Safe Radon Levels: What the Numbers Mean and When to Act
The short answer: there is no known safe level of radon. The EPA recommends taking action at 4 pCi/L (picocuries per liter), while the World Health Organization sets its threshold lower at 2.7 pCi/L. Both agencies agree that any radon exposure carries some lung cancer risk. But practical guidance exists for every level, and knowing your number tells you exactly what to do next.
Radon Levels Chart
This chart breaks down radon test results into clear categories with recommended actions. Radon is measured in picocuries per liter (pCi/L) in the United States.
| Radon Level (pCi/L) | Risk Category | Smoking Equivalent | Recommended Action |
|---|---|---|---|
| Below 1.3 | Average outdoor level | Minimal | No action needed |
| 1.3 to 2.0 | Below average indoor level | Minimal | No action needed. Retest in 2 to 5 years |
| 2.0 to 4.0 | Below EPA action level, above WHO threshold | Comparable to 4 to 8 cigarettes per day | Consider a long-term test to confirm. Mitigation is optional but beneficial |
| 4.0 to 8.0 | Above EPA action level | Comparable to 8 or more cigarettes per day | Mitigate within a few months |
| 8.0 to 20.0 | High | Comparable to half a pack to one pack per day | Mitigate promptly |
| Above 20.0 | Very high | Comparable to 1 to 2 packs per day | Take action immediately. Limit time in lowest level of home until system is installed |
The average outdoor radon level in the U.S. is about 0.4 pCi/L. The average indoor level is approximately 1.3 pCi/L. About 1 in 15 homes in the U.S. has radon levels at or above the EPA action level of 4 pCi/L.
Understanding Your Radon Test Results
If you just got your radon test results back, here is how to read them. Your result is a single number measured in pCi/L. Compare it to the chart above to find your category.
A few things to keep in mind when reading your results:
- Short-term tests (2 to 7 days) give a snapshot, not a long-term average. Radon levels fluctuate daily and seasonally.
- If your short-term test comes back between 2 and 4 pCi/L, a follow-up long-term test (90 days or more) gives a more accurate picture of your actual exposure.
- If your short-term test is above 4 pCi/L, the EPA recommends following up with either a second short-term test or a long-term test. If both tests average above 4, mitigate.
- Test results above 8 pCi/L warrant action without waiting for a second test.
EPA vs. WHO: Why the Guidelines Differ
The EPA and WHO set different action levels, and the reason comes down to practicality versus pure health science.
The WHO recommends action at 2.7 pCi/L (100 Bq/m³). This threshold is based strictly on health risk data. Research shows that lung cancer risk increases measurably at levels above 2.7 pCi/L, even for nonsmokers.
The EPA recommends action at 4.0 pCi/L (148 Bq/m³). The EPA chose this level because it is technically achievable through mitigation in most homes. Reducing radon below 2 pCi/L is not always possible, especially in areas with high natural radon. The EPA set its action level where mitigation could reliably deliver results.
That said, the EPA does not consider 4 pCi/L “safe.” The agency states clearly that homeowners should “consider fixing” their home if levels are between 2 and 4 pCi/L. The 4 pCi/L threshold is a practical action trigger, not a safety guarantee.
For context: living in a home with 4 pCi/L of radon carries roughly the same lung cancer risk as the risk of dying in a car accident over a lifetime. At 8 pCi/L, the risk is comparable to smoking 8 cigarettes every day. The EPA estimates that radon causes about 21,000 lung cancer deaths per year in the United States, making it the leading cause of lung cancer among nonsmokers.
What Affects Your Radon Level
Radon levels are not constant. They change with the seasons, the weather, and how your home is built. Understanding these factors helps explain why a single test is a snapshot, not a definitive reading.
Season
Radon levels tend to be highest in winter and lowest in summer. In cold months, the warm air inside your home rises and escapes through the upper floors and roof (the stack effect), pulling soil gas, including radon, in through the foundation. Closed windows and doors in winter also reduce ventilation, allowing radon to accumulate.
Weather and Barometric Pressure
Falling barometric pressure can draw more radon from the soil. Rain-saturated ground traps radon near the surface, pushing more of it toward your foundation. Wind can create pressure differentials around your home that pull soil gas indoors. A single-day radon reading during a low-pressure storm can be significantly higher than the weekly average.
Home Ventilation
Homes that are tightly sealed for energy efficiency can trap radon more effectively. Opening windows dilutes radon but is not a practical long-term solution. Mechanical ventilation systems (like HRVs) can help but are less effective than dedicated radon mitigation.
Foundation Type
Homes with basements and slab-on-grade foundations have more contact with the soil and tend to have higher radon levels than homes with open crawl spaces. Cracks in the foundation, gaps around pipes, and sump pits all provide entry points for radon.
