What pCi/L measures
Picocuries per liter (pCi/L) measures radioactive decay events in a liter of air. One pCi/L is about 2.2 radioactive decays per minute per liter. Every one of those events inside your lungs releases alpha particles that can damage cell DNA. The unit is a rate of radiation, not a concentration of a chemical — which is why there is no threshold below which risk is exactly zero.
EPA radon action levels
Scale shown in picocuries per liter (pCi/L).
Green is at or below the typical US indoor average. Amber is the range where the EPA suggests considering mitigation. Red is at or above the 4.0 pCi/L action level, where the EPA recommends fixing the home.
The 4.0 pCi/L action level
The EPA recommends that homes measuring 4.0 pCi/L or higher be fixed. This is not a biological cliff edge — it is a practical threshold chosen because mitigation reliably and affordably achieves levels below it. Above 4.0, the recommendation is unambiguous: mitigate.
The 2.0 pCi/L consider-action level
Between 2.0 and 3.9 pCi/L, the EPA suggests considering mitigation. Risk in this band is real but lower, and most mitigation systems can bring a home in this range down to well under 1.0 pCi/L. Whether it is worth it depends on how much time is spent on the lowest level, whether anyone in the home smokes (which multiplies radon risk substantially), and whether children live there.
How levels map to action
pCi/L reference table
Levels reflect EPA guidance for residential indoor air.
| Level | What it means | Recommended action |
|---|---|---|
| 0.0 – 1.9 pCi/L | At or below typical US indoor average (1.3 pCi/L) | No action needed. Retest in about two years. |
| 2.0 – 3.9 pCi/L | Elevated but below the action level | EPA suggests considering mitigation. A long-term test helps confirm. |
| 4.0 – 7.9 pCi/L | At or above the EPA action level | Mitigation recommended. |
| 8.0 – 19.9 pCi/L | Two to five times the action level | Mitigate promptly. |
| 20+ pCi/L | Substantially elevated | Treat as urgent; mitigate as soon as possible. |
The outdoor and average-indoor baselines
| Reference point | Approximate level |
|---|---|
| Average outdoor air | 0.4 pCi/L |
| Average US indoor air | 1.3 pCi/L |
| EPA consider-action level | 2.0 pCi/L |
| EPA action level | 4.0 pCi/L |
| WHO reference level (≈100 Bq/m³) | 2.7 pCi/L |
You cannot reduce a home to outdoor levels, and no reputable contractor should promise it. A well-designed system typically lands between 0.5 and 2.0 pCi/L.
Converting between units
Much of the world measures radon in becquerels per cubic meter (Bq/m³). To convert: 1 pCi/L = 37 Bq/m³. So 4.0 pCi/L is 148 Bq/m³, and the WHO's 100 Bq/m³ reference level is about 2.7 pCi/L — stricter than the EPA's action level.
Should you mitigate?
Should I mitigate?
Guidance based on EPA action levels. Your specific situation may vary.
Mitigation recommended
You are at or above the EPA action level of 4.0 pCi/L. The EPA recommends fixing homes at this level.
Common questions
It is below the action level, not risk-free. The EPA suggests considering mitigation anywhere above 2.0 pCi/L, and there is no level of radon exposure known to be entirely without risk.
Related services
Keep learning
- Radon Health Risks: What the Evidence ShowsRadon is the second leading cause of lung cancer in the United States. The mechanism is well understood, and the risk scales with concentration and time.
- Radon Testing Methods ComparedEvery radon test measures the same thing, but the method determines how much you can trust the number and what decisions it can support.