A county average, not a house reading

EPA's zone map was built to help state and local governments target resources and building codes — a screening tool for policy, not a substitute for testing an individual home. A Zone 1 county can and does contain homes that test well under the action level, and a Zone 3 county can and does contain homes that test above it. Foundation type, ventilation, and the specific geology under one house all vary within a county, sometimes sharply.

Why EPA recommends testing everywhere

Because zone cannot predict any single house reliably, EPA's own guidance is that every home should be tested regardless of its county's zone — testing is inexpensive relative to the alternative and is the only way to actually know. Harris County, TX is rated Zone 3 by this site, but that does not mean every home there is safe from elevated radon; it means the countywide average is predicted low.

What actually changes the odds within a house

Basements and slab-on-grade construction typically see higher readings than raised foundations, since radon enters through contact with soil and rock. Ventilation, cracks in the foundation, and even the season a test is taken can shift a result. None of this is captured at county grain, which is exactly why a zone is a starting point for awareness, not a final answer.

Where to go next

See the range this plays out across: Loving County, TX, Logan County, KY, Middlesex County, MA.

What Radon Testing and Mitigation Actually Cost · The US Counties EPA's Radon Map Can't Rate Directly