The EPA considers any radon level at or above 4 pCi/L (picocuries per liter) dangerous enough to require action. At that threshold, the agency recommends installing a mitigation system to reduce radon in your home. But there is no known safe level of radon exposure, which is why the EPA also recommends considering fixes for levels between 2 and 4 pCi/L. Radon is the second leading cause of lung cancer in the United States, responsible for an estimated 21,000 lung cancer deaths each year.
The Official Action Levels
In the U.S., the EPA sets the action level at 4 pCi/L. If a home tests at or above that number, the recommendation is to install a radon reduction system. Between 2 and 4 pCi/L, mitigation is still worth considering, especially if you smoke or plan to stay in the home long term.
International guidelines are stricter. The World Health Organization and the International Atomic Energy Agency set maximum reference levels at 300 Bq/m³ for homes, which converts to roughly 8 pCi/L. For workplaces, the ceiling is 1,000 Bq/m³. The WHO’s reference level is higher than the EPA’s action level, but the EPA’s 4 pCi/L threshold is not a “safe” line. It’s a practical trigger for action, acknowledging that any exposure carries some risk.
Why Radon Is Harmful
Radon is a radioactive gas that seeps into buildings from the natural breakdown of uranium in soil and rock. You can’t see it, smell it, or taste it. When you breathe it in, radon and its decay products release alpha particles, a form of radiation that damages lung tissue at the cellular level. These particles can alter DNA directly, but research published in Environmental Health Perspectives found they also cause damage indirectly: alpha particles generate chemical signals that travel to nearby cells and trigger DNA changes even in cells that were never hit by radiation. This “bystander effect” means the zone of damage extends beyond what the particles physically touch.
There are no immediate symptoms from radon exposure. No coughing, no headaches, no warning signs. Lung cancer from radon typically develops 5 to 25 years after exposure, which is why testing is the only way to know if your home has a problem.
How Smoking Multiplies the Risk
Radon is dangerous on its own, but the combination of radon and smoking is far worse than either risk alone. The numbers illustrate this clearly. Research from the Utah Radon Lab calculated the lifetime risk of radon-related lung cancer for a 40-year-old living in their current home for another 10 years.
At 4 pCi/L, a current smoker who does nothing has a 1.69% lifetime chance of radon-related lung cancer. That may sound small, but for context, a nonsmoker at the same radon level faces just 0.09%, roughly 19 times lower. If the smoker quits but doesn’t fix the radon, risk drops to 0.81%. If they fix the radon but keep smoking, it only drops to 1.35%. Quitting smoking delivers a bigger risk reduction than mitigation alone.
At 10 pCi/L, the stakes rise sharply. A smoker who does nothing faces a 3% lifetime risk. Combining quitting with radon mitigation cuts that to 0.9%, a 70% reduction. For nonsmokers at 10 pCi/L, the risk is 0.16%, which mitigation would lower by nearly 40%. The takeaway: if you smoke and have elevated radon, both quitting and mitigating matter, but quitting smoking is the single most impactful step.
How to Test Your Home
Radon levels fluctuate throughout the day, across seasons, and with weather changes. That variability makes testing method and duration important. Short-term tests run anywhere from 2 to 10 days using active electronic monitors, while long-term tests use passive detectors (small devices you leave in place) for three months or more.
Short-term tests work well as an initial screen. Most experts recommend a minimum of four days, and a full week is better. For homes with low readings (below about 2 pCi/L), a one-week test can predict the annual average with over 95% confidence. But at moderate levels, short-term tests only predict the annual average about 50% of the time. For homes in high-risk zones or with borderline results, a three-month long-term test is the gold standard for accuracy.
Before and during a short-term test, you need to maintain closed-building conditions: keep windows and exterior doors shut (except for normal entry and exit) starting at least 12 hours before the test begins. Place the test device in the lowest lived-in level of your home, since radon concentrations are highest closest to the ground. Avoid placing it in a kitchen, bathroom, or near drafts.
If a short-term test comes back at 4 pCi/L or higher, the standard protocol is to follow up with either a second short-term test or a long-term test before committing to mitigation. If the result is very high, say above 8 or 10 pCi/L, most professionals recommend moving straight to mitigation without waiting for a confirmation test.
What Mitigation Looks Like
The most common and effective fix is called sub-slab depressurization. A contractor drills a small hole through the basement floor or foundation slab, inserts a pipe, and attaches a fan that pulls radon-laden air from beneath the house and vents it above the roofline before it can seep inside. The system runs continuously and is about the size of a small appliance.
This approach reduces radon levels by 50 to 99%, according to the EPA. Most homes with levels in the 4 to 10 pCi/L range can be brought well below 2 pCi/L. Installation typically takes a day, and the systems are relatively quiet. Ongoing costs are limited to the electricity for the fan, usually comparable to running a light bulb. The fan will eventually need replacement, but most last several years.
After installation, you should retest to confirm the system is working. Periodic retesting every two years or so ensures the system continues to perform, since soil conditions and building settling can change over time.
Radon Levels by Context
- Below 2 pCi/L: Low risk. No action needed, though zero risk does not exist at any level.
- 2 to 4 pCi/L: Consider mitigation, especially if you smoke, have young children, or plan to live in the home long term.
- 4 pCi/L and above: The EPA action level. Mitigation is recommended.
- 8 pCi/L and above: Exceeds the WHO reference level for homes. Mitigation should be a priority.
- 10+ pCi/L: Significantly elevated. Smokers at this level face a 3% lifetime risk of radon-related lung cancer. Fix promptly.
Radon is present in virtually every home at some concentration. About 1 in 15 U.S. homes has levels at or above the EPA’s action level. The only way to know your number is to test, and inexpensive kits are available at hardware stores or through state radon programs, often for free.