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What state is highest in radon?

By Radon411 Editorial TeamUpdated How we research

Short answer

There is no official ranking of states by measured radon, and the CDC testing figures the site holds are county figures. By EPA's Map of Radon Zones, a prediction, Iowa and North Dakota are the only states with every county in Zone 1, the highest predicted potential, followed by Wyoming, Montana and Colorado.

Next step: a test is the only way to know your home's level. How to test your home

Those counts come from the data Radon411 holds: Iowa has 99 counties and North Dakota 53, and every one is Zone 1. Next by the share of counties in Zone 1 are Wyoming (21 of 23), Montana (49 of 56) and Colorado (53 of 64). Across all 3,231 U.S. counties and county-equivalents in the data, 1,071 are Zone 1.

A zone is a prediction for an area, not a measurement of any home. EPA's zone map page says the map identifies areas with the greatest potential for elevated indoor radon levels and should not be used to determine if individual homes need to be tested. EPA's zone map document says homes with elevated levels of radon have been found in all three zones, and the Citizen's Guide says high radon levels have been found in every state.

What to do next: open your state's page for its county zones and any reported testing figures, then test your own home, since the state you live in does not tell you your level.

The states with the most Zone 1 counties

States by share of counties in EPA Zone 1, from the EPA zone data Radon411 holds. Predicted potential, not measured levels.
StateCountiesZone 1 countiesZone 2Zone 3
Iowa999900
North Dakota535300
Wyoming232120
Montana564970
Colorado6453110
Minnesota8768190
Maine161240
Pennsylvania6749171
South Dakota6648180
Indiana9257350

At the other end, every county in Hawaii (5) and Louisiana (64) is in the same data, the lowest predicted potential. That still does not clear a home there: EPA says homes with elevated levels have been found in all three zones. The radon map lists every state's zone counts.

Why there is no measured ranking

EPA's zone map page says its map is based on indoor radon measurement data, geology, aerial radioactivity, soil parameters, and foundation types, and that additional radon testing data may be accessed through the CDC Environmental Health Tracking Network, from Tracking Program-funded states and six national radon testing laboratories. Those testing figures, as Radon411 holds them, are published per county; there is no statewide CDC figure to rank, so the site does not add counties up into one.

What state programs say about their own states

Three state programs' statements about radon in their state, each in its own terms
StateWhat the state program says
MinnesotaThe average radon level in Minnesota is more than three times higher than the U.S. radon level; more than two in five homes have radon levels that pose a significant health risk
PennsylvaniaAbout 40 percent of Pennsylvania homes have radon levels above the EPA action level of 4.0 pCi/L
North CarolinaMore than three-quarters of all North Carolina counties have elevated levels of radon

Each statement measures something different, a state average, a share of homes, a share of counties, so they cannot be lined up as a ranking. They do show that state programs treat radon as a statewide concern.

Questions about radon by state

Which state has the most radon?

There is no official ranking of states by measured radon. By EPA's zone map, Iowa and North Dakota are the only states with every county in Zone 1, the highest predicted potential.

Is Iowa the highest radon state?

On EPA's zone map, Iowa ties with North Dakota: all 99 of Iowa's counties are Zone 1 in the data Radon411 holds. That is a prediction, not a measured ranking. See the Iowa page.

Which states have the least radon?

By EPA's zone map, every county in Hawaii and Louisiana is Zone 3, the lowest predicted potential. EPA still says all homes should be tested regardless of geographic location.

Is radon a problem in every state?

The Citizen's Guide says high radon levels have been found in every state.

Where can I see radon data for my state?

Each state page on Radon411 lists its counties' EPA zones and any reported CDC testing figures. Start from the radon map or the state radon programs list.

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