The short answer
Our ZIP-area lookup uses Census ZCTAs and county relationships. One area may relate to multiple counties; confirm the property’s county before interpreting its county radon context.
ZIP and ZCTA are different concepts
The Census Bureau creates ZIP Code Tabulation Areas to approximate the geographic distribution of ZIP Codes. They are statistical areas. A ZCTA lookup should not be presented as an exact postal-address determination.
Follow the join
The lookup begins with a five-digit ZCTA, finds related counties in the Census relationship file, and attaches each available EPA county zone. When more than one county is related, we show the possibilities rather than silently choosing one.
An illustrative example
Imagine a ZCTA that overlaps County A and County B. If the county zone records differ, one ZIP-area label cannot describe both as a single zone. The correct next step for an address-specific question is to confirm the property’s county, then read the corresponding county context.
Handle a missing result
An absent lookup is not evidence of low radon potential. It can reflect unsupported geography or an unavailable join. Use the county lookup or the original sources. A home test remains the way to assess indoor radon.
Your next steps
- Five-digit entry → Census ZCTA.
- ZCTA → one or more county relationships.
- County identifier → EPA potential zone.
- Property question → confirm county and test the home.
Common questions
Is a ZCTA identical to a postal ZIP Code?
No. Census ZCTAs are geographic representations of ZIP-code distribution, built from Census blocks.
Why is my valid ZIP Code missing from the lookup?
Not every valid ZIP Code has a ZCTA. Missing coverage cannot establish a radon zone or a home’s radon level.
Sources and editorial record
Prepared with AI assistance from the linked sources. Independent human review pending. “Public Atlas Editorial” is a publication label. First edition: 2026-09-28. Scheduled source review: 2026-10-28. Data examples update with their source snapshots; the guide text is versioned separately.
Changes to this guide
- 2026-09-28 — First AI-assisted edition published with source links and review status disclosed.
- 2026-09-29 — Added focused reader questions and concise answers with source links; checked primary-source guidance and retained pending human review.