Radon Risks by State: An Overview
Radon risk varies dramatically across the US. Iowa and Pennsylvania sit on geology that produces some of the highest indoor radon levels in the world. Louisiana and Hawaii barely register. But every state has homes that test above the EPA action level, and that is the point this guide is here to make.
This is not a comprehensive state-by-state database. RadonLookup already gives you county-level data for every county in the country. This guide explains the patterns, the geology, and why some states show up in the data the way they do.
What drives state-level radon risk
Three geological factors explain most of the variation between states:
- •Uranium content in bedrock and soil. Uranium decays into radium, which decays into radon. States with granite, shale, and phosphate-rich bedrock tend to produce more radon. The Reading Prong formation in the mid-Atlantic and glacial deposits in the Midwest are major sources.
- •Soil permeability. Sandy and gravelly soils allow radon to move easily from bedrock to the surface. Clay-heavy and waterlogged soils slow it down. This is why flat, wet-soil states like Louisiana have lower levels.
- •Climate and housing patterns. Cold-weather states keep homes sealed for more of the year, reducing ventilation and allowing radon to accumulate. Basement construction is also more common in northern states, adding a high-concentration living space in direct contact with soil.
Why state averages can mislead
States with the highest radon risk
These states have the largest proportion of Zone 1 counties and the highest measured indoor levels:
The highest statewide radon levels in the country. Almost every county is Zone 1.
The Reading Prong geological formation creates intense radon hotspots across eastern and central PA.
Northern and western Ohio are heavily Zone 1. Glacial till deposits create uranium-rich soil.
High altitude + uranium-bearing geology. Front Range counties are particularly elevated.
Widespread Zone 1 coverage. Granite bedrock and cold-weather sealed homes amplify exposure.
Much of the state sits on uranium-rich geological formations. Testing rates remain low.
Other states with significant Zone 1 presence include North Dakota, South Dakota, Indiana, Illinois, Kansas, Montana, and parts of New York and New Jersey.
States with the lowest radon risk
Flat terrain, alluvial soils, and high water table limit radon entry. Almost entirely Zone 3.
Volcanic geology produces minimal radon. The lowest state-level risk in the US.
Similar geology to Louisiana. Zone 3 statewide, but individual homes can still test elevated.
Low-risk states are still not no-risk states. The EPA has documented homes above 4 pCi/L in every US state. If you live in a low-risk state and have never tested, a one-time check is still reasonable, especially if you have a basement or crawl space.
What this means for you
State-level data is useful for understanding the big picture, but it does not tell you what is happening in your home. Radon is driven by your specific foundation, soil, and construction, not your state border.
- •If you are in a high-risk state and have never tested, it should be a priority.
- •If you are in a low-risk state, a one-time baseline test is still cheap insurance.
- •Regardless of state, retest every two years or after major renovations.
Search your county on RadonLookup to see county-specific data: EPA zone, CDC measured average, and local contractors.