Data
Data dictionary
Every term used across the maps, reports and tables, defined plainly and technically.
Updated October 2, 2026
Predicted
Maps and places
What the predicted maps classify, and the geography a figure belongs to.
EPA Zone
A rating from 1 to 3 that EPA assigns to every U.S. county describing how much radon potential is predicted in that area, not the radon level in any specific building.
Technical: EPA's Map of Radon Zones classifies counties into Zone 1 (predicted average indoor screening level greater than 4 pCi/L), Zone 2 (2 to 4 pCi/L) or Zone 3 (less than 2 pCi/L), based on indoor radon measurements, geology, aerial radioactivity, soil permeability and foundation type data aggregated at the county level.
Related: Geologic radon potential, Action level, Measurement
Geologic radon potential
A USGS assessment of how likely the underlying geology of an area is to produce radon.
Technical: USGS radon potential assessments combine indoor radon measurements, geology, aerial radioactivity and soil permeability to estimate an area's geologic potential to produce radon. Like an EPA zone, this describes the area, not any specific building, and both agencies note it is not a substitute for testing.
Related: EPA Zone, Measurement
ZCTA vs. ZIP code
A ZIP code is a USPS mail-routing code. A ZCTA is the Census Bureau's approximation of that ZIP code's area, built from census blocks: they are related but not identical.
Technical: USPS ZIP codes describe mail delivery routes, not fixed land areas, and can change without notice. The Census Bureau publishes ZIP Code Tabulation Areas (ZCTAs) as a statistical approximation for mapping and reporting. Radon411 reports data at the county level for a ZCTA/ZIP, because that is the level at which EPA zone and CDC data are actually published, and a ZIP or ZCTA page always says so.
Related: EPA Zone, Geologic radon potential
Units
Units and the action level
How radon is measured, and the level at which EPA recommends fixing a home.
pCi/L
Picocuries per liter, the standard U.S. unit for reporting a radon test result.
Technical: A picocurie is one trillionth of a curie, a unit of radioactivity. pCi/L measures the concentration of radioactive decay from radon gas in a liter of air. It is the primary consumer unit used in EPA guidance and on U.S. test kit results.
Related: Bq/m³, Action level, Measurement
Bq/m³
Becquerels per cubic meter, the metric unit for radon concentration, used internationally.
Technical: A becquerel is one radioactive decay event per second. 1 pCi/L equals approximately 37 Bq/m³. EPA's zone thresholds are published in both units: 4 pCi/L is about 150 Bq/m³, and 2 pCi/L is about 75 Bq/m³.
Related: pCi/L, Action level
Action level
The radon level, 4 pCi/L, at which EPA recommends fixing a home.
Technical: EPA's action level of 4 pCi/L is the threshold used throughout EPA's radon guidance and in the EPA zone framework's Zone 1 definition. EPA also identifies 2 to 4 pCi/L as a range in which fixing the home should be considered, and states that any radon exposure carries some risk that decreases as the level decreases.
Related: pCi/L, EPA Zone, Measurement
Measured
Test results
The words used for reported tests and the figures drawn from them.
Measurement
A radon test, the only way to know the actual radon level in a specific building.
Technical: In EPA's usage, "measurement" refers to a completed radon test with a result, as distinct from a prediction (an EPA zone or USGS potential rating). EPA states directly that testing is the only way to know the radon level in a specific building.
Related: Screening measurement, EPA Zone
Screening measurement
A single short-term test result, treated as an initial read rather than a final answer.
Technical: A short-term test (2 to 90 days, and a minimum of 48 hours under EPA's guidance) is less likely than a long-term test to tell you your year-round average radon level, because radon levels tend to vary from day to day and season to season. If a first result is high, EPA's Citizen's Guide says to follow up with either a long-term test or a second short-term test.
Related: Testing, Measurement
Buildings tested
The count of individual buildings with a reported radon test result feeding a given statistic.
Technical: The denominator behind any percentage or summary statistic drawn from reported test data. A statistic based on a handful of tested buildings carries far less statistical weight than one based on thousands, and CDC's own tracking data notes this: always read the sample size alongside the figure it produced.
Related: Median, Maximum, Measurement
Pre-mitigation
A radon test taken before a mitigation system is installed.
Technical: A pre-mitigation measurement establishes the baseline radon level a mitigation system needs to reduce. EPA recommends that the mitigation contractor review the radon measurement results before beginning any radon-reduction work.
Related: Post-mitigation, Mitigation, Measurement
Post-mitigation
A radon test taken after a mitigation system is installed, to confirm it is working.
Technical: Post-mitigation testing verifies that an installed system has reduced the indoor radon level. EPA's Consumer's Guide says a post-mitigation test should be done within 30 days of system installation, and recommends that you also get an independent follow-up radon measurement.
Related: Pre-mitigation, Mitigation
Median
The middle value in a set of test results when they are lined up from lowest to highest.
Technical: For a county's reported test results, the median is the value with an equal count of results above and below it. A few very high readings can pull the average (mean) up, and the median is less affected by them, which is why Radon411 reports the median for a set of reported results.
Related: Maximum, Buildings tested, Measurement
Maximum
The single highest test result reported for an area.
Technical: The maximum reported value in a set of test results for a county or state. A high maximum reflects one reported reading, not the typical level in that area, and should always be read alongside the median and the count of buildings tested, never on its own.
Related: Median, Buildings tested
In the home
Testing and fixing
The two things a homeowner does about radon.
Testing
Placing a device in a building to measure its radon level over a set period.
Technical: EPA's guides describe short-term devices (2 to 90 days, and EPA says all radon tests should be taken for a minimum of 48 hours), long-term devices (more than 90 days) and continuous monitors that log readings throughout the test period. Device placement, duration and closed-house conditions all affect the result's reliability.
Related: Screening measurement, Measurement
Mitigation
Work done to a building to reduce its indoor radon level, primarily with a vent pipe and fan system.
Technical: EPA's Consumer's Guide calls active subslab suction the most common and usually the most reliable radon reduction method, and submembrane suction the most effective way to reduce radon levels in crawlspace homes: a pipe drawing soil gas from beneath the foundation, or from under a plastic sheet, and venting it outdoors via a fan. Where a state has no mitigation regulations, EPA recommends that the system conform to EPA's most current voluntary consensus-based standards.
Related: Pre-mitigation, Post-mitigation
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