My Radon Test Result Is Near 4 pCi/L — What Does That Mean?

What Is the EPA Action Level?

The EPA recommends that homeowners take action to reduce radon levels when a long-term average is at or above 4 picocuries per liter of air (pCi/L). This is called the action level. It is not a safety threshold — there is no known completely safe level of radon — but it is the practical line at which the EPA considers mitigation cost-effective based on risk reduction models.

Below 4 pCi/L, the EPA also recommends considering mitigation when levels fall between 2 and 4 pCi/L. Levels below 2 pCi/L are considered low concern, though the EPA notes that even these levels carry some risk over a lifetime of exposure.

The action level of 4 pCi/L was set by the EPA in 1992 and has not been changed since, despite some researchers and public health groups — including the World Health Organization, which uses 2.7 pCi/L — recommending a lower threshold. For the purposes of U.S. homeowner decisions, the 4 pCi/L line remains the operative standard.

The Four Decision Zones

The Borderline Zone: What Does 4.2 or 3.8 pCi/L Mean?

The most common scenario that sends homeowners searching is a result that lands just above or just below the action level — results like 3.6, 4.1, 4.2, or 4.8 pCi/L. These results require a slightly more nuanced response than the clear-cut cases.

If Your Result Is 3.5–3.9 pCi/L

You are just below the EPA action level but squarely in the "consider mitigating" zone. The EPA recommends thinking about mitigation here — and there are good reasons to act even below 4 pCi/L:

• A short-term test result of 3.8 pCi/L likely represents a long-term average somewhat higher or lower — seasonal variation can swing levels by 1–2 pCi/L in either direction.
• If the test was conducted in summer (warmer months, windows open), the actual heating-season average is likely higher than your test result.
• Mitigation at this level costs the same as at 5 pCi/L — approximately $800–$2,000. The risk reduction is real at any elevated level.

Recommended action at 3.5–3.9: Conduct a long-term test (90-day minimum, winter/fall preferred) to get a true annual average. If the long-term result confirms above 3.0, most radon specialists recommend mitigating.

If Your Result Is 4.0–7.9 pCi/L

You are above the EPA action level. The guidance is clear: mitigate. The question most homeowners in this zone have is timing — specifically, whether to re-test first or proceed directly to mitigation.

If you used a short-term test (2–7 days), EPA guidance says to conduct a second short-term test before deciding on mitigation. If both tests average 4 pCi/L or above, mitigation is recommended. However, at 5 pCi/L or above on a short-term test, many radon specialists and the most recent AARST guidance suggest proceeding directly to mitigation — the probability that a second test would come back below 4 pCi/L is low, and the cost of a second test ($15–$30 for a mail-in test kit) is trivial compared to the delay.

If Your Result Is 8.0–20 pCi/L

At this level, the long-term health risk is significant and documented. EPA compares this exposure level (for a non-smoker) to having dozens of chest X-rays per year. Mitigation should happen within weeks, not months. A second test is not necessary — the appropriate response is to immediately contact two or three certified radon contractors, get quotes, and schedule installation.

Seasonal Variation: Why Your Test Date Matters

Radon levels in a home are not constant. They fluctuate based on:

• Season and temperature. During winter heating season, homes are closed up and the stack effect (warm air rising, drawing soil gases in from below) is strongest. Radon levels in most northern and midwest homes are 25–50% higher in winter than summer.
• Wind and barometric pressure. Low-pressure weather systems pull more soil gas into homes. High-wind days can reduce levels temporarily.
• Ventilation habits. Homes with open windows test significantly lower than homes with closed windows under identical subsurface radon conditions.

Short-Term vs. Long-Term Tests: What You Have Changes the Answer

Short-Term Test (2–7 days)

Charcoal canisters or electret ion chambers. Quick, inexpensive ($15–$30 for a mail-in kit). The result is a snapshot — not your annual average. Short-term results are more variable and can swing high or low based on temporary conditions. EPA allows using short-term tests for initial screening but recommends confirming with a second short-term or a long-term test before making mitigation decisions.

Long-Term Test (90+ days)

Alpha track detectors left in place for a full season or full year. Provides the most accurate estimate of your actual annual average exposure. The result is what your lungs actually experience over time, smoothing out day-to-day and week-to-week variation. If your home is in a Zone 1 or Zone 2 county and you have time, a long-term test before mitigation decisions is ideal — unless your short-term result is 8 pCi/L or above, in which case act now.

Should You Re-Test Before Mitigating?

The practical answer depends on your result and how you tested:

• Short-term result below 4.0 pCi/L: Run a long-term test (90-day alpha track) before concluding you're safe. Test during fall/winter if possible.
• Short-term result of 4.0–6.9 pCi/L: Run a second short-term test under closed-house conditions. If both average 4+ pCi/L, mitigate.
• Short-term result of 7.0–9.9 pCi/L: Proceed to mitigation. A confirmatory second test is optional — at this level, the right decision is mitigation regardless.
• Any result 10+ pCi/L: Do not wait for a second test. Contact a certified contractor now.

What Is the Actual Health Risk?

Radon is the second leading cause of lung cancer in the United States, responsible for approximately 21,000 deaths per year according to EPA estimates. It causes about 2,900 deaths among non-smokers — making it the leading cause of lung cancer death in people who have never smoked.

The risk is a lifetime cumulative exposure model, not an acute toxicity situation. Spending one year at 4 pCi/L does not cause immediate harm. The risk is calculated over years and decades of exposure. This is why the priority for mitigation is homes where people sleep — basements used as primary bedrooms carry more risk than unfinished basements used only for storage.

The Decision Framework: What to Do Right Now

1. Identify your test type. Short-term (2–7 days) or long-term (90+ days)? Was it conducted under closed-house conditions? Was it placed at the lowest livable level of your home?
2. Look up your result on the table above. Below 2: retest in 2 years. 2–3.9: run long-term test. 4–7.9: confirm and mitigate. 8+: mitigate without further delay.
3. Account for season. If you tested in summer (May–August) with windows open, your winter-season level is likely higher. Add a mental "bump" to your result for planning purposes.
4. Use the Action Level Advisor tool. Enter your result and state to get state-specific guidance, contractor licensing requirements, and permit notes for your area.
5. Get quotes. Contact 2–3 NRPP or NRSB certified contractors. A mitigation quote is free and there is no obligation. Knowing the cost changes the calculation — for most homeowners, mitigation is a few hundred to a couple thousand dollars and lasts the life of the home with minimal maintenance.

Frequently Asked Questions