Radon risk zone by county

EPA Radon Zone 1

EPA's highest-priority radon zone — counties with predicted average indoor radon ≥4 pCi/L (the action level).

pCi/L (picocuries per liter)

Standard US unit for indoor radon concentration. EPA action level: 4 pCi/L; mitigation considered: 2-4 pCi/L.

EPA Radon Zones — what 1, 2, and 3 mean and when you should test

EPA classifies every US county into Zone 1 (high), Zone 2 (moderate), or Zone 3 (low) for radon. Plain-English guide to the classifications and at-home testing.

EPA SDWIS — how to read drinking-water violations and what they actually mean

Plain-English guide to EPA SDWIS drinking-water violations: health-based vs. monitoring vs. reporting violations, MCLs, and what triggers a public notice.

Radon and real estate — what disclosure laws and home tests actually require

What home sellers must disclose about radon by state, when home tests are required, and how to interpret radon-mitigation systems if a home you're buying has one.

Radon disclosure during real-estate transactions — state-by-state

Whether sellers must disclose known radon results varies by state. Quick map of mandatory-disclosure states + what to ask for during inspection.