The EPA’s action level for radon is 4 pCi/L (picocuries per liter). If your home tests at or above that number, the EPA recommends installing a radon reduction system. But there is no known safe level of radon exposure, so the EPA also recommends considering mitigation for homes between 2 and 4 pCi/L.
What the Numbers Mean
Radon is measured in picocuries per liter of air (pCi/L) in the United States, or in becquerels per cubic meter (Bq/m³) in most other countries. The 4 pCi/L action level equals roughly 150 Bq/m³. That threshold isn’t a safety line. It’s an action trigger, meaning the EPA considers any radon exposure a health risk but views 4 pCi/L as the point where the cost and effort of fixing the problem clearly outweigh the risk of leaving it alone.
For context, outdoor air typically contains very low radon levels, around 0.4 pCi/L. Indoor air in the U.S. averages higher because buildings trap radon that seeps up from the soil. If your test comes back at 2 pCi/L, your home is below the action level but still carries some risk. At 1.3 pCi/L or lower, you’re in the range of a typical indoor background level and there’s little practical benefit to mitigation.
How Other Countries Set Their Limits
The U.S. action level is one of the stricter guidelines in the world, but not the strictest. Canada’s guideline is 200 Bq/m³ (about 5.4 pCi/L), which was lowered from 800 Bq/m³ in 2007. The World Health Organization recommends an action level of 100 Bq/m³ (about 2.7 pCi/L), which is actually tighter than the EPA’s. These differences reflect varying policy judgments about feasibility and cost, not disagreements about the science. All major health organizations agree that lower is better.
Why Radon Is Dangerous
Radon is a radioactive gas that forms naturally as uranium in soil and rock breaks down. It’s colorless and odorless, so the only way to know your level is to test. When you breathe radon in, its decay products release tiny bursts of radiation called alpha particles inside your lungs. These particles damage DNA in lung tissue, and over years of exposure, that damage can lead to lung cancer. Radon is the leading cause of lung cancer among non-smokers in the United States.
What makes radon especially concerning is that the radiation doesn’t need to hit a cell’s DNA directly to cause harm. Research has shown that alpha particles can trigger chemical signals in nearby tissue that damage DNA in cells that were never directly hit. This “bystander effect” means even relatively low doses can cause genetic changes across a wider area of lung tissue than you might expect from the radiation dose alone.
If you smoke, the risk compounds dramatically. Smokers exposed to elevated radon face an estimated 10 to 20 times greater risk of lung cancer compared to non-smokers with the same radon exposure. The combination of tobacco smoke and radon is far more dangerous than either one alone.
How to Test Your Home
Radon test kits fall into two categories: short-term and long-term. Short-term kits measure radon over 2 to 90 days and give you a quick snapshot. Long-term kits measure for more than 90 days and provide a much more accurate picture of your home’s year-round average, since radon levels fluctuate with seasons, weather, and how often you open windows. The longer the test runs, the more reliably it reflects what you’re actually breathing day to day.
For a first screening, a short-term test is a reasonable starting point. You can buy charcoal-based kits at most hardware stores for under $20. Place the kit in the lowest lived-in level of your home (usually the basement or ground floor), keep windows and exterior doors closed as much as possible during the test, and mail it to the lab when the testing period ends. If the result comes back at 4 pCi/L or higher, the standard recommendation is to follow up with either a second short-term test or a long-term test to confirm before investing in mitigation.
What Mitigation Looks Like
If your home needs radon reduction, the most common and reliable method is called active subslab suction. A contractor installs a pipe through the foundation slab and connects it to a small fan that continuously draws radon-laden air from beneath your home and vents it outside before it can enter your living space. The same principle works for homes with crawlspaces, where a plastic membrane is laid over the soil and a fan pulls radon out from underneath it.
These systems are effective. Subslab suction and crawlspace depressurization can reduce radon levels by 50 to 99 percent, depending on the home’s construction and soil conditions. A typical installation costs between $800 and $2,500 and takes less than a day. The fan runs continuously and uses about as much electricity as a lightbulb. After installation, you should retest to confirm levels have dropped below 4 pCi/L, ideally below 2.
Radon and Home Sales
Radon testing often comes up during real estate transactions. The Federal Housing Administration requires that all prospective homebuyers receive an inspection form that includes a section on radon testing, following a recommendation from both the EPA and the U.S. Surgeon General that all homes be tested. State-level requirements vary. Some states mandate radon disclosure if the seller has test results, while others have no specific radon laws at all.
If you’re buying a home, requesting a radon test during the inspection period is straightforward and inexpensive. A result above 4 pCi/L gives you leverage to negotiate mitigation as part of the sale, and most sellers are willing to address it since the fix is relatively cheap compared to other home repairs. If you’re selling, having a recent test result (or a mitigation system already installed) removes a potential obstacle and shows buyers the home’s air quality is addressed.