# Radon Levels by County — Dataset Methodology

**Publisher:** Cedar Rapids Radon Mitigation Research  
**Site identity:** Cedar Rapids Radon Mitigation Research is the independent research and reference section of cedarrapidsradonmitigation.com.  
**Dataset version:** 2026-07-24.1  
**Verification date:** 2026-07-24  
**Primary radon source:** U.S. Environmental Protection Agency, *EPA Map of Radon Zones*  
**EPA source file vintage:** September 2025  
**Underlying EPA map model:** 1993  
**Current geography reference:** Deduplicated 50-state-and-DC county set derived from the U.S. Census Bureau 2025 County Adjacency File

## What this dataset is

This is an original, versioned compilation of EPA county and county-equivalent radon-potential source rows for the 50 states and the District of Columbia, joined only where a direct normalized state-and-name match exists to a deduplicated Census Bureau 2025 county and county-equivalent comparison set. It adds EPA definitions, state summaries, source metadata, an Iowa and Linn County extract, stable verification fields, and an explicit geography-status audit.

The dataset does **not** create new radon measurements. The `epa_radon_zone` field is a county-level potential classification, not a measured county average and not a result for a particular building.

EPA defines:

- Zone 1: highest potential; average indoor radon levels may be greater than 4 pCi/L.
- Zone 2: moderate potential; average indoor radon levels may be between 2 and 4 pCi/L.
- Zone 3: low potential; average indoor radon levels may be less than 2 pCi/L.

EPA states that homes with elevated radon have been found in all three zones and that the map should not determine whether an individual home is tested.

## Reproducible transformation

1. Use the spreadsheet identified on EPA's Map of Radon Zones page as the September 2025 XLS release.
2. Read the worksheet named `filtered-raw data`.
3. Retain each row whose `Zone` field is exactly 1, 2 or 3; omit title rows, state headings, national summary rows and blank rows.
4. Preserve EPA's source fields for county-and-state label, county label, state, region and zone; remove the spurious leading period introduced in the parsed XLS county-label text, which is absent from EPA's official JSON/source labels.
5. Keep county and independent-city rows distinct in the county file; combine them only under the appropriate jurisdiction for state-level totals.
6. Attach EPA's published zone meanings and the release's source, vintage and verification fields, including EPA's exact Zone 3 descriptor, `Low potential`.
7. Build the Census comparison set from the 2025 County Adjacency File's primary County Name and County GEOID fields, limit it to the 50 states and the District of Columbia, exclude Puerto Rico and the Island Areas, and deduplicate it by five-digit county GEOID.
8. Normalize punctuation and spacing solely to test direct state-and-name matches against that comparison set; keep a separate clean display field.
9. Assign a current Census GEOID only where the state-and-name match is unique and direct.
10. Preserve unresolved EPA labels and leave current GEOIDs blank; do not transfer, split, average or infer a zone into a renamed, divided, merged or newly defined geography.
11. Generate the national distribution, state summary, Iowa extract and two-sided geography audit from the resulting versioned file.
12. Generate JSON equivalents from the public CSV files and calculate SHA-256 checksums for every distributed release file.

## Final-audit checks performed July 24, 2026

- Confirmed that EPA's current Map of Radon Zones page still identifies the XLS download as September 2025 and describes the map as a 1993 potential model.
- Recomputed all national, state, Iowa and geography-audit totals from the public release CSVs.
- Confirmed **3,144** EPA source rows: **1,072 Zone 1**, **1,035 Zone 2** and **1,037 Zone 3**.
- Confirmed **3,127** direct 2025 Census name matches and **17** legacy or unmatched EPA source labels.
- Confirmed **17** current Census geographies without a direct EPA source-name match.
- Confirmed **51 jurisdictions** — the 50 states and the District of Columbia — with the jurisdiction totals summing to the national totals.
- Confirmed Iowa has **99 of 99** source rows in Zone 1 and Linn County is Zone 1 with GEOID **19113**.
- Confirmed Iowa and North Dakota are the only states in which every EPA source row is Zone 1.
- Corrected missing state-name values on the current-Census side of the public geography-audit file.
- Corrected the public Zone 3 descriptor from `Lower potential` to EPA's exact `Low potential` wording.
- Removed a spurious leading period introduced in the parsed XLS county-label text; no zone, geography or substantive source value changed.
- Regenerated the national JSON, Iowa extract, state-summary JSON, geography-audit JSON and checksum manifest from the published files.

The source workbook is not redistributed in this package. Its SHA-256 digest at extraction was `fa8f6c0920dbdc4b274c631bfb095b18fd755e847a683c3be65b620bc9a3bc54`. The final audit verified the official source page, release vintage, zone definitions and public release calculations; the checksum manifest covers the files actually distributed by this publication.

## Verification tier

- `★` — extracted from or read directly against the named primary source and programmatically checked for this release.

Every published record in the EPA classification layer and every row in the measured-source index is `★`. No provisional verification tier appears in the public release.

## Geography limitation

The EPA source retains legacy county and county-equivalent labels. Seventeen EPA source names have no direct match in the 2025 Census county file. The release preserves the original EPA labels and leaves current GEOIDs blank for those rows. The separate audit lists both the unmatched EPA source geographies and the current Census geographies without a direct EPA source-name match.

The identical totals do not establish one-to-one compatibility: the EPA source has 3,144 zone rows, and the deduplicated 50-state-and-DC Census comparison set has 3,144 unique county or county-equivalent GEOIDs, but the compositions differ.

## Observed test results

Observed county test results belong in a separate evidence layer. A measured value should be published only when an official state program, CDC Tracking indicator or named laboratory source supplies the measurement period, test or building count, pre- versus post-mitigation context, unit, suppression rules, measure definition, retrieval date and geography identifier.

Never substitute an EPA zone number or potential range for an observed pCi/L average.

## Primary source URLs

- EPA Map of Radon Zones: https://www.epa.gov/radon/epa-map-radon-zones-0
- U.S. Census Bureau 2025 County Adjacency File: https://www.census.gov/geographies/reference-files/time-series/geo/county-adjacency.2025.html
- CDC Radon Testing data documentation: https://www.cdc.gov/environmental-health-tracking/php/data-research/radon-testing.html