The EPA radon zone map, explained

Our lookup is built on EPA’s Map of Radon Zones. Knowing how that map was made helps you use it well, and understand why it is only a starting point.

Why it exists

Congress directed EPA, under the Indoor Radon Abatement Act, to identify areas of the United States with the potential for elevated indoor radon. EPA released the Map of Radon Zones in 1993. EPA states its purpose plainly: to help national, state, and local organizations target their resources and to help building code officials decide where radon-resistant building practices are most needed.

How zones were assigned

EPA’s fact sheet explains that each county was assigned a zone based on five factors:

• indoor radon measurements,
• geology,
• aerial radioactivity,
• soil permeability, and
• foundation types.

These were combined into a radon potential assessment for geologic provinces, then adapted to county boundaries.

The three zones

• Zone 1: predicted average indoor screening level greater than 4 pCi/L (highest potential).
• Zone 2: predicted average between 2 and 4 pCi/L (moderate potential).
• Zone 3: predicted average less than 2 pCi/L (low potential).

In EPA’s current spreadsheet, the counties are split roughly evenly among the three zones.

The limits EPA stresses

EPA’s own text is clear that the map “should not be used to determine if individual homes need to be tested.” Three reasons:

1. It is an average. A county-wide prediction hides large variation from house to house.
2. It is old. The map dates from 1993 and has not been redrawn. States have collected many more test results since.
3. Elevated homes exist everywhere. EPA notes there are many thousands of homes with elevated radon in Zone 2 and Zone 3 counties.

Better local data

EPA encourages supplementing the map with state or local data. Many state radon programs publish test results by county or ZIP code, and CDC’s Environmental Public Health Tracking Network hosts radon testing data from participating states and labs. EPA also publishes state-by-state supporting documents describing the data and confidence behind each state’s map.

How to use the zone well

• Use it to gauge how common radon is in your area and how urgent testing is.
• Use it for community planning and building codes.
• Do not use it to skip testing. Test every home, and fix at 4 pCi/L or higher.

Check your county in the ZIP lookup or browse by state.