Local Geology
Radon comes from the natural decay of uranium in soil and rock. Granite, shale, and phosphate-rich soils produce more radon. Two houses on the same street can have very different radon levels depending on the soil composition directly beneath each foundation.
Because of all these variables, a long-term test (90 days or more) gives a much better picture of your actual radon exposure than a short-term test. If your short-term result is in the 2 to 4 pCi/L range, a long-term test is the smartest next step.
What to Do at Each Level
Here are specific steps based on your radon test results:
Below 2 pCi/L
Your home is below both the EPA and WHO action levels. No mitigation is needed. Retest every 2 to 5 years, or sooner if you make significant changes to your home (like finishing a basement, adding an addition, or sealing the foundation). Radon levels can change over time.
2 to 4 pCi/L
Your home is below the EPA action level but above the WHO threshold. Consider running a long-term test (90+ days) to confirm the level. Mitigation at this range is optional but reduces your risk. If your long-term test confirms levels above 2 pCi/L, mitigation is a reasonable investment, especially for homes with children or long-term occupants.
4 to 8 pCi/L
Your home is above the EPA action level. Mitigate within a few months. Contact a certified radon mitigation professional to install a system. Most homes in this range are brought below 2 pCi/L after mitigation. If you ran a short-term test, you can do a second short-term test to confirm before scheduling installation, but do not delay beyond a few months.
8 to 20 pCi/L
Your home has high radon levels. Mitigate promptly. At these concentrations, every month of delay increases your cumulative exposure significantly. Contact a certified radon professional and schedule installation as soon as possible. A second test is not necessary at this level.
Above 20 pCi/L
Your home has very high radon levels. Take action immediately. While waiting for mitigation, limit the time you and your family spend in the lowest level of the home (where radon concentrations are highest). Keep windows open on the lowest level if weather permits. This is a temporary measure; professional mitigation is the only real solution. Contact a certified radon professional today.
Post-Mitigation: What to Expect
After a radon mitigation system is installed, here is what happens next.
Immediate reduction: A properly installed sub-slab depressurization system begins working right away. Most systems reduce radon levels by 80% to 99%.
Typical post-mitigation levels: Homes that tested between 4 and 20 pCi/L before mitigation typically drop to 1 to 2 pCi/L after installation. Homes with very high pre-mitigation levels (above 20 pCi/L) usually reach 2 to 4 pCi/L, though many get below 2.
Post-installation testing: Retest your home 24 to 48 hours after the system is installed and running. Use a short-term test placed in the lowest livable area of the home. This confirms the system is working as expected.
Ongoing monitoring: Retest every 2 years to make sure the system continues to perform. Check the system’s manometer (a U-tube gauge on the pipe) monthly to verify the fan is creating suction. If the fluid levels in the manometer are equal, the fan may have failed and needs replacement.
Fan replacement: Radon fans typically last 5 to 10 years. Replacement costs $200 to $400 including labor. The PVC piping lasts indefinitely.
If levels remain above 4 pCi/L after mitigation: Contact your installer. The system may need an additional suction point, better foundation sealing, or a more powerful fan. A qualified professional can diagnose and correct the issue.
Take Action
Whether your radon test results show 3 pCi/L or 30 pCi/L, knowing your number is the first step. If your level calls for mitigation, the process is straightforward: a certified professional can install a system in a single day, and the cost typically runs $800 to $2,500 depending on your home.
Find a certified radon professional near you to test your home or install a mitigation system.
Frequently Asked Questions
What is a safe level of radon in a home?
There is no known safe level of radon. The EPA recommends taking action at 4 pCi/L, while the WHO recommends action at 2.7 pCi/L. The EPA also suggests homeowners consider mitigation between 2 and 4 pCi/L. The lower your radon level, the lower your risk.
What does a radon level of 3 pCi/L mean?
A radon level of 3 pCi/L is below the EPA action level of 4 pCi/L but above the WHO threshold of 2.7 pCi/L. The EPA recommends considering mitigation in this range. A long-term test (90 days or more) will give you a more accurate reading to help decide.
How often should I test my home for radon?
Test your home every 2 years, even if previous results were low. Also retest after any major renovation, especially if you finish a basement or modify the foundation. If you have a mitigation system, retest every 2 years to confirm it is still working.
Can radon levels change over time?
Yes. Radon levels fluctuate with seasons, weather, and changes to your home. Levels tend to be highest in winter due to closed windows and the stack effect. A home that tests at 3 pCi/L in summer might test at 5 pCi/L in winter. Long-term tests give the most reliable average.
What radon level is normal after mitigation?
Most mitigation systems reduce radon to 1 to 2 pCi/L. A properly installed system reduces levels by 80% to 99%. Retest 24 to 48 hours after installation to confirm, then retest every 2 years. If levels remain above 4 pCi/L, contact your installer for adjustments.