Data
Methodology
Which public agencies the numbers come from, what each kind of number means, how often it is refreshed, and how mistakes are fixed.
Updated October 2, 2026
The rule
Predicted, measured and calculated: three kinds of number
Radon411 publishes three different kinds of figure and never blends them. Every number on the site is labelled with which of these it is.
Predicted
A forecast for an area
A classification or model output for an area as a whole, never a measurement of a specific building. EPA's radon zones and USGS's geologic radon potential are both predicted.Measured
Tests that were carried out
Results from radon tests that were actually carried out and reported to a public health agency: CDC's testing statistics, and the results a state publishes itself. They describe tests reported, not a random sample of every home.Calculated
Worked out by Radon411
Something Radon411 calculated from the above, always labelled as calculated, with its method set out on this page.
Predicted
EPA radon zones and USGS geologic potential
EPA assigns every U.S. county to one of three zones based on a model of predicted average indoor screening levels for the area: Zone 1 is greater than 4 pCi/L, Zone 2 is 2 to 4 pCi/L, Zone 3 is less than 2 pCi/L. This is a prediction for the area, not a measurement, and not a prediction for any specific home. EPA is explicit that the zone map is not intended to determine whether an individual home should be tested, that elevated homes occur in all three zones, and that every home should be tested regardless of zone. Every place Radon411 shows a zone, it shows that caveat alongside it.
USGS classifies geology, not counties, into low, moderate and high radon potential polygons. These polygons do not follow county lines, so Radon411 never collapses a county to a single class; instead it records what share of a county's area falls in each class, so a report can say something like "62% of the county falls in a high geologic potential area" rather than assigning the whole county one label.
Measured
Reported tests, and who the denominator is
CDC testing statistics
CDC's testing measure reports counts and percentages among tests that were reported to CDC, broken down by year and result band. The population it describes is buildings that were tested and reported, which is not the same as homes in the county. Every percentage Radon411 shows states that denominator, for example "58% of the radon tests reported to CDC for this county in 2021 were at or above 4 pCi/L," never "58% of homes."
A state's own results
Where a state publishes its own radon results and its terms allow reuse, Radon411 shows them as the state published them, at the geography the state printed: a county, or a ZIP code. They are kept apart from CDC's figures and labelled with the state agency's name. Where a state reports fewer than 5 tests for a place, the count is shown but the figures are not. A figure with an impossible value, such as a share above 100%, is held back and never shown.
Calculated
The figures Radon411 calculates
Each is labelled Calculated by Radon411 wherever it appears.
Counts
Tests per result band
CDC publishes the share of reported tests in each result band, not a count per band. Where a county report shows a per-band count, it is the published band percentage times the published test count, rounded, with the arithmetic stated under the chart. It is shown only when the test count and all three band percentages are present, and the shares beside it are always CDC's own.Points
A point inside each place
Each county and state's representative point: a point guaranteed to fall inside the boundary, used for maps and distances.Neighbours
Real neighbours
County adjacency, computed from where county boundaries actually touch, not a fixed radius or a population-based idea of nearby.
What we do not do
Why there is no Radon411 risk score
A number that blends a prediction with a measurement, or that implies home-level risk from area-level or reported-test data, would misrepresent what the underlying sources actually say. Radon risk in any specific building can only be known by testing that building, so Radon411 does not publish a single proprietary risk score. Calculated figures are allowed, but only when they are labelled and their method is set out, as above.
Updates
How often the data is refreshed, and how mistakes are fixed
Refreshed on a schedule
The public datasets are checked every week, and a new release is taken in when its publisher issues one. The state licensing lists behind the directory and the state radon program pages are checked once a month. The data sources page shows the date each source was last brought in or checked.
Corrected when it is wrong
If a figure or a sentence looks wrong, the form on the corrections page sends it to us. Each report is checked against the agency's own publication. Where the error is confirmed, the page is fixed and its date changes.
Fine print
Missing data, and counties, ZCTAs and ZIP codes
How missing data is handled
A missing value shows as a dash, never a zero and never a blank that could be misread as zero. Where a county has no EPA zone and no CDC statistics on file, its report says so plainly, rather than guessing or leaving out the caveat that would normally go with a zone.
County versus ZCTA versus ZIP code
A USPS ZIP code is a mail-routing construct with no fixed boundary. A Census ZIP Code Tabulation Area (ZCTA) is a generalized statistical approximation of ZIP code service areas, and does have a boundary. A ZIP code search is resolved through the ZCTA of the same code to the county or counties it lies in, and opens a county report. There are no ZIP code pages.
Where a state publishes results by ZIP code, Radon411 keeps them at the ZIP code the state printed. It never spreads them over a county, and never presents a county figure as a ZIP code's own.
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