Methodology

Last updated: September 2026

Every figure on this site is computed from the public datasets below by a script we publish with the site's source. Nothing is hand-entered.

1. Predicted radon levels (model estimates)

Source: National Radon Exposure Model, file ZIPCODE_Average_Radon_2001_2021.csv, license CC0 1.0. The model is a random forest trained on more than six million radon measurements from independent laboratories (2001-2021) using roughly 200 geological, meteorological, architectural and socioeconomic predictors. For each of 29,215 ZIP codes in the conterminous US it gives a predicted long-term mean radon concentration in pCi/L for three settings, each with a standard deviation:

These are estimates of an area average. They are not measurements of any home, and individual homes commonly differ from the area mean by several times. We label them "predicted (model estimate)" everywhere. Alaska and Hawaii are outside the model.

2. EPA radon zones

Source: EPA Map of Radon Zones spreadsheet (EPA 402/A-16/001), public domain. The 1993 map assigns each county Zone 1 (predicted average indoor screening level above 4 pCi/L), Zone 2 (2-4) or Zone 3 (below 2). The file has no FIPS codes, so we match counties by normalized name to Census 2020 counties; renamed or split Alaska census areas and Oglala Lakota County, SD are mapped explicitly, with successor areas inheriting the old zone. All 3,143 current counties received a zone; four EPA rows for geographies that no longer exist (Yellowstone National Park, MT; Bedford, Clifton Forge and South Boston cities, VA) are dropped. The EPA itself notes that homes with high radon exist in all three zones.

3. Geography: ZIP to county

We use the Census Bureau's 2020 ZCTA-to-county relationship file. Each ZIP is assigned to the one county containing the largest share of its land area (its "primary county"). 29,218 of the model's 29,231 ZIPs matched a 2020 ZCTA; 13 did not and are omitted. A ZIP that straddles counties is counted only once, in its primary county.

4. County and state figures

A county's predicted basement, upstairs and screening values are the mean of its ZIPs' predictions weighted by each ZIP's 2010 Census population (the most recent ZCTA population available without an API key), so the figure describes where people live. The county SD shown is the population-weighted mean of the ZIP SDs - a typical ZIP-level uncertainty, not a formal county interval - and we also show the min-max range of ZIP predictions. 3,085 counties have predictions; 58 have none (Alaska, Hawaii, and 23 small counties or independent cities where no ZIP has most of its land) and show the EPA zone only. State and national averages are population-weighted across counties: the national predicted basement average is 1.8 pCi/L.

4b. City figures

City pages cover the 1,000 most-populous Census places (incorporated cities plus census-designated places, ranked by the Vintage 2023 population estimate, or the 2010 Census for CDPs, which the estimates program does not cover). A city's ZIP codes come from the Census 2020 ZCTA-to-place relationship file; ZIPs holding less than 1% of the place's land are dropped unless the place covers at least 20% of the ZIP. The city's predicted basement and upstairs means are the mean of those ZIPs' model predictions weighted by each ZIP's share of the city's land - not a new estimate, and labeled "predicted (model estimate)" like every other figure. Its primary county is the one holding the largest share of that land, and the EPA zone shown is that county's. 4 places in Alaska and Hawaii have no model coverage and show the zone only.

5. Grades

Grades use the EPA's own thresholds applied to the predicted basement mean: High ≥ 4 pCi/L (EPA action level), Moderate 2 to 4 (EPA "consider fixing"), Low below 2. A grade describes the typical home in an area and is a guide to how urgently to test, nothing more.

6. Measured state test results

County pages in six states additionally show measured results from the state health or environmental department, kept visually separate from the model and always labeled with source and years:

Measured means usually run higher than the model because testing is voluntary and often basement-focused; where a source mixes floors the opposite can happen. Each county page states which way the gap runs. Full field definitions, aggregation rules and CSV downloads are on the Data & Downloads page. The Minnesota source is MDH Public Health Data Access.

7. Which pages exist

Every state and county has a page, as does each of the 1,000 largest cities. ZIP pages are generated only for ZIPs predicted at or above 2.5 pCi/L in the basement or that are the most populous ZIP in their county; all other ZIPs appear in their county's table and in the homepage lookup.

Known limitations

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