programs prioritize outreach and resources — not to predict radon levels in any specific home.

Zone 1 States (Highest Risk)

The highest-risk counties are concentrated in specific geological regions. States with substantial Zone 1 county coverage include:

Northern Plains and Upper Midwest: Iowa, Minnesota, South Dakota, and North Dakota sit largely in Zone 1. Iowa consistently records among the highest average indoor radon levels in any state — approximately 8 pCi/L on average, roughly twice the EPA action level.

Appalachian and Mid-Atlantic region: Pennsylvania, Ohio, and West Virginia have extensive Zone 1 coverage, driven by uranium-bearing shale and granite formations. Pennsylvania’s Reading Prong — a band of uranium-rich rock extending from southeastern Pennsylvania into New Jersey and New York — is one of the most studied high-radon geological features in the country.

Mountain West: Colorado, Wyoming, and Montana have Zone 1 counties concentrated in areas underlain by uranium-rich granite and ore deposits. Colorado’s Front Range communities have documented some of the highest individual home readings ever recorded.

Eastern Midwest: Illinois, Indiana, and Michigan have significant Zone 1 county populations, particularly in glaciated regions where uranium-bearing glacial till has been deposited.

Zone 2 and Zone 3: Moderate and Low Risk Areas

Zone 2 counties cover much of the mid-Atlantic coast, parts of the South, and portions of the Pacific coast. Average indoor levels in these areas are predicted between 2 and 4 pCi/L — meaningful risk, but below the action threshold on average.

Zone 3 counties — including most of Florida, coastal areas of the Gulf states, and parts of California — have predicted averages below 2 pCi/L. Florida’s geology (limestone and sand rather than uranium-bearing crystalline rock) produces relatively little radon.

Why Geology Matters More Than Geography

Radon potential is fundamentally a geological question. The key variables are:

• Uranium content in bedrock and soil. Granite, black shale, phosphate rock, and certain uranium ore formations are the primary radon sources. Sedimentary limestone and sandstone typically produce less.
• Soil permeability. Loose, coarse soils allow radon to migrate more easily than clay-rich soils that trap the gas.
• Depth to bedrock. Homes built directly on or near uranium-bearing rock tend to have higher radon potential.

This geology-first reality explains why Zone maps are guidelines, not guarantees. A home in a Zone 1 county built on low-permeability clay over non-uraniferous rock may test below 1 pCi/L. Conversely, a home in a Zone 3 county with a foundation sitting in fractured uranium-bearing rock may test above 10 pCi/L.

Should You Test Even in Low-Risk Zones?

Yes. The EPA recommends testing all homes regardless of zone designation for several reasons:

• Zones are averages, not predictions. The zone map tells you something about the regional average — not about your specific house.
• High-radon homes exist in every zone. State radon programs consistently find elevated readings in Zone 2 and Zone 3 counties. Zone 3 does not mean radon-free.
• Building features matter. A tight, energy-efficient home in a Zone 2 county can accumulate more radon than a drafty older home in a Zone 1 county, simply because it retains soil gases more effectively.
• Testing is inexpensive. A short-term charcoal canister test costs $15 to $30. Knowing your actual exposure takes a weekend and minimal effort.

The only way to know your home’s radon level is to test it. Find local radon testing and mitigation contractors in your area to get started.