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What Radon Levels Mean: The EPA 4 pCi/L Action Level Explained
You got a radon result back: 2.7, or 6.1, or 14. Now what? This guide translates the number using EPA's own guidance and risk estimates, so you can decide what to do next.
What "pCi/L" means
Radon in U.S. homes is measured in picocuries per liter of air (pCi/L), a measure of radioactive decay. Other countries often use becquerels per cubic meter; EPA lists 4 pCi/L as equal to 150 Bq/m³. For context, EPA estimates the average indoor level in U.S. homes is about 1.3 pCi/L, and outdoor air is about 0.4 pCi/L.
The key thresholds
| Your level | What EPA recommends |
|---|---|
| 4 pCi/L or higher | Fix your home. This is the "action level." |
| 2 to under 4 pCi/L | Consider fixing. Risk still exists at these levels. |
| Under 2 pCi/L | Lower risk; reducing further is often hard. Retest after remodeling or changes in how you use the home. |
SC DES describes the action level as "the point where the risk of radon exposure justifies the cost of repairs" and notes there is no completely safe level of radon.
EPA's lifetime risk estimates
EPA's Citizen's Guide estimates how many people out of 1,000 could get lung cancer if exposed to a given level over a lifetime:
| Radon level | Never smoked | Smokers |
|---|---|---|
| 20 pCi/L | About 36 | About 260 |
| 10 pCi/L | About 18 | About 150 |
| 8 pCi/L | About 15 | About 120 |
| 4 pCi/L | About 7 | About 62 |
| 2 pCi/L | About 4 | About 32 |
| 1.3 pCi/L (U.S. average indoor) | About 2 | About 20 |
Source: EPA, A Citizen's Guide to Radon (2016). Lifetime estimates; they are not predictions for any individual.
Two takeaways: risk climbs steadily with level, and smoking multiplies it. For more on health effects, see radon health risks for Upstate homeowners.
One test isn't the final word
A single short-term result is a snapshot. EPA's method is to confirm a 4+ result with a follow-up test and act on the long-term result or the average of two short-term tests. Our radon testing checklist walks through it.
How common are high results around Greenville?
From homeowner kits returned to the SC Radon Program through June 2022, SC DES reports an average of 3.4 pCi/L in Greenville County, 3.0 in Pickens, 2.6 in Spartanburg and 1.7 in Anderson. SC DES cautions that these kits weren't necessarily used under ideal conditions. See the SC DES county table. For what that means locally, read is radon a problem in Greenville County?
If your level is high
SC DES says radon levels in most homes can be reduced to 2 pCi/L or below, usually with active soil depressurization. Read about radon mitigation for Upstate homes.
References: EPA, A Citizen's Guide to Radon (2016); SC DES Radon Program; SC DES average radon by county (2022).
Radon services in Greenville
Quick answers
What is the EPA action level for radon?
4 picocuries per liter (pCi/L). EPA recommends fixing your home at or above that level, and considering a fix between 2 and 4 pCi/L.
What is a normal radon level?
EPA estimates the average U.S. indoor level is about 1.3 pCi/L and outdoor air is about 0.4 pCi/L. There is no known completely safe level.
More radon guides
- Is Radon a Problem in Greenville County, SC?
- How to Test Your Greenville Home for Radon
- What Radon Mitigation Typically Costs in Upstate SC
- Radon Tests When Buying or Selling a Home in Greenville
- Radon Health Risks: What Upstate Homeowners Should Know
- Radon in Crawlspace Homes: Testing and Fixes That Work
- How to Choose a Radon Mitigation Company in Greenville, SC