What Radon Levels Actually Look Like Near You

How this data was built — and what we left out on purpose

This map comes directly from RADON & CO's own closed-test records — real NRPP-certified 48-hour continuous radon monitor tests, updated as we complete more of them. Before anything went on the map, we made a few deliberate calls about what to include:

• Homes with an active, powered mitigation system are excluded from the area averages. An active system typically suppresses a reading down near or below 1–2 pCi/L regardless of what the soil underneath is actually producing — blending that engineered number into a neighborhood average would understate the real geological risk. Homes with a passive rough-in system (a code-required vent stub with no fan) are included, since that's usually a builder default rather than a response to a known problem.
• Duplicate QA pairs are collapsed into one reading. When we run two monitors at the same property for quality-assurance cross-checking, that's one test event, not two data points.
• Our own test address and incomplete tests are excluded entirely.
• Results are grouped by zip code, not by individual address or exact test count. This map is meant to show a general local picture, not to identify any specific property or client.
• Circle size reflects a rough, 3-tier sense of how much data backs a zip code's average — not an exact test count. It's a fixed-width bucket, not a rank, so you can tell "more" from "less" but not compare two zip codes precisely.

Why Montgomery County and the surrounding area matter for radon

Montgomery County is mapped by the EPA as Radon Zone 1 — the agency's highest-risk classification, predicting a county-wide average indoor screening level above the 4 pCi/L action level. Parts of central and western Maryland, including Frederick County, sit in what's sometimes called Maryland's "Radon Belt," with some of the highest average readings recorded in the state. Zone maps like these are built from limited historical sampling and geological modeling — real, current, address-level test results like the ones behind this map are a meaningful complement to that picture, which is part of why we make this data public.

The one caveat that matters more than any color on this map

Radon is produced by uranium decay in the soil and rock directly under a specific house — it does not respect zip code or county lines. Two homes across the street from each other can test completely differently depending on foundation type, slab cracks, sump design, and what's happening in the ground beneath that exact lot. A good zip-code average is a reason to be curious, never a reason to skip testing your own home.