1. What pCi/L Means
Radon test results are reported in picocuries per liter (pCi/L). This unit measures the radioactivity of radon gas in each liter of air in your home. One picocurie represents 2.2 radioactive decay events per minute per liter of air.
You do not need to understand the physics to interpret your results. What matters is how your number compares to these benchmarks:
Some international reports use Becquerels per cubic meter (Bq/m3). To convert: 1 pCi/L = 37 Bq/m3. The WHO action level of 100 Bq/m3 equals 2.7 pCi/L.
2. The Numbers: What Each Range Means
Here is a straightforward guide to what your test result means and what action to take:
| Result | Risk Level | Recommended Action |
|---|---|---|
| Below 2 pCi/L | Low | No action needed. Retest every 2 years. |
| 2.0 - 3.9 pCi/L | Moderate | Consider mitigation. Follow-up long-term test recommended. |
| 4.0 - 7.9 pCi/L | High | Mitigate. EPA recommends action at 4.0+. |
| 8.0 - 19.9 pCi/L | Very High | Mitigate soon. This is significantly above the action level. |
| 20+ pCi/L | Extreme | Mitigate immediately. Consider temporary measures while scheduling. |
Context Matters
A single test result is a snapshot. If you tested during winter, your result reflects peak levels. If you tested in summer, your winter levels could be 40-60% higher. A result of 3.0 pCi/L in July might mean 4.5+ pCi/L in January. Factor in the season when interpreting borderline results.
3. The EPA Action Level Explained
The EPA set 4.0 pCi/L as the "action level" in 1986. This is not a safety threshold. It does not mean 3.9 is safe and 4.1 is dangerous. It is an administrative guideline that represents the level at which the EPA determined mitigation is both practical and cost-effective.
Important context about the 4.0 pCi/L threshold:
The Borderline Trap
Many homeowners get a result of 3.5 pCi/L and think "below the action level, I am fine." But that 3.5 was likely measured during a specific season under specific conditions. A winter retest might return 5.5 pCi/L. If your result is between 2 and 4, do not assume you are in the clear. Consider a long-term test to get a more representative annual average.
4. Reading a Professional CRM Report
A professional continuous radon monitor (CRM) test gives you much more data than a DIY kit. Here is what you will see on a typical CRM report:
Average Reading
This is the most important number. It represents the average radon concentration across the entire test period (typically 48 hours). This is the number you compare to the EPA action level. Most mitigation decisions are based on this average.
Hourly Data
CRM reports include hourly or bi-hourly readings displayed in a graph or table. You will typically see significant variation. A home with a 4.0 pCi/L average might show hourly readings ranging from 1.5 to 8.0 pCi/L. This fluctuation is completely normal and reflects natural radon dynamics.
What the Hourly Data Tells You
Tamper Indicators
Professional CRM reports include tamper indicators: motion sensors and tilt sensors that flag if the device was moved during the test. For real estate transactions, tamper-free results are essential for credibility.
5. Understanding DIY Kit Results
A DIY charcoal test kit gives you a single number: the average radon concentration during the exposure period. The lab report typically includes:
DIY kits do not provide hourly data or tamper indicators, which is why they are not accepted for real estate transactions. But for initial screening of your own home, they are a reasonable starting point.
6. Why Results Fluctuate
If you have tested your home more than once, you may have gotten different results. This is normal. Radon levels are influenced by multiple factors:
This variability is why the EPA recommends retesting every two years and why long-term tests provide a better picture of your true annual exposure than any single short-term test.
7. What to Do After Your Test
Based on your results, here is the recommended path forward:
Below 2 pCi/L: Low Risk
No action needed. Retest every 2 years or after major renovations. Check your zip code periodically with our radon risk lookup.
2.0 - 3.9 pCi/L: Borderline
Consider a follow-up long-term test (90+ days) to confirm. If you tested in summer, retest in winter. Mitigation is reasonable at this level, especially if the WHO guideline of 2.7 concerns you. Get a cost estimate with our mitigation cost calculator.
4.0+ pCi/L: Above Action Level
The EPA recommends mitigation. Contact a professional radon contractor for an assessment. Most systems are installed in one day and reduce radon by 90-99%. Use our cost calculator to estimate your investment.
