Understanding pCi/L and Bq/m3
A picocurie is a unit of radioactivity representing a very small number of radioactive decay events per second, and pCi/L expresses how many of those events occur, on average, in a liter of air. It's the standard unit used in United States radon reporting, appearing on virtually every test report from a home inspection or mitigation contractor.
Becquerels per cubic meter (Bq/m3) is the metric unit used more commonly outside the U.S., measuring decay events per second in a cubic meter of air. To convert roughly, 1 pCi/L equals about 37 Bq/m3, so a home measuring 4.0 pCi/L corresponds to roughly 148 Bq/m3, useful context if you encounter international radon guidance.
EPA's action level and what it actually means
EPA's action level of 4.0 pCi/L is the threshold at which EPA recommends fixing a home, based on a combination of health risk and technical and economic feasibility of mitigation, not a hard line below which radon becomes harmless. EPA is explicit that there is no known safe level of radon exposure, since risk increases with concentration and duration of exposure rather than dropping to zero below any particular number.
For results between 2 and 4 pCi/L, EPA states that homeowners should consider fixing their home, acknowledging that mitigation still meaningfully reduces long-term exposure even below the formal action level. This is why some homeowners in Indianapolis choose to mitigate homes testing in that 2-4 range rather than waiting until levels reach 4.0 or higher.
- Below 2 pCi/L: generally considered lower risk, though no level is entirely risk-free
- 2 to 4 pCi/L: EPA suggests considering mitigation
- 4.0 pCi/L and above: EPA recommends fixing the home
- There is no known safe level of radon exposure at any concentration
Why levels vary so much between and within homes
Radon levels are influenced by the specific soil conditions beneath a given home, the foundation type, and the pathways available for soil gas to enter, which is why two similar homes on the same street can show quite different results. Central Indiana's glacial till soils and the specific fractures or gravel layers beneath an individual lot both play a role in how much radon is available to migrate upward.
Levels also vary seasonally and even hour to hour within the same home, generally rising during colder months when Indiana's heating season creates stronger stack effect, pulling more soil gas indoors as warm indoor air rises and escapes, drawing replacement air in from the foundation. This is part of why a single test needs a defined protocol rather than a spot check.
Comparing a result to context, not just the number alone
A result of 3.8 pCi/L and a result of 12 pCi/L are both technically below or above different thresholds, but they represent meaningfully different risk levels and often different mitigation approaches. Very high readings, well above the action level, may point to a particularly strong soil gas source or multiple entry points and can sometimes require a more robust suction system design than a home just barely over 4.0.
It's also useful to compare a short-term result to any prior test if one exists, since a level that has changed significantly from a previous reading can be a signal worth investigating on its own, separate from where the number falls relative to the action level.
What a result means for your next decision
A result below 2 pCi/L generally means continuing with the standard EPA-recommended retesting interval of at least every two years is reasonable. A result between 2 and 4 pCi/L is a point at which many homeowners choose to consult with a mitigation professional even though it's not a hard requirement. A result at or above 4.0 pCi/L is when EPA recommends fixing the home, and most Indianapolis homeowners at this level move forward with mitigation.
Whatever the number, EPA and CDC identify radon as the second leading cause of lung cancer in the United States, behind smoking, and note that risk compounds for smokers, which is useful context for weighing how quickly to act on a given result rather than treating any threshold as an all-or-nothing cutoff.
