Your radon test came back with a number. Now you need to know what that number means — not just whether it is above or below an arbitrary threshold, but what the actual health risk is at that concentration, what the EPA recommends at each level, and what your realistic options are. This guide translates pCi/L into plain language.
What Is pCi/L?
Picocuries per liter (pCi/L) is the standard U.S. measurement unit for radon concentration in air. One picocurie represents approximately 2.2 radioactive disintegrations per minute in one liter of air. The measurement reflects how much radon decay activity is occurring in the air you breathe.
For context: the average outdoor radon level in the U.S. is approximately 0.4 pCi/L. The average indoor level is 1.3 pCi/L — already elevated above outdoor air simply because buildings concentrate radon that enters from the soil. EPA considers 4.0 pCi/L the action level at which mitigation is recommended.
The EPA Action Level: 4.0 pCi/L
The EPA’s 4.0 pCi/L action level is not a bright line between “safe” and “dangerous.” It is a practical threshold chosen to balance risk reduction with the cost and feasibility of mitigation. EPA has also established a 2.0 pCi/L “consider mitigating” level — acknowledging that even at concentrations between 2.0 and 4.0 pCi/L, radon exposure contributes meaningfully to lifetime lung cancer risk.
The World Health Organization (WHO) uses a lower reference level of 2.7 pCi/L (100 Bq/m³), reflecting evidence that significant risk exists below EPA’s 4.0 threshold. Many European countries use the WHO reference level or lower values in their national radon programs.
Health Risk at Each Concentration Level
EPA publishes risk estimates for radon exposure using lifetime lung cancer risk per 1,000 people exposed continuously at each concentration level. These estimates apply to never-smokers — smokers face dramatically compounded risk because radon decay products and tobacco smoke synergistically damage lung tissue.
| Radon Level (pCi/L) | Estimated Lung Cancer Deaths per 1,000 Never-Smokers | EPA Recommendation |
|---|---|---|
| 0.4 (outdoor average) | ~0.4 | Baseline — outdoor air |
| 1.3 (indoor average) | ~1.0 | National average |
| 2.0 | ~1.5 | Consider mitigating |
| 4.0 | ~2.9 | Mitigate |
| 8.0 | ~5.8 | Mitigate without waiting for confirmatory test |
| 20.0 | ~14.7 | Mitigate immediately |
For comparison: radon at 4.0 pCi/L carries roughly the same lifetime lung cancer risk as having 200 chest X-rays per year, or smoking approximately 8 cigarettes per day according to EPA risk comparisons. At 20 pCi/L, the risk approaches that of smoking a pack per day.
What to Do at Each Level
Below 2.0 pCi/L
No action required. Retest in 2 years, or after any significant renovations that affect the foundation or HVAC system. If your result is below 1.3 pCi/L, your home is below the national indoor average.
2.0–3.9 pCi/L
EPA recommends considering mitigation. This is not a mandate — mitigation at this level is a personal risk decision. Factors that strengthen the case for mitigation even below 4.0 pCi/L:
- Smokers in the household (radon and tobacco risk multiply, not add)
- Young children who will spend decades in the home
- Plans to finish a basement or spend more time in the lower level
- Result was from a short-term test in favorable conditions — actual annual average may be higher
Mitigation in this range typically costs the same as mitigation at 10 pCi/L — the system is the same. The only question is whether the risk reduction justifies the investment at your specific level.
4.0–7.9 pCi/L
At or above the EPA action level. EPA recommends mitigation. If the result was from a short-term test, conduct a confirmatory long-term test or second short-term test before proceeding — unless you want to mitigate without waiting, which is always safe to do. If confirmed above 4.0 pCi/L, install an active radon mitigation system.
8.0 pCi/L or Higher
Mitigate without waiting for a confirmatory test. At this concentration, the cumulative risk from continued exposure while conducting additional testing is not justified by the modest additional certainty a second test provides. Contact a certified radon mitigator and schedule installation.
Post-Mitigation Results: What to Expect
A properly installed active Sub-Slab Depressurization system typically reduces radon levels by 85–99%. Common post-mitigation results:
- A home at 12 pCi/L before mitigation commonly achieves 0.5–1.5 pCi/L after a single-point ASD installation with good aggregate conditions
- A home at 4.5 pCi/L commonly achieves 0.3–0.8 pCi/L
- Post-mitigation results above 4.0 pCi/L indicate insufficient suction coverage, unsealed entry pathways, or an undersized fan — and warrant a contractor callback
EPA recommends post-mitigation testing 24 hours after system activation (if using a continuous monitor) or placing a short-term test at least 24 hours post-installation and running it for 48 hours minimum. The target is below 4.0 pCi/L; most installations achieve below 2.0 pCi/L.
Frequently Asked Questions
Is 3.9 pCi/L safe?
It is below the EPA action level of 4.0 pCi/L, so EPA does not mandate mitigation. However, the risk difference between 3.9 and 4.0 pCi/L is negligible — they represent essentially the same health risk. EPA recommends “considering mitigation” at 2.0 pCi/L, so at 3.9 pCi/L you are in the range where mitigation is a reasonable personal risk decision even if not required.
What is a safe radon level?
There is no radon level that carries zero risk — even outdoor radon (0.4 pCi/L) contributes some cumulative exposure. The EPA action level of 4.0 pCi/L represents a pragmatic threshold for mandatory action, not a definition of “safe.” Many health organizations, including the WHO, recommend action at 2.7 pCi/L or lower. Reducing radon levels as low as reasonably achievable is always the goal.
My test result is in WL, not pCi/L. How do I convert?
Working level (WL) is an older measurement unit still used in some occupational and commercial radon standards. To convert: 1 WL equals approximately 200 pCi/L of radon in equilibrium. EPA’s 4.0 pCi/L action level corresponds to approximately 0.02 WL. Most modern residential tests report in pCi/L.
My result is 2.5 pCi/L — should I mitigate?
EPA recommends considering mitigation at this level. The decision is yours. Key factors: whether you have smokers in the home (dramatically compounded risk), whether you are planning to spend significantly more time in the lower level (finishing a basement), the age of occupants, and your personal risk tolerance. Mitigation at 2.5 pCi/L will typically cost the same as mitigation at 8.0 pCi/L and will reduce levels to 0.3–0.8 pCi/L.
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