What Level of Radon Is Unsafe?
By Daniel SmokeDirector of Operations
Radon is measured in picocuries per liter (pCi/L), a unit of radioactivity per volume of air. No level of radon is completely without risk. But the EPA has established an action level of 4 pCi/L, above which mitigation is strongly recommended. Understanding what your test result means helps you decide on next steps.
The EPA Action Level: 4 pCi/L
The EPA recommends taking action to reduce indoor radon levels when they are at or above 4 picocuries per liter (pCi/L). This level was chosen based on the feasibility of mitigation. The EPA's goal is to reduce indoor radon to as close to outdoor levels as possible (typically about 0.4 pCi/L). Practically, though, most mitigation systems can reliably bring levels below 2 pCi/L. At 4 pCi/L, a non-smoker faces approximately a 7 in 1,000 lifetime lung cancer risk from radon. A smoker faces approximately 62 in 1,000.
The 2–4 pCi/L Range
The EPA also recommends considering mitigation for levels between 2 and 4 pCi/L. This is particularly true for households with smokers or young children who will be spending decades in the home. While the absolute risk at these levels is lower, mitigation is often cost-effective and can reliably bring levels below 2 pCi/L. If your short-term test result falls in this range, a long-term test provides a more accurate picture of your average annual exposure before making a mitigation decision.
High Levels: Above 10 pCi/L
Levels above 10 pCi/L are considered high and warrant prompt mitigation without waiting for a confirming long-term test. At 20 pCi/L, a non-smoker's lifetime lung cancer risk is approximately 36 in 1,000, comparable to a significant occupational hazard. Very high levels (above 100 pCi/L) are rare but have been documented in some geologically active areas and in buildings with structural features that concentrate soil gas. These situations require professional mitigation design.
Radon in Water vs. Air
Radon in domestic water from private wells can contribute to airborne radon levels, particularly in bathrooms and laundry rooms when water is agitated. It can also be ingested directly. The EPA's proposed maximum contaminant level for radon in public water supplies is 300 pCi/L for systems with multimedia programs and 4,000 pCi/L for others. The airborne contribution from water use is generally a small fraction of total indoor radon in most homes with municipal water supplies. But it may be more significant for homes on private wells with high radon in water.
What Mitigation Can Achieve
A properly installed sub-slab depressurization system typically reduces radon levels by 50% to 99%. Homes with pre-mitigation levels of 20–30 pCi/L can often be reduced to below 2 pCi/L with a well-designed system. Post-mitigation testing, conducted 24 hours or more after the system is installed and activated, confirms the achieved reduction. NRPP and NRSB certified contractors are required to provide post-mitigation testing results and will modify the system if results remain above the action level.
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