Radon test results are reported as a concentration of radon gas in air, measured in picocuries per liter (pCi/L) in the United States or becquerels per cubic meter (Bq/m³) internationally. The key number to know: the EPA recommends fixing your home if levels reach 4 pCi/L (150 Bq/m³) or higher. Below that, you’re in a gray zone where mitigation is still worth considering.
Understanding the Units
If you tested your home in the U.S., your result will almost certainly be in pCi/L. If you’re looking at international guidelines or used a European-made detector, you may see Bq/m³ instead. The conversion is straightforward: 10 Bq/m³ equals 0.27 pCi/L. To go the other direction, 1 pCi/L equals about 37 Bq/m³.
Both units measure the same thing: how much radioactive radon gas is present in a given volume of air. The number itself is what matters, not the unit, so just make sure you’re comparing your result against the right scale.
What the Numbers Mean
Radon results fall into a few practical ranges:
- Below 2 pCi/L (75 Bq/m³): Considered low. No action is typically needed, though no level of radon is completely risk-free.
- 2 to 4 pCi/L (75–150 Bq/m³): The EPA recommends you consider mitigation in this range. The risk is modest but real, especially over decades of exposure.
- 4 pCi/L (150 Bq/m³) or higher: The EPA action level. You should have a mitigation system installed.
- Above 8 pCi/L (300 Bq/m³): This is the upper reference level used by many countries internationally. At these concentrations, mitigation becomes urgent.
The World Health Organization sets its reference level at 100 Bq/m³ (about 2.7 pCi/L), which is stricter than the U.S. action level. Some countries follow the WHO recommendation, while others set their own thresholds up to 300 Bq/m³ for homes or even 1,000 Bq/m³ for workplaces.
Why Your Risk Depends on More Than the Number
A radon reading of 4 pCi/L carries very different risks depending on whether you smoke. Out of 1,000 smokers exposed to 4 pCi/L over a lifetime, roughly 62 could develop lung cancer. That’s about five times the risk of dying in a car crash. Among 1,000 nonsmokers at the same level, about 7 would develop lung cancer, a risk comparable to dying in a car crash. Smoking and radon together are far more dangerous than either one alone because radon’s radioactive particles attach to the damaged tissue that cigarette smoke creates in the lungs.
This is why there’s no truly “safe” radon level. Even below 4 pCi/L, cumulative exposure over years contributes to lung cancer risk. If your result is 3 pCi/L and you plan to live in your home for 20 or 30 years, mitigation is a reasonable investment.
Short-Term vs. Long-Term Test Results
How you interpret your number depends heavily on what type of test produced it. Short-term tests run anywhere from two days to a week using an active electronic monitor. Long-term tests use passive detectors (small devices you leave in place) for three months to a full year. The two types of tests don’t always agree, and understanding why will help you read your results correctly.
Short-term tests are good at catching a snapshot, but radon levels fluctuate constantly. Research comparing short-term and long-term measurements at the same locations found that long-term levels were about 1.3 times higher than short-term readings. That means a 48-hour test can underestimate your actual annual exposure. For low readings below about 2 pCi/L (75 Bq/m³), a one-week short-term test predicts annual averages with better than 95% confidence. But at moderate levels, that accuracy drops to around 50%, making a follow-up long-term test necessary.
The practical takeaway: if your short-term test comes back below 2 pCi/L, you can be fairly confident your home is low-risk. If it’s between 2 and 4 pCi/L, a long-term test of at least three months will give you a much more reliable picture. If it’s above 4 pCi/L, the EPA recommends doing a second short-term test to confirm before installing mitigation. Results above about 13.5 pCi/L (500 Bq/m³) on a short-term test strongly suggest your annual average will also exceed the action level, so you can act without waiting for long-term confirmation.
Why Readings Fluctuate Day to Day
If you’re watching a continuous radon monitor, you’ll notice the number changes from hour to hour. This is normal. Radon seeps into your home from the soil beneath it, and several environmental factors push and pull at that flow. Outdoor air temperature is the single biggest influence on indoor radon. When it’s cold outside and warm inside, the temperature difference creates a chimney effect that draws soil gases (including radon) up through your foundation.
Barometric pressure also plays a role. When atmospheric pressure drops, as it does before a storm, radon escapes from the ground more easily. Wind creates pressure differences around your house that can push more soil gas inside through one side of the foundation. Studies have found negative correlations between both wind speed and atmospheric pressure and indoor radon, meaning lower pressure and higher winds tend to pull more radon indoors.
These fluctuations are exactly why single-day readings can be misleading. A reading taken during a low-pressure winter evening might be twice what you’d see on a breezy summer afternoon. If you’re using a continuous monitor, look at the average over the entire testing period rather than fixating on any single peak or valley.
How to Get the Most Accurate Reading
Placement matters. Test in the lowest lived-in level of your home, since radon concentrations are highest closest to the ground. A basement you use as a family room counts; a crawl space you never enter does not. Place the detector at least 20 inches off the floor, away from exterior walls, and away from drafts, humidity, or direct sunlight.
During a short-term test, keep windows and exterior doors closed as much as possible (normal entry and exit is fine). Run your heating or cooling system normally, but don’t operate fans that bring in outside air. These closed-house conditions give you a realistic worst-case reading.
If your first test comes back elevated, don’t panic over one number. Confirm it. A second short-term test or, better yet, a three-month long-term test will tell you whether that initial reading reflects your true average or just an unlucky few days of low pressure and cold weather. Your annual average is the number that actually corresponds to your health risk, because lung cancer from radon is driven by years of cumulative exposure, not brief spikes.