No level of radon is technically “safe,” but the EPA sets 4 pCi/L (picocuries per liter) as the action level, the concentration at which you should take steps to reduce radon in your home. Below that number, the cancer risk from radon is low but not zero. The average outdoor air concentration is about 0.4 pCi/L, which serves as a useful baseline for comparison.
The 4 pCi/L Action Level
The EPA’s action level of 4 pCi/L is not a line between “safe” and “dangerous.” It’s a practical threshold. Above it, mitigation is strongly recommended. Between 2 and 4 pCi/L, the EPA still encourages homeowners to consider reducing levels, since long-term exposure at those concentrations carries some risk.
The World Health Organization uses a stricter reference level of 100 Bq/m³, which translates to roughly 2.7 pCi/L. The International Atomic Energy Agency sets a maximum reference of 300 Bq/m³ (about 8.1 pCi/L) for homes. So depending on which standard you follow, the threshold for action ranges from about 2.7 to 4 pCi/L. If your reading comes back under 2 pCi/L, your risk is close to the outdoor background level and most guidelines consider that acceptably low.
How Radon Causes Lung Cancer
Radon is a radioactive gas that seeps up from soil and rock into buildings through cracks in foundations, gaps around pipes, and other openings. It’s odorless and invisible, so you can’t detect it without a test. When you breathe it in, radon’s decay products release alpha particles, a form of radiation with enough energy to penetrate the lining of your airways and damage the DNA inside cells. Unlike other types of radiation, alpha particles tend to cause clustered damage to DNA strands, which makes the resulting breaks harder for your cells to repair correctly. Errors in that repair process can lead to mutations that eventually trigger cancer.
Radon is the second leading cause of lung cancer after smoking, responsible for an estimated 21,000 deaths per year in the United States.
Risk Levels for Smokers vs. Non-Smokers
At the average indoor radon concentration of about 1.3 pCi/L, a person who has never smoked has roughly a 2 in 1,000 chance of developing lung cancer from radon exposure over a lifetime. A smoker exposed to that same level faces a 20 in 1,000 chance, ten times the risk.
This gap exists because nicotine and radon interact at the cellular level. Research on human lung cells shows that nicotine speeds up DNA repair after alpha particle damage, but that faster repair comes at a cost: it’s sloppier. Cells exposed to both nicotine and alpha radiation show significantly more chromosome translocations, a hallmark of cancer development, compared to cells exposed to radiation alone. Nicotine also appears to loosen the structure of DNA packaging in ways that make these dangerous rearrangements more likely. On top of that, the combination ramps up inflammatory signaling in lung tissue, creating an environment that further promotes tumor growth.
Overall, about 62 out of every 1,000 smokers will die of lung cancer, compared to 7.3 out of 1,000 for people who have never smoked. Radon exposure amplifies that disparity at every concentration level.
How to Test Your Home
Radon levels vary dramatically from house to house, even between neighbors, so the only way to know your level is to test. Hardware stores and online retailers sell do-it-yourself test kits for under $20. There are two types:
- Short-term kits measure radon over 2 to 7 days and give you a quick snapshot. These are useful for an initial screening.
- Long-term kits measure radon for over 90 days and capture seasonal fluctuations, giving you a more accurate year-round average.
If a short-term test comes back at or above 4 pCi/L, a follow-up test (either another short-term or a long-term kit) confirms whether mitigation is needed. If the result is between 2 and 4, a long-term test gives you a better picture before deciding on next steps.
Where to Place the Kit
Place the test in the lowest livable level of your home. That usually means the basement if it’s finished or could reasonably be used as living space. Choose a room where people spend time, like a family room, bedroom, or den. Avoid kitchens, bathrooms, laundry rooms, and hallways, where humidity, drafts, or ventilation can skew results. The kit should sit at least 20 inches above the floor, away from exterior walls, drafts, and heat sources.
What to Do if Your Level Is High
Radon mitigation typically involves a contractor installing a vent pipe and fan system that draws radon from beneath your foundation and releases it above the roofline, where it disperses harmlessly into outdoor air. Most systems reduce indoor radon by up to 99%, often bringing levels well below 2 pCi/L. Installation generally takes less than a day and costs between $800 and $2,500 depending on your home’s construction and location.
After a system is installed, retesting confirms it’s working. The EPA recommends testing again every two years or after any major renovation that could change airflow patterns in your home, since radon levels can shift over time as soil conditions and building seals change.