The EPA recommends mitigation at 4.0 pCi/L or above. This is the EPA's official action level nationwide, including for Ohio homes — Ohio does not set its own separate legal threshold, so the federal 4.0 pCi/L guidance is the number Ohio homeowners and radon professionals work from.
What exactly is 4.0 pCi/L?
pCi/L stands for picocuries per liter of air — a measure of radioactive decay, and specifically how much radon gas is present in the air you're breathing. For context, outdoor ambient air runs around 0.4 pCi/L, and the U.S. national average for indoor air is about 1.3 pCi/L. Ohio's statewide average is notably higher, around 7.9 pCi/L, roughly double the national indoor average, due to the state's underlying geology. The EPA's 4.0 pCi/L figure is what's called an "action level" — a policy line that weighs the health risk of radon exposure against how reliably homes can be measured and how effectively that exposure can realistically be reduced nationwide.
Is there a "safe" level below 4.0 pCi/L?
Not technically. The EPA is explicit that there is no fully safe level of radon exposure — risk scales with concentration and duration, and it doesn't drop to zero just below the action line. The World Health Organization uses a more conservative reference level of 2.7 pCi/L. That's why some Ohio homeowners with readings in the 2-4 pCi/L range choose to mitigate anyway, even though it isn't the EPA's formal recommendation threshold — it comes down to personal risk tolerance, particularly in a household with long-term occupants or young children.
How much does mitigation actually reduce radon levels?
An active sub-slab depressurization system — the standard mitigation approach, and the same underlying design used for EPA-recommended new construction — typically reduces indoor radon by 80% to 99%, often bringing a home that tested above 4.0 pCi/L down below 2.0 pCi/L. The system works by drawing soil gas from beneath the foundation through a sealed vent pipe and fan, venting it above the roofline before it can enter the living space.
What should I do if my Ohio home tests above 4.0 pCi/L?
Confirm the result with a proper short-term test if you haven't already (minimum 48 hours, closed-building conditions — see our radon testing guide), then schedule mitigation. See our radon mitigation systems page for how an install works, or radon mitigation cost for transparent Ohio pricing. Because roughly half of Ohio homes test above the action level, a high reading is common — not a sign of anything unusual about a specific property.
How radon compares to other everyday exposures
To put the 4.0 pCi/L threshold in context: outdoor ambient air runs about 0.4 pCi/L, and the U.S. national indoor average is about 1.3 pCi/L. A home at 4.0 pCi/L has roughly ten times the outdoor background level of radon inside it, and roughly three times the national indoor average — which is part of why the EPA set the action line where it did, rather than at the outdoor baseline. Ohio's statewide indoor average of 7.9 pCi/L means a large share of the state's housing stock sits well above the action level before any mitigation is done.
Why a single test result isn't always the final word
Because radon fluctuates with weather and season, a reading that comes back just above or just below 4.0 pCi/L on one 48-hour test can shift on a retest taken at a different time of year — winter closed-building conditions in particular tend to push readings higher than a mild-weather test. A result close to the line is a reasonable case for either a confirmatory retest or a long-term test (90+ days) to get a more stable year-round picture before deciding.
Why 4.0 pCi/L specifically, and not a round number like 5.0 or a stricter one like 2.0
The EPA's 4.0 pCi/L action level isn't an arbitrary round number — it reflects a deliberate balance between the health risk radon poses at that concentration and what's realistically achievable at a national scale, factoring in how reliably homes can be measured and how consistently mitigation systems perform. A stricter number, like the WHO's 2.7 pCi/L reference level, would flag more homes for action and reduce cumulative population risk further, but would also mean many more mitigations for a smaller marginal reduction per system. The EPA's choice represents where regulators judged the cost-benefit line to sit for a nationwide recommendation — individual homeowners are still free to set their own personal threshold below it.
What actually happens once mitigation is scheduled
An active sub-slab depressurization install is a targeted, single-visit job for most homes: a small entry point is core-drilled through the slab (typically in a basement, utility area, or garage — chosen to minimize visible impact), a suction pit is created beneath it, and PVC vent pipe is routed from that point to an exterior or attic-routed termination above the roofline, with an in-line fan mounted along the run. The whole process is designed to disturb as little of the finished space as possible, and most installs are completed and fan-tested in a single day.
Confirming the system worked
Once the system is running, a follow-up short-term test — the same 48-hour, closed-building protocol used for the original test — confirms the actual reduction achieved. This isn't optional paperwork: it's the only way to verify the specific home's result landed where it was supposed to, since soil conditions and house construction both affect how much a given system design reduces radon in practice.
Does every home above 4.0 pCi/L need the same kind of system?
Most single-family homes use the sub-slab depressurization approach described above, but the exact configuration — number of suction points, pipe routing, fan sizing — depends on the home's foundation type (basement, crawlspace, or slab-on-grade all handle differently) and how the initial test reading compares to the action level. A home testing well above 4.0 pCi/L may need a more robust system design than one testing just over the line, which is why a site evaluation, not just the test number alone, determines the right system for a specific property.
What if you're right on the line — 3.5 to 4.5 pCi/L?
Readings in this narrow band are common and worth thinking through deliberately rather than reflexively rounding up or down. Because radon fluctuates seasonally, a home testing 3.8 pCi/L in summer could plausibly test above 4.0 pCi/L in winter, when closed-building conditions are in effect for months rather than just the 48-hour test window — Ohio's climate makes this a realistic scenario, not a hypothetical. A long-term test (90+ days, ideally spanning a winter period) gives the clearest picture for a borderline result. Given that mitigation reliably brings levels down to a fraction of the action level regardless of whether the starting point was 4.1 or 8.0 pCi/L, many homeowners in the borderline range choose to mitigate rather than gamble on seasonal variation, particularly if they plan to stay in the home long-term.
Getting a straight answer for your specific number
General guidance is useful context, but the right decision for your household depends on your actual test result, your home's construction, and how long you plan to live there. That's a conversation, not a formula.
How Radon Eliminator determines the right mitigation approach
Rather than applying a single default design to every home that tests above 4.0 pCi/L, a proper mitigation plan starts with a site evaluation: foundation type, sump pit location if present, number of distinct foundation sections, and where soil gas is most likely concentrating based on the test result and the home's construction. That evaluation determines suction-point count and pipe routing before any drilling happens, so the system is sized to the specific home rather than a one-size-fits-all install.
Key takeaways on what radon level requires mitigation
- The EPA recommends mitigation at a radon level of 4.0 pCi/L or above; there is no fully safe radon level below that line.
- Ohio's average indoor radon level, about 7.9 pCi/L, is roughly double the national average.
- A radon level near the 4.0 pCi/L line can shift seasonally, so a borderline reading is a reasonable case for a retest.
- Mitigation typically reduces a home's radon level by 80-99%, often to well below 2.0 pCi/L.
Test result at or near 4.0 pCi/L and not sure whether to mitigate? Call Radon Eliminator at 216-480-9580 to talk through your specific number